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Versions: 00 01 02 03 04 05 06 07                          Informational
Network Working Group                                        E. Rescorla
Internet-Draft                                                RTFM, Inc.
Intended status:  Standards Track                       G. Lebovitz, Ed.
Expires:  August 31, 2010                         Juniper Networks, Inc.
                                             Internet Architecture Board
                                                       February 27, 2010

                 A Survey of Authentication Mechanisms


   Authentication is a common security issue for the design of Internet
   protocols.  A wide variety of authentication technologies are
   available.  A common problem is knowing which technology to choose or
   which of a variety of essentially similar implementations of a given
   technique to choose.  This memo provides a survey of available
   authentication mechanisms and guidance on selecting one for a given

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on August 31, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the

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   document authors.  All rights reserved.

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   it for publication as an RFC or to translate it into languages other
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  The Authentication Problem . . . . . . . . . . . . . . . . . .  7
     2.1.  Authorization vs. Authentication . . . . . . . . . . . . .  7
     2.2.  Authentication Building Blocks . . . . . . . . . . . . . .  8
     2.3.  Clients and Servers  . . . . . . . . . . . . . . . . . . .  8
   3.  Basic Authentication, Attacks, and Counter Measures  . . . . .  9
     3.1.  Password Sniffing  . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Post-Authentication Hijacking  . . . . . . . . . . . . . .  9
     3.3.  Online Password Guessing (aka Brute Force Attack)  . . . . 10
     3.4.  Offline Dictionary Attack  . . . . . . . . . . . . . . . . 10
       3.4.1.  Shadow Passwords . . . . . . . . . . . . . . . . . . . 11
       3.4.2.  Iteration  . . . . . . . . . . . . . . . . . . . . . . 11
       3.4.3.  Salting  . . . . . . . . . . . . . . . . . . . . . . . 11
       3.4.4.  Stronger Passwords . . . . . . . . . . . . . . . . . . 12
       3.4.5.  Encrypted Channel  . . . . . . . . . . . . . . . . . . 12
     3.5.  Phishing . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.6.  Case Study: HTTP Basic Authentication  . . . . . . . . . . 13
       3.6.1.  Password Caching . . . . . . . . . . . . . . . . . . . 13
       3.6.2.  Proactive authentication . . . . . . . . . . . . . . . 13
     3.7.  List of Systems that Use Passwords in the Clear  . . . . . 13
   4.  One Time Passwords . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  Case Study: S/Key and OTP  . . . . . . . . . . . . . . . . 15
       4.1.1.  Race Conditions  . . . . . . . . . . . . . . . . . . . 16
     4.2.  Case Study: SecurID  . . . . . . . . . . . . . . . . . . . 17
     4.3.  List of One-Time Password Systems  . . . . . . . . . . . . 17
   5.  Challenge/Response . . . . . . . . . . . . . . . . . . . . . . 18
     5.1.  Offline Attacks on Challenge/Response  . . . . . . . . . . 19
     5.2.  Password File Compromise . . . . . . . . . . . . . . . . . 19
     5.3.  Case Study: CRAM-MD5 . . . . . . . . . . . . . . . . . . . 20
     5.4.  Case Study: HTTP Digest  . . . . . . . . . . . . . . . . . 21
       5.4.1.  Message Integrity  . . . . . . . . . . . . . . . . . . 21
       5.4.2.  Replay Attack  . . . . . . . . . . . . . . . . . . . . 22
       5.4.3.  Downgrade Attack . . . . . . . . . . . . . . . . . . . 22
     5.5.  List of Challenge-Response Systems . . . . . . . . . . . . 23
   6.  Anonymous Key Exchange . . . . . . . . . . . . . . . . . . . . 23
     6.1.  Case Study: SSH Password Authentication  . . . . . . . . . 24
     6.2.  List of Anonymous Key Exchange Mechanisms  . . . . . . . . 25
   7.  Zero-Knowledge Password Proofs . . . . . . . . . . . . . . . . 25
     7.1.  Intellectual Property  . . . . . . . . . . . . . . . . . . 26
     7.2.  List of Zero Knowledge Password Proof Systems  . . . . . . 26
   8.  Server Certificates plus User Authentication . . . . . . . . . 26
     8.1.  Case Study: Passwords over HTTPS . . . . . . . . . . . . . 28
       8.1.1.  Authentication State . . . . . . . . . . . . . . . . . 28
     8.2.  List of Server Certificate Systems . . . . . . . . . . . . 30
   9.  Mutual Public Key Authentication . . . . . . . . . . . . . . . 30
     9.1.  Password Equivalence . . . . . . . . . . . . . . . . . . . 31

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     9.2.  Authentication between Unknown Parties . . . . . . . . . . 31
     9.3.  Key Storage  . . . . . . . . . . . . . . . . . . . . . . . 32
     9.4.  Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     9.5.  Password Derived Keys  . . . . . . . . . . . . . . . . . . 32
     9.6.  Case Study: SMTP over TLS  . . . . . . . . . . . . . . . . 33
     9.7.  List of Mutual Public Key Systems  . . . . . . . . . . . . 33
   10. Generic Issues . . . . . . . . . . . . . . . . . . . . . . . . 33
     10.1. Channel Security Protocols . . . . . . . . . . . . . . . . 33
       10.1.1. Limited Authentication Options . . . . . . . . . . . . 34
       10.1.2. Limited Application Integration  . . . . . . . . . . . 34
       10.1.3. List of Channel Security Protocols . . . . . . . . . . 34
     10.2. Authentication Frameworks  . . . . . . . . . . . . . . . . 35
       10.2.1. Downgrade Attacks  . . . . . . . . . . . . . . . . . . 37
       10.2.2. Multiple Equivalent Mechanisms . . . . . . . . . . . . 37
       10.2.3. Channel Bindings . . . . . . . . . . . . . . . . . . . 40
       10.2.4. Excessive Layering . . . . . . . . . . . . . . . . . . 41
       10.2.5. List of Authentication Frameworks  . . . . . . . . . . 42
   11. Sharing Authentication Information . . . . . . . . . . . . . . 42
     11.1. Authentication Services  . . . . . . . . . . . . . . . . . 42
     11.2. Single Sign-On . . . . . . . . . . . . . . . . . . . . . . 43
     11.3. Case Study: RADIUS . . . . . . . . . . . . . . . . . . . . 43
     11.4. Case Study: Kerberos . . . . . . . . . . . . . . . . . . . 44
     11.5. List of Authentication Server Systems  . . . . . . . . . . 44
   12. Guidance for Protocol Designers  . . . . . . . . . . . . . . . 44
     12.1. Know what you're trying to do  . . . . . . . . . . . . . . 44
       12.1.1. What's my threat model?  . . . . . . . . . . . . . . . 45
       12.1.2. How many users will this system have?  . . . . . . . . 45
       12.1.3. What's my protocol architecture? . . . . . . . . . . . 45
       12.1.4. Do I need to share authentication data?  . . . . . . . 46
     12.2. Use as few mechanisms as you can . . . . . . . . . . . . . 46
     12.3. Avoid simple passwords . . . . . . . . . . . . . . . . . . 47
     12.4. Avoid inventing new frameworks . . . . . . . . . . . . . . 47
     12.5. Use the strongest mechanisms you can . . . . . . . . . . . 47
     12.6. Consider providing message integrity . . . . . . . . . . . 48
   13. Scenarios  . . . . . . . . . . . . . . . . . . . . . . . . . . 48
     13.1. Capability Considerations  . . . . . . . . . . . . . . . . 48
       13.1.1. Neither side has a public/private key pair . . . . . . 49
       13.1.2. One side has an authenticated key pair . . . . . . . . 49
       13.1.3. Both sides have authenticated key pairs  . . . . . . . 50
     13.2. Architectural Considerations . . . . . . . . . . . . . . . 50
       13.2.1. Simple Connection  . . . . . . . . . . . . . . . . . . 50
       13.2.2. Proxied Client/Server  . . . . . . . . . . . . . . . . 50
       13.2.3. Store and Forward  . . . . . . . . . . . . . . . . . . 52
       13.2.4. Multicast  . . . . . . . . . . . . . . . . . . . . . . 52
   14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 53
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 53
     15.2. Informative References . . . . . . . . . . . . . . . . . . 53

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   Appendix A.  IAB Members at the time of this writing . . . . . . . 59
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60

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1.  Introduction

   Authentication is perhaps the most basic security problem for
   designers of network protocols.  Even the early Internet protocols
   such as TELNET [RFC0854] and FTP [RFC0959], which provided no other
   security services, made provision for user authentication.
   Unfortunately, these early authentication systems were wholly
   inadequate for the Internet Threat Model [RFC3552] and a vast array
   of other authentication mechanisms have been introduced in an attempt
   to close these holes.

   The most striking thing about these security mechanisms is how many
   of them are essentially similar.  There are only 7 basic classes of
   authentication protocol but there are a large number of slightly
   different protocols with essentially the same security properties.
   This memo surveys the space of authentication mechanisms, describes
   the basic classes and provides examples of protocols which fit into
   each class.  This document is aimed at protocol designers, more so
   than system deployers.

   In section 2 we will review the problem space around authentication.
   It will define what we are trying to accomplish when using
   authentication mechanisms, will contrast authorization and
   authentication, and review the basic building blocks of

   In section 3 we introduce basic password authentication systems,
   describe attacks that formed against them, and the counter measures
   that addressed such attacks.  These descriptions will lay a
   foundation of understanding and terminology useful for the mechanisms
   described in the sections that follow section 3.

   The next six sections, starting with section 4, each describe a
   single class of authentication technology.  In each case, we first
   describe the technology in general, with possible subsections
   describing security or implementation issues that are generic to this
   technology.  Once we have described the technology in general we then
   provide one or more case studies:  descriptions of specific protocols
   which use this authentication technology and the various security or
   implementation issues that are specific to that protocol.  Thus, each
   section uses the following pattern.

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        A Mechanism
        A.x risk
          (description and countermeasures)
        A.y risk
          (description and countermeasures)
        A.z Case Study: Specific Protocol
          (description of the protocol)
        A.z.x Protocol Specific problems.
        A.w List of known Protocols/Systems that use this mechanism

   In order to understand the pros and cons of each mechanism, it's
   important to have a clear idea of the threat model for the
   environment in which your protocol will be deployed.  [RFC3552]
   provides more information on threat modelling.

2.   The Authentication Problem

   The authentication problem is simple to describe but hard to solve:
   Two parties (or devices/daemons/endpoints/etc.) are communicating and
   one wishes to establish its identity to another, or establish the
   identity of another.  The basic scenario is exemplified by TELNET
   [RFC0854].  A client (on behalf of a user) wishes to remotely access
   resources on a TELNET server.  The user has an account on the server
   and the server remembers the user's authentication information but
   the client itself may have no long-term storage and only limited
   computational capabilities.  The client side of the credentials must
   be able to be carried by the user, either on a small device or in his
   memory.  Note that the major technical division here is between
   situations in which there is any client-side storage (for instance
   for cryptographic keys) and ones where any authentication material
   must be memorized.

2.1.   Authorization vs. Authentication

   AUTHORIZATION is the process by which one determines whether an
   authenticated party has permission to access a particular resource or
   service.  Although often tightly bound, it is important to realize
   that authentication and authorization are two separate mechanisms.
   Perhaps because of this tight coupling, authentication is sometimes
   mistakenly thought to imply authorization.  Authentication simply
   validates the identity of a party -- that the party really is who
   they claim to be; authorization defines whether they can perform a
   certain action.

   Authorization necessarily relies on authentication, but
   authentication alone does not imply authorization.  Having already

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   successfully passed some authentication step, but before granting
   permission to perform an action, the authorization mechanism must be
   consulted to determine whether that action is permitted.  This
   document is solely concerned with the first step, authentication.

2.2.   Authentication Building Blocks

   The classic formulation of authentication is that there are three
   different elements that can be employed, alone or in combinations

   1.  Something you have--a physical token like a key.
   2.  Something you know-- something known only to you, not recorded
       anywhere where another could obtain and use it, a secret, e.g., a
   3.  Something you are--some physical characteristic unique to you,
       e.g. a thumbprint.

   The best authentication mechanisms combine two or more of these
   mechanisms.  For instance, if you use a driver's license or a
   passport to authenticate, that's something you have (the license) and
   something you are (your resemblance to the picture on the license).
   In practice, biometric authentication mechanisms work poorly over the
   Internet.  Biometric authentication mechanisms work best where the
   relying party can directly verify the presence of the person being
   authenticated.  In general this is not possible over the Internet
   because the relying party does not control the authenticating party's
   computer.  Thus, it is difficult to distinguish real authentications
   from replay attacks mounted by attackers who have captured the user's
   biometric information.  So the best Internet authentication
   mechanisms will involve a token plus a secret.

2.3.   Clients and Servers

   Most of the protocols which run on the Internet are inherently
   asymmetric, with one peer taking the role of the client and the other
   the server.  Because the servers are generally fixed machines with a
   fixed IP address and the clients may have any IP address, many
   protocols (such as SSH or TLS) operate by attempting to authenticate
   the server first and, if that succeeds, to then authenticate the
   client.  This occurs because the client wants to disclose it's
   identity (and certainly its credentials) only to the "real" server,
   not an imposter.  When such systems are used in peer-to-peer
   contexts, it is still necessary for one peer to take on the client
   role and one the server.  Typically, the party which spoke first (the
   initiator) is treated as the client.

   Even protocols which are peer to peer (such as IKE) require one party

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   to speak first.  In such cases, it's appropriate to refer to that
   party as the initiator and the other as the responder.

3.  Basic Authentication, Attacks, and Counter Measures

   This section will describe the early forms of authentication systems,
   the attacks they attracted, and the counter measures taken.  These
   descriptions will lay a foundation of understanding and terminology
   that is useful for understanding the mechanisms in the sections that
   follow it.

   The most basic form of authentication is for the client to provide a
   username/password pair to the server.  The server then verifies the
   password against the user's stored credentials.  If they match, the
   server allows the client to access the resource.

   The most primitive approach is for the server to simply store the
   user's username and password in a file on the server's disk.  This
   has the serious problem that if the password file is somehow
   compromised, the attacker has immediate access to all user passwords
   and can log in as any user.  The standard approach, first described
   by Wilkes [Wil68], is to store the output of a one-way function
   (typically a cryptographic message digest (see [RFC4949] for a
   definition of terms like this)) of the password instead of the
   password itself.  When the server needs to verify a password, it
   computes the function using the password as input and compares the
   output against the stored output.  Because the function is one way,
   the server cannot recover the user's password from the password file.
   Therefore, anyone gaining access to the password hash file also
   cannot recover the user's password from the password file.

3.1.   Password Sniffing

   The simplest attack against passwords delivered by client is simple
   password sniffing.  The attacker arranges to intercept traffic
   between the client and the server (this is relatively easy,
   especially if the attacker is on the same network as one of the
   endpoints).  Since the password traverses the network in the clear,
   the attacker is easily able to recover the password and can use it
   for any future authentications.

3.2.   Post-Authentication Hijacking

   An attacker who can hijack network connections need not know the
   user's password at all.  He can simply wait for the user to complete
   his authentication and then take over the connection.  This attack is
   more difficult to mount than password sniffing, but as we'll see

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   later, it can be useful when stronger authentication schemes are

   This attack can be prevented.  The connection must use some form of
   per-packet (or per segment) cryptographic authentication which can be
   neither mimicked nor guessed by a man-in-the-middle.  More on this

3.3.   Online Password Guessing (aka Brute Force Attack)

   Extensive experience [Klein90] shows that users choose bad passwords.
   Common choices include the user's real name, login name, date of
   birth, and simple dictionary words.  An attacker with no special
   capabilities can therefore attack a server by simply trying known or
   common usernames and common passwords.  This technique was used to
   great effect by the Morris worm [Worm88]

   The standard countermeasure to this attack is to make it difficult
   for the attacker to try a large number of passwords.  This can be
   done by incorporating a LIMITED TRY capability.  After some number of
   failed attempts, the system simply locks the account and the user
   cannot log in even with the correct password.  Unfortunately, simple
   limited try provides the attacker with an easy denial-of-service
   (DoS) attack--he can lock any account simply by performing failed

   A superior approach is to incorporate a delay.  For instance, the
   system might allow the user to immediately try 3 passwords, but after
   three failures lock the account for 60 seconds, increasing the delay
   (up to some fixed maximum) for each failure.  This is a less
   effective countermeasure than simple limited try but resists the DoS
   attack better.

3.4.   Offline Dictionary Attack

   Even if digested password files are used, it still often possible for
   an attacker who recovers the digested password, or password file, to
   discover user's passwords.  The attacker can mount an OFFLINE
   DICTIONARY ATTACK on the password or password file.  A dictionary
   attack uses the fact that users tend to choose words rather than
   random strings in order to narrow the scope of exhaustive search.
   The attacker simply runs through each word (and common variations) in
   sequence, comparing the digest of the trial word against the digest
   in the password file.  There are a number of programs available to
   mount this sort of attack, including the classic Crack [Crack]

   An OFFLINE DICTIONARY ATTACK can occur in two ways.  First, using

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   sniffing, an attacker who sits along the path of packets between two
   hosts can observe the transmission, extract a digested password, and
   work offline to find a matching password.  A second variation occurs
   when an attacker recovers the entire digested password file for a

   There are four basic countermeasures to offline dictionary attack.

3.4.1.   Shadow Passwords

   The first countermeasure to an offline dictionary attack is to deny
   attackers the password digest.  In the original UNIX systems, reading
   the password file was the only way to get information about users and
   therefore the password file had to be publicly readable.  Later
   systems introduced SHADOW PASSWORDS, whereby the password file
   contained a dummy password and a second copy of the password file
   containing the encrypted passwords was unreadable except to root.
   Thus, unprivileged user processes would consult the ordinary password
   file (now containing dummy passwords) to get user information (such
   as name, home directory, etc) but only privileged processes can read
   the encrypted passwords.  Of course, sometimes an attacker can
   convince a privileged process (via bugs) to give him a copy of the
   file, thus allowing him to attack it.

3.4.2.   Iteration

   The second type of countermeasure is to make search slower.  One
   approach is to simply make the hash function slower.  The original
   UNIX crypt() function did this by repeating the basic operation
   (based on DES) 25 times.  (The designers also slightly modified the
   operation so that it couldn't be done with ordinary DES hardware.)
   The idea here is that noone will notice a second or so delay on login
   but that making each guess take a second will seriously slow down an
   attacker.  To compensate for the speed of modern computers, rather
   more iterations are currently required each year.

3.4.3.   Salting

   If a simple hash of the password is stored in the password file, then
   an attacker can attack all the passwords in the file in parallel.  He
   simply generates the hash of each candidate and then compares it
   against each stored hash.  In order to prevent this attack, many
   systems SALT the hash with some random value (which is different for
   each user).  Thus, instead of storing simply H(password) they store
   salt || H(salt || password), with the result that even two users who
   have the same password will in general not have the same stored
   password hash.  One interesting innovation is to use a secret salt.
   This requires the attacker to try all possible salts, automatically

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   slowing down the process (thereby making iteration unnecessary).

3.4.4.   Stronger Passwords

   The reason that dictionary attacks are so easy is that users choose
   bad passwords.  Even the 8 character UNIX password space allows 2^56
   possible passwords--a search space that is impractical for most
   attackers to search.  One obvious countermeasure is to force users to
   choose stronger passwords.  This can be done reactively by running a
   password cracker on your system or proactively by forcing users to
   use stronger passwords when they set them.  It's also possible to
   force users to use randomly generated passwords.  Unfortunately,
   unguessable passwords are often less memorable, causing users to
   write them down.  It's not clear that this is an improvement.
   Security-conscious people are often willing to use complex mnemonics
   to help remember random passwords but ordinary users are not.  One
   welcome innovation on this front is the replacement of the old UNIX
   DES-based crypt() function with an MD5-based function that accepts
   longer passwords, allowing the user to have a meaningful but still
   harder to guess password.

   After a lengthy discussion about passwords and their etropy, NIST
   800-63 [SP800-63], Appendix A, page 52 suggests a system that uses:

   o  a minimum of 8 character passwords, selected by subscribers from
      an alphabet of 94 printable characters,
   o  required subscribers to include at least one upper case letter,
      one lower case letter, one number and one special character, and;
   o  Used a dictionary to prevent subscribers from including common
      words and prevented permutations of the username as a password.

3.4.5.  Encrypted Channel

   Another countermeasure is to deny inline attackers a view of the
   password.  TLS is an example of a protocol that provides for this.
   Using certificates (described in a different section below), the
   client first verifies the identity of the server, then the two
   establish an encrypted channel.  Once the encryption is in place, the
   HTTP authentication occurs in which the client's password digest is
   sent to the server.  The TLS encryption prevents an attacker from
   seeing the authentication digest, and thus from attempting an OFFLINE

3.5.  Phishing

   Even an attacker with no access to the victim's network can capture a
   user's password with a social engineering attack (often called
   PHISHING).  In the basic attack, the attacker sends the victim an

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   email posing as some site that the victim has a relationship with
   (e.g., eBay, a bank, a web-based email service, or a corporate IT-
   Support Helpdesk) and containing a URL [RFC4248], [RFC4266] for the
   user to dereference.  When the user dereferences the URL he is
   prompted for his password, which he often supplies.  (See also

3.6.   Case Study: HTTP Basic Authentication

   HTTP basic authentication [RFC2617] is the original HTTP
   authentication mechanism.  It's a simple username/password scheme.
   The server prompts the client with a request for authentication (in a
   WWWAuthenticate header).  The client responds with the password in an
   Authorization header.  The password is base-64 encoded but this
   doesn't provide any security, just protection from damage due to
   transport reencoding.

3.6.1.   Password Caching

   Any reasonable Web page fetch consists of a number of HTTP fetches,
   each of which may requires HTTP authentication.  Requiring the user
   to type in his password for each such fetch would be prohibitively
   intrusive.  Accordingly, web clients typically cache the user's
   password for some time (generally for the lifetime of the browser

   In some cases, the browser will cache password on disk so that the
   user never has to type in the password again.  This practice
   introduces a new security problem:  protection of the user's cached
   passwords.  These passwords can be encrypted on disk (under another
   password) but users often find this inconvenient and so the passwords
   are often stored on the disk in the clear.  This is dangerous on
   multiuser machines, even ones which provide strong file permissions,
   since administrators can still read such cache files.

3.6.2.   Proactive authentication

   Requesting a page, receiving an authentication challenge and
   rerequesting with a password introduces an extra round-trip.  This
   latency can be quite significant if the original request was large,
   such as with a file PUT.  Thus, many clients proactively send their
   cached passwords whenever accessing any URL deeper than the URL for
   which they were originally prompted.

3.7.   List of Systems that Use Passwords in the Clear

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      FTP(when none of GSSAPI-KRB5, SRP, or TLS are negotiated)
      TELNET (when neither AUTH or STARTTLS are used)
      HTTP (basic authentication)
      SASL (password mode)
      POP (among others mechanisms)
      IMAP (among other mechanisms)
      (too many others to mention)

   The next six sections each describe a single class of authentication
   technology, according to the format described in the introduction
   Section 1

4.   One Time Passwords

   The simplest approach to preventing sniffing attacks on passwords is
   to use ONE TIME PASSWORDS.  In its basic form, the user is provided
   with a password (or list of passwords) that can only be used once,
   making replay attack impossible.  The passwords are still transmitted
   in the clear, but since each one can only be used once, a sniffed
   password cannot be used as an authenticator.

   The major use of one-time password systems is to improve the security
   of protocols which previously used password authentication.  One-time
   password schemes can be designed such that they require no changes to
   the client software and only minimal changes to the server software.

   One-time passwords are generally used in one of two ways:  a single
   one-time password used to bootstrap a trust relationship, or a
   continuous use of one-time passwords, i.e. new one-time password used
   at each login.

   A one-time password may be used to bootstrap a trust relationship.
   For example, a user might be given a single one-time password to
   access a system.  Once authenticated to that system, the user will be
   asked to supply a new, personalized password that will be used from
   then on.  Similarly, a one-time password can be used to authenticate
   a first exchange of long-term keys (e.g. asymmetric keys) between two
   parties.  This system is particularly good when many users or end-
   point machines will be connecting to a well-known, central system,
   but the user/end-points are not pre-known to the server.  An example
   is a smart-phone-based email client doing a first time registration
   from the a new smart phone to the provider's server.  The phone will
   ship with the provider's server's IP address, and public key. pre-
   installed The user will be given the one-time password and enter it
   via the smart phones UI on the first connection only.  The user
   initiates a secure connection to the server, and uses the server's

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   verification material to confirm the server's identity.  It then uses
   the one-time password to authenticate itself to the server.  It will
   then send it's public key (from a pre-computed asymmetric key pair)
   to the server, again, authenticated by the one-time password.  The
   server verifies the received messages using the one-time password,
   and confirms the identity of the host.  It can then trust the
   credentials, the public key, passed to it by the host.  Henceforth
   the two parties will authenticate one to the other using their public
   keys.  The one-time password is expired and discarded.

   The second main use of one-time passwords occurs when the user
   (generally) has either a physical password list or a token that
   computes the password, but the client software does not need to be
   replaced and the wire protocol is unchanged.

   The remainder of this section will describe three specific case
   studies of continuous use one-time password implementations available
   today:  S/Key, OTP and SecurID.  None of these one-time password
   schemes are very useful for automated authentication, since they only
   provide a limited number of keys.  Using automated authentication
   with S/Key or OTP it is easy to quickly use up a large number of
   keys.  SecurID provides an essentially infinite number of keys but
   they are changed too infrequently to be usable in most automated

   As with ordinary passwords, one time password mechanisms are subject
   to a number of active attacks.  However, even if the attacker
   captures a specific authenticator via an active attack, he can use it
   only once, not indefinitely.

4.1.   Case Study: S/Key and OTP

   S/Key [RFC1760], invented by Neil Haller and Phil Karn, is a
   straightforward one time password system that uses some clever
   implementation tricks.  One-Time Passwords (OTP) [RFC2289] is the
   successor protocol to S/Key, standardized by the IETF.  In S/Key, the
   one time passwords are constructed by iteratively hashing a public
   seed and a secret.  Thus:

        P[0] = H(Seed,Secret)
        P[i] = H(P[i-1]).

   Passwords are used in reverse order.  This allows the server to
   simply store the last password that it received (P[i]).  The client
   will next authenticate with P[i-1].  The server can verify a password
   by hashing it and checking to see if it matches the stored password.
   Once authentication is complete, the server simply deletes the old
   password and stores the new one.

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   S/Key uses a special password encoding that's designed to make it
   easy for users to type passwords without errors.  The 64-bit one-time
   password is broken up into a sequence of six 11-bit values (with the
   remaining two bits being used as a checksum).  Each 11-bit value is
   used as an index into a fixed dictionary of 2048 short words.  Thus,
   a password might look like:


   This encoding is intended to be easier to type than base64 or
   hexadecimal.  (Though hexadecimal is defined as well).

   S/Key can be used in two modes.  In the first, the client is simply
   provided with a list of passwords on a piece of paper.  He uses one
   at a time and crosses them off as he goes.  In this case, the Secret
   is usually cryptographically random.  In the second mode, the client
   has a token or a computer program that he uses to calculate the
   appropriate S/Key key.  In this case, the Secret is generally some
   user-memorable password which the user keys into the program or

   S/Key scheme has a number of nice properties.  First, the password
   file need not be kept secret, since going from P[i] to P[i-1]
   requires reversing the message digest, which is believed to be
   computationally infeasible.  (Note:  if a text password is used as
   the secret then the password file is still subject to dictionary
   attack, but a passive attacker who recovers ANY S/Key authenticator
   can mount a dictionary attack on it (by iteratively hashing the
   potential seed), so it's not that important to keep the password file
   per se secret).

   Second, it's easy for the user to rekey:  He simply creates a new
   Secret, generates a set of keys and sends the last one to the server.
   Note that it's of course possible for an active attacker to hijack a
   connection and rekey with a key of his choice, thus one time
   passwords are in general a poor choice when active attack is part of
   the threat model.

4.1.1.   Race Conditions

   S/Key has an interesting security flaw:  Consider a protocol where
   passwords are transmitted one character at a time.  A passive
   attacker might wait for the victim to log in and then create his own
   login connection at the same time.  The attacker would then echo the
   victim's password character for character, until there was only one
   character left.  At this point the attacker would simply guess the
   last character and then complete the authentication.  This attack is
   relatively simple to mount because nearly all the words in the S/Key

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   dictionary are 4-characters long and the number of words with any
   given 3-letter prefix is generally quite small (2 or 3).

   The standard countermeasure to this attack is to only allow one
   pending authentication for a given user at any given time.  In order
   to prevent DoS attacks, there must be at timeout on any such pending
   connection.  OTP implementations are required to implement this or
   some other countermeasure.

4.2.   Case Study: SecurID

   Probably the most commonly deployed commercial one time password
   implementation is SecurID, sold by RSA Security Inc. Instead of using
   a fixed list of keys, SecurID uses a time-dependent, six digit key.
   The user has a token with an LCD displaying a pseudo-random number.
   That number changes at an interval between 30 seconds and 2 minutes
   and is synchronized with an authentication server located at the
   server.  SecurID also has the advantage that it employs two-factor
   authentication (as describe above Section 2.2), combining something
   you have, the token, with something only you know, a personal
   identification string, or PIN.

   In order to authenticate the user enters both his PIN and the time-
   dependent key (they can be concatenated so that this is transparent
   to the client program.)  For example where the user's PIN is
   "2Enl0*/b" and the OTP on the token currently reads "041 980", the
   user will simply enter in the password field of the application
   "2EnI0*/b041980".  This value will then be used as the password for
   whatever authentication function the client and server are using.
   The server verifies the password and checks that the time-dependent
   key is correct for the current time and only then allows login.  If
   sent in the clear (versus being used in some challenge/response
   mechanisms; see next section), it's clearly possible for an attacker
   to capture the password and replay it but without the token he
   (theoretically) can't generate the right time-dependent key (unless
   the replay is executed within the same time window as the current
   value).  When sent using a cryptographic message digest function,
   this weakness is mitigated.

4.3.   List of One-Time Password Systems

   Note:  any system that uses passwords can be adapted to use one-time

      Single One-Time-Password used for bootstrapping
      S/Key [RFC1760]

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      OTP [RFC2289]
      SecurID [RFC2808]

5.   Challenge/Response

   CHALLENGE/RESPONSE mechanisms fix the sniffing problem associated
   with ordinary passwords.  The basic idea is simple:  the verifying
   party provides a random (or at least unique) challenge and the
   authenticating party returns some function of the shared key and the
   challenge.  Generally this function is some sort of message digest.
   In the simplest form it is H(challenge || key), where H is a
   cryptographic message digest and "||" denotes message concatenation.
   A better design is probably to use HMAC [RFC2104] which has stronger
   security guarantees.

   Challenge/response mechanisms are resistant to simple sniffing
   attacks but in general have all the other security problems of
   ordinary password systems.  Additionally, they are vulnerable to
   another form of offline dictionary attack and are more vulnerable to
   password file compromise than correctly implemented password in the
   clear systems.

   Challenge/response mechanisms can be completely hardened against
   offline dictionary attacks by the use of a sufficiently large
   randomly-generated shared key instead of a password.  Such a password
   is of course difficult for a user to memorize but is quite useful if
   it can be statically configured on both sides of a connection.

   Unlike simple password mechanisms, challenge/response mechanisms can
   be designed which provide both mutual authentication and secure key
   exchange.  Such systems can be made resistant to most forms of active
   attack, and depending on the strength of the shared key, passive
   attacks as well.

   There also explicit challenge response systems, where users are
   stepped through a challenge and response exchange where they must use
   a one-time-password token system.  For example, as a user attempts to
   login to a Telnet session, the server sends a challenge.  The user
   sees this challenge appear on the screen.  Holding an electronic one-
   time-password token, she uses its key pad to key in that challenge.
   The token responds by displaying a one-time response to that
   challenge.  The user concatenates the token's response with her PIN
   and sends it back over the Telnet session login.  The server checks
   that both the correct PIN and one-time-password are used.

   A challenge-response system can also be turned into a secure channel
   protocol by using the shared key to establish cryptographic keys

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   which are then used to encrypt the traffic.  In this context, a
   CHANNEL is a security association between two endpoints through which
   further security exchanges may occur.  A secure channel would be
   offer encryption, authentication and integrity-checking on a per-
   packet basis.  TLS-PSK [RFC4279] is one example of such a system.

5.1.   Offline Attacks on Challenge/Response

   Although a passive attacker cannot mount an ordinary sniffing attack,
   he can combine sniffing with an offline dictionary attack.  The
   attacker simply captures a single challenge/response exchange and
   then dictionary searches the password space until he finds a password
   that produces the correct response for a given challenge.  With high
   probability (though not certainty) this will be the correct password.
   This problem is inherent in all simple challenge response mechanisms
   and cannot be fixed without public-key technology.  This problem is
   inherent unless public-key methods are incorporated within the
   challenge-response protocol, as will be discussed in Sections 6 and
   7, or the challenge-response transactions are carried over secure
   channels (which themselves must be authenticated).

5.2.   Password File Compromise

   Challenge/response mechanisms also introduce a new problem:  PASSWORD
   EQUIVALENCE.  In order to locally compute (for verification purposes)
   the appropriate response for a given challenge, the server must store
   the user's password locally.  Thus, if the password file is
   compromised, the attacker can directly log in to the server, without
   even needing to crack the password file.  We'll call this property

   A more serious variant of the same problem occurs if users use the
   same password on multiple systems.  Compromise of one system can thus
   lead to compromise of many.  This is called STRONG PASSWORD
   EQUIVALENCE.  This risk should not be overstated--compromise of an
   ordinary password system can still lead to attack if the attacker
   completely compromises the system and can capture people's passwords
   when they login--but is nevertheless worse in challenge/response than
   with ordinary passwords.  The standard countermeasure is to use a
   two-stage digesting process, such as:



   The server stores STORED instead of the password.  (Making STORED
   effectively the password).  The server then gives the client both
   SALT and CHALLENGE, allowing the client to compute RESPONSE from the

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   password alone.  Note that the two-stage process only prevents
   compromise of one system from affecting others.  Compromise of a
   password file still allows immediate access to the target system.

   SCRAM [I-D.newman-auth-scram] describes one defense against this sort
   of attack.  The server stores a hashed version of the password, and
   must prove that it knows it using challenge-response.  The client
   then provides the preimage for the hashed password, thus
   demonstrating that it knew the original password.  With this system,
   an attacker who recovers the password file can immediately
   impersonate the server to the client, but not the client to the
   server.  However, if he impersonates the server to the client he can
   capture the preimage and can then impersonate the client to the

5.3.   Case Study: CRAM-MD5

   CRAM-MD5 [RFC2195] is a challenge/response authentication extension
   for IMAP [RFC3501] CRAM-MD5 is a classic challenge/response system:
   the server provides a presumably random challenge and the client
   transmits an HMAC of the challenge using the shared key as the HMAC
   key.  The interaction looks like this:

        1 S: * OK IMAP4 Server
        2 C: A0001 AUTHENTICATE CRAM-MD5
        3 S: + PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9uLm1jaS5uZXQ+
        4 C: dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNGQzODkw
        5 S: A0001 OK CRAM authentication successful

   The second message from the server (message 3) is the base-64
   encoding of the string "<1896.697170952@postoffice.reston.mci.net>".
   This string must be in the form of an email address [RFC5322] and is
   intended to be globally unique.  The client's response (message 4) is
   computed using HMAC-MD5(password,challenge) and then base-64 encoded
   for transmission in message 4.

   CRAM-MD5 is an improvement on the password-in-the-clear mechanisms
   that it replaces but still has all the security flaws of basic
   challenge/response mechanisms.  In particular, it is vulnerable to
   postauthentication hijacking and is strongly password equivalent.

   CRAM-MD5 has some interesting security properties with respect to
   server password file compromise.  The RFC encourages servers to store
   a pre-initialized HMAC context rather than than the client's
   password.  Since the password has already gone through the MD5
   compression function, it is believed to be infeasible to recover the
   password from the context.  However, since the HMAC context is
   sufficient to compute any response without knowing the key, an

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   attacker who recovers the context can impersonate the client without
   knowing the key.  This context will be the same for all servers which
   share the same password.  The result of these facts is that an
   attacker who recovers the password file from such a server can attack
   any other server which (1) uses CRAM-MD5 and (2) has a user with the
   same password.  However, it cannot attack other users with the same
   password on machines with a different authentication mechanism (since
   that would require direct access to the password rather than the HMAC

5.4.   Case Study: HTTP Digest

   HTTP Digest Authentication [RFC2617] is a replacement for HTTP's
   notoriously weak Basic Authentication mechanism, which used passwords
   in the clear.  Digest Authentication is a challenge/response
   mechanism with some additional features to prevent hijacking attacks
   and remove strong password equivalence, as well as to reduce round
   trip time for multiple requests.

   The basic Digest Authentication interaction takes two round trips.
   In the first, the client requests some document and is rejected.  The
   server's rejection (a 401 Unauthorized) contains an indication that
   it supports Digest Authentication, a realm string, and a random
   challenge.  The client's subsequent request includes a message digest
   over the password, the challenge, and part of the HTTP Request.

   HTTP Digest offers two types of integrity check (the field specifying
   them is called "qop" for quality of protection).  The "auth" scheme
   covers only the request URI.  The "auth-int" scheme protects the URI
   and the message body, but not the message headers since they may be
   changed in transit by proxies or other intermediaries.  Negotiation
   of the qop is simple:  the server offers a set of acceptable qop
   values and the client chooses one.

5.4.1.   Message Integrity

   As previously noted, simple challenge/response schemes without
   associated channel security allow an attacker to hijack the
   connection after authentication has occurred.  Since each HTTP
   request must be individually authenticated, an attacker who takes
   over the channel cannot transmit new unauthenticated requests over
   that channel.  However, an attacker might attempt to intercept an
   authenticated request and mount a cut-and-paste attack, leaving the
   authenticator but changing the contents.  This attack is prevented by
   including the URI in the message digest.

   Unfortunately, the URI isn't the only piece of security relevant
   information in the HTTP request.  Both the headers and the body are

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   potentially sensitive.  For instance, if HTTP POST is used, FORM
   input values will be in the message body.  The auth-int qop value
   protects this information, but it is not widely deployed.  None of
   the qop values protect the headers.

   It's worth noting that Digest provides protection only for the
   request.  No authentication is provided for the server, nor is
   message integrity provided for the response.  It's technically
   possible to provide this feature using a shared key, as is done in
   S-HTTP [RFC2660], but Digest doesn't do so.

   Digest deployment has been somewhat spotty in the past.  For
   instance, the popular Netscape Navigator 4 versions did not support
   it.  More recently, Internet Explorer 7.0 +, Mozilla Firefox 2.0+,
   Netscape 7+ all support digest authentication.  SIP [RFC3261]
   requires Digest authentication and it is near universal there.

5.4.2.   Replay Attack

   Many HTTP requests are idempotent.  In such cases, replay attacks are
   not a problem since the attacker doesn't get any information that he
   would not get by sniffing the original request.  However, many HTTP
   transactions have side effects and in such cases preventing replay is
   important.  Unfortunately, the conventional approach of requiring a
   separate challenge/response exchange for each authentication would
   double the number of round-trips for each transaction.

   HTTP Digest provides two features to avoid these round trips.  First,
   the server can provide a new nonce in a response header.  This nonce
   must be used for the next client request.  This feature interacts
   poorly with request pipelining so HTTP Digest also allows the client
   to issue multiple requests using a given server challenge by using a
   request sequence number (the "nonce-count").

5.4.3.   Downgrade Attack

   HTTP Digest suffers from two types of downgrade attack.  In the first
   type of attack, the attacker forces the peers to agree on Basic
   authentication rather than on Digest.  There is no realistic way to
   protect against this attack, other than simply refusing to send Basic
   at all--note that the server refusing to accept it does not help,
   since the attacker can impersonate the server.

   In the second Downgrade attack, the attacker forces the peers to
   negotiate a qop of "auth" instead of "auth-int".  The downgrade
   attack would then presumably be followed by an integrity attack on
   the client request.  This attack could be prevented by requiring the
   client to include a digest of the server's offered qop values in the

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   client's authenticator.  However, that is not the case with the
   current scheme.

5.5.   List of Challenge-Response Systems

      APOP [RFC1939]
      HTTP Digest [RFC2617]
      AKA [AKA]
      CRAM-MD5 [RFC2195]
      Kerberos password-based authentication [RFC4120]

6.   Anonymous Key Exchange

   All three of the mechanisms mentioned so far can be hardened against
   passive attacks by the use of anonymous key exchange.  Essentially,
   the peers arrange for a secure channel using a key establishment
   mechanism that does not authenticate either side.  Public key
   algorithms such as Diffie-Hellman and RSA can be used in this way.
   Once the key is established you can encrypt all the traffic. and any
   data which is transmitted over the channel is secure from
   eavesdroppers.  This includes data such as passwords or

   The problem with this system is that it's subject to what's called a
   man-in-the-middle (MITM) attack.  Because the cryptographic key
   establishment mechanism is unauthenticated, it is possible for an
   attacker to intercept communications between the peers (say Alice and
   Bob) and pose as Alice to Bob and Bob to Alice.  The attacker can
   then forward traffic between them and get access to whatever's being

   The MITM attack on Diffie-Hellman key exchange is shown in the
   following figure.  Yc, Ya, and Ys are used to denote the client,
   attacker, and server public keys respectively.

        Client                Attacker                Server
        ------                --------                -----
        Yc -&gt;
                                 Ya -&gt;
                                                       &lt;- Ys
                              &lt;- Ya

   At the end of this exchange the client thinks that the server's
   public key is Ya and the server thinks that the client's public key
   is Ya.  However, in reality both have established a shared secret
   with the attacker.  Thus, when the password is transmitted over
   channel the attacker sees it.

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   So we see that with if only one side of the connection is
   CRYPTOGRAPHICALLY authenticated this attack cannot be detected.Thus,
   how much security you believe that anonymous key exchange adds to
   your protocol depends on your threat model.  Active attacks are
   generally more difficult to mount than passive attacks but by no
   means impossible [Bell89]

   All of these mechanisms use public key cryptography to perform the
   initial anonymous key exchange.  As a result, performance can be
   unacceptably slow if one side (e.g., a handheld device) is heavily
   constrained.  Such concerns were more relevant when the Diffie-Helman
   technology first came to commercial maturity in the late 1990s.  As
   processing speeds increase per Moore's Law, and as smart phones are
   given more and more powerful processors, this concern reduces.  Most
   Internet servers are fast enough to keep up with the normal number of
   required authentications and hardware acceleration solutions are
   readily available.  This is not to say that performance is of now
   concern.  Moore's law has to some extent been counter-balanced by an
   increase in the size of common keys. 2048-bit keys are now quite
   common and the potential for even larger keys has lead to an
   increased interest in elliptic curve cryptography.

6.1.   Case Study: SSH Password Authentication

   Secure Shell (SSH) provides a number of authentication mechanisms,
   but the first step is always to establish a secure channel between
   the client and the server.  SSH is designed not to require
   certificates:  the server merely provides a raw public key to the
   client.  As a countermeasure to man-in-the-middle attack, the SSH
   client caches the server's public key and generates a warning or
   error (depending on the implementation) if that key changes.

   In theory, caching the public key protects against MITM attack at any
   time other than the initial connection to the server.  In practice,
   when users encounter the error that the key has changed, they may
   simply override the warning or delete the cache entry when the error
   occurs, assuming, correctly, that the likely case is that the server
   administrator has just reset the public key (e.g. by reinstalling the
   software without preserving the old key).

   A very careful user can obtain complete security against MITM attacks
   by obtaining the server's key fingerprint (a message digest of the
   key) out of band and comparing that to the fingerprint of the key the
   server offers.  Machines and their user interfaces can easily be made
   to perform this check in a predictable way.  For example, if one
   machine uses SSH as a secure channel for management of a second
   machine, the application on the first machine can prompt the
   administrator for the second's fingerprint and not continue until the

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   string is received.  It can then check upon every connection that
   what it sees from the second machine matches what was entered.

   SSH bootstraps off of the system's login mechanisms so it will
   support either passwords in the clear or one time password
   authentication.  Note that in either case if an attacker mounts a
   successful man in the middle attack, he will be able to hijack the
   connection post-authentication, just as he would have if the
   transaction was performed in the clear.  This vulnerability can be
   alleviated with careful protocol design, as we'll see in the next
   case study.

   Another option is to combine a one-time-password with public keys.
   In such a system, a host first authenticates itself to the other
   using the one-time-password, then, once the first SSH channel is
   established, securely passes a public key for long term use.  On the
   next connection, the public key will be used, and the fingerprint of
   that key will be used to validate it.

6.2.   List of Anonymous Key Exchange Mechanisms

      SSH (password mode) [RFC4251]
      IPsec using IKEv2 with unauthenticated public keys, aka BTNS
      SSL/TLS (anonymous keying) [RFC5246]

7.   Zero-Knowledge Password Proofs

   All of the mechanisms mentioned so far depend on some sort of shared
   key.  If that shared key is a user-derived password, then it's
   possible for the attacker to mount an offline dictionary attack on
   the password, either completely passively (as with CRAM-MD5) or with
   a single MITM attack (as with TLS anonymous DH).  However, a rather
   clever class of protocols known as Zero Knowledge Password Proofs
   (ZKPPs) makes it possible to use user-generated passwords without
   fear of offline dictionary attack

   The earliest (and simplest) ZKPP is EKE [Bell92], designed by Steve
   Bellovin and James Merritt.  EKE is based on Diffie-Hellman, but
   instead of sending the key shares (the public keys) in the clear they
   are encrypted using a password.  The protocol looks like this.

        Client                             Server
        ------                             -----
        Name, E(Password, Ya)) ->
              <- E(Password, Yb),E(K,Challenge-b)
        E(K,Challenge-a || Challenge-b) ->

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              <-                E(K, Challenge-a)

   Where K is the DH shared secret == g(Xa * Xb) mod p, E(blah, blah) is
   encryption using the elements within the paranthesis, Ya = public key
   of client, Yb = public key of server, and Challenge-a and Challenge-b
   are random stings.

   Note that EKE as described above is insecure against password file
   compromise, since the server must store the password.  Augmented EKE
   [Bell94] describes a protocol that is secure against this.  A large
   number of other ZKPPs have been proposed, including PDM [KP01], SPEKE
   [Jab96], and SRP [RFC2945].  These protocols are all roughly
   equivalent, offering slightly different combinations of security,
   performance, and message count.

7.1.   Intellectual Property

   From a technical perspective, ZKPPs dominate the anonymous key
   exchange mechanisms described in Section 6.  Their performance is
   roughly equivalent and their security guarantees are superior.  The
   major ZKPPs are EKE, A-EKE, SPEKE, and SRP. there are a number of
   Intellectual Property Rights in this area, some of which are on file
   with the IETF (www.ietf.org/ipr).

7.2.   List of Zero Knowledge Password Proof Systems

      EKE [Bell92]
      A-EKE [Bell94]
      PDM [KP01]
      SPEKE [Jab96]
      SRP [RFC2945]

8.   Server Certificates plus User Authentication

   If you can authenticate one side of the connection (typically a
   server) then it becomes far easier to provide strong authentication.
   Anonymous key exchange, cleartext passwords, one time passwords, and
   challenge/response protocols can all run over an authenticated and
   encrypted channel.  In such a system, there's no need to worry about
   active attack, so the authentication protocols don't need to be
   hardened against it.

   Providing an encrypted channel with authentication for the server
   dramatically reduces the security advantage enjoyed by more
   complicated schemes over simple passwords.  Since the marginal
   security benefit of such systems is so modest when compared to the
   increased implementation and deployment complexity, common practice

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   when server authentication is available is to first establish the
   encrypted channel, and then send simple passwords over the encrypted
   channel.  This includes systems such as passwords over SSL/TLS and
   passwords over IPsec VPNs.

   In addition to making the overall authentication problem simpler,
   hosting one's application protocol over an encrypted and
   authenticated channel has a number of other security benefits.
   First, a properly designed channel security protocol removes the
   threat of post-authentication hijacking (described in Section 3.2).
   Second, it provides confidentiality and message integrity for the
   rest of the application traffic, which is in general a good thing.

   This approach is especially applicable in client server systems where
   the server is well known and the clients are many.  Examples include
   a web site being hit by many users, a remote access gateway serving
   many remote workers, or a content service being connected to by many
   subscribing applications.  This approach is less well suited to peer-
   to-peer or mesh connections.

   The primary difficulty with this approach is that providing
   certificate-based server authentication is not straightforward.  The
   first problem is that the server machine must have a certificate,
   which entails some effort, configuration, and cost.  The use of self-
   signed certificates can ease the operational and cost issues while
   preserving security as long as the shal-1 fingerprint of that
   certificate's key is both listed in the self-signed certificate and
   delivered in a secure and trusted way to the end-point.  Self-signed
   certificates without explicit verification aren't acceptable in this
   case (rather, they reduce one to the anonymous key exchange scenario
   described in Section 6).

   The more serious problem is establishing what the server side name in
   the certificate ought to be.  Common practice (stemming from practice
   in HTTPS [RFC2818]) is to have the server's certificate contain the
   server's fully qualified domain name (FQDN), either in the Common
   Name or subjectAltName fields, but this is unacceptable if the server
   does not have a domain name.  One can also put the server's IP
   address in the subjectAltName, but this is inappropriate if that IP
   address might change.  Further, use of IP address is insufficient in
   cases where the "server" is actually a service intended to appear to
   users as one server, but in reality virtualized across several
   servers and IP addresses.  Any protocol which uses this mechanism
   must specify a mechanism for determining the server's expected domain

   One concern here is what happens if the server has a certificate that
   has the wrong name or that is signed by a Certificate Authority that

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   the user's client does not recognize.  Many such (web browser)
   clients present a dialogue warning the user that the connection may
   be under attack and offering to let him ignore the error.  The common
   Internet user will have no idea what this means, the implications,
   how to determine if the threat is real or just a configuration error,
   and therefore will not know how to react.  The easiest path to their
   immediate goals is usually to chose the option to ignore the error.
   Obviously, if users do this routinely (and it is widely believed that
   they do) then they can be subject to an active attack.

8.1.   Case Study: Passwords over HTTPS

   Despite the existence of Digest Authentication, the dominant form of
   strong HTTP authentication is passwords with HTTP over SSL/TLS
   (HTTPS).  As mentioned above, this mechanism has superior security
   properties to Digest (provided that the server has a real
   certificate) and is easier to deploy, especially if the server wants
   to use SSL/TLS for channel security in any case.

   There are actually two ways to use passwords over HTTPS.  The first
   is to use HTTP's built in authentication mechanisms (either Digest or
   Basic) over an HTTPS connection.  The second is to perform password
   authentication at the application layer, using an HTML form to prompt
   for the password.  The form method is far more popular, primarily
   because it allows the application designer far greater control over
   when and how authentication occurs.  In particular, the designer can
   give the password dialog any look he chooses.

   In general, if form-based authentication is used, the only available
   option is to use simple passwords, since HTML has no facilities for
   performing arbitrary computation or challenge/response passwords.
   Theoretically, one could perform these operations in a JavaScript or
   Java program, but in practice this is generally not done.

8.1.1.   Authentication State

   When Basic or Digest Authentication is used, the client can simply
   transmit an authenticator with every request.  However, if
   authentication is performed using an HTML form, this approach is
   impractical, since it would require client interaction for every page
   fetch.  Three approaches for solving this problem are generally
   proposed.   The Token Problem

   In general, all HTTP authentication state carrying schemes involve
   providing the client with some token which it can then present to
   authenticate future requests.  This token must be constructed in such

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   a fashion that it is impossible for the client to tamper with it and
   obtain access to resources that they would not otherwise be able to

   There are two basic techniques for constructing tokens.  The first is
   to have the token be self-authenticating, e.g. by having it be the
   user's information signed or MAC-ed with a key known only to the
   server.  The second is to have it be an index into some database of
   authenticated users stored on the server.  Note that these indices
   must be unpredictable to prevent one user from guessing another
   user's token.  The self-authenticating approach has the advantage
   that it does not require persistent storage on the server but the
   disadvantage that there is no way to mark a token invalid or update
   it (although they can of course contain an expiry time).  When
   multiple servers are involved, self-authenticating tokens have the
   additional advantage that they do not require inter-server
   communication.   URL Rewriting

   The most general but also most difficult approach is to dynamically
   rewrite all URLs provided to the client after authentication has
   occurred.  One might, for instance, pass all pages through a CGI
   script, where the arguments include the real page to be accessed and
   the authenticator token. an example of such a URL is:


   The CGI script would then use the authenticator argument to determine
   the client identity, recover the actual target page and perform the
   authentication checks.  Using a CGI script this way is inconvenient
   since it requires replicating the server's access control
   infrastructure.  A less intrusive approach involves having a server
   plugin unwrap the target URL early in the server's processing
   pipeline, before the access control checks are performed.  This
   allows the server to perform its normal authentication checks based
   on the unwrapped identity.

   The primary difficulty with URL rewriting is that it all pages must
   be dynamically generated.  Either each page must be generated by a
   script which embeds the appropriate URLs or the server must
   postprocess pages to embed them.  Either approach makes the system
   more complex and therefore adds instability.  However, before the
   introduction of cookies, URL rewriting was essentially the only
   option for token passing.

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   The inconvenience of URL rewriting lead to the introduction of HTTP
   Cookies [RFC2695].  Essentially, an HTTP cookie is a token issued by
   the server and transmitted by the client with requests.  The cookies
   can be labeled to be transmitted only when resources matching various
   prefixes are dereferenced, including resources on another server.
   Browsers generally persistently cache cookies between invocations.
   Cookies are the method of choice for carrying HTTP state information
   and can be used to carry all kinds of state besides authentication
   information.  Note, however, that since cookies can be used to
   transmit information from one server to another, they have been the
   focus of privacy concerns [RFC2965].  Accordingly, some users choose
   not to accept or transmit cookies.

   Note that [RFC2964] specifically recommends against the usage of
   cookies for carrying authentication and authorization information.
   Nevertheless, this practice is nearly universal on the Web.   HTTPS Session Binding

   Each TLS/SSL session has a session identifier, which is used for
   resuming the session without a full handshake.  These session IDs are
   unique for any given server, so server administrators often think to
   use the session ID as a search key for the user's information.  This
   is a bad idea.  The fundamental problem is that there's no guarantee
   that any given session will be resumed.  The client need not offer to
   resume a session and the server need not accept, or may flush its
   session cache at any time.  Thus, using the session ID as a
   persistent identifier is unwise.

8.2.   List of Server Certificate Systems

      HTTP over TLS (HTTPS) [RFC2818]
      SMTP over TLS [RFC3207]
      XMPP over TLS [RFC3290]
      SIP over TLS [RFC3261]
      IPsec (under some conditions)
      SSH (under some conditions)

9.   Mutual Public Key Authentication

   If both client and server have certificates, then the peers can use
   mutual certificate authentication.  This is done by having both
   client and server establish that they know the private keys
   corresponding to their certificates.  A wide variety of protocols
   offer this functionality, including SSL, IPsec, and SSH (SSH actually

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   offers mutual authentication with pre-arranged public keys).

   The two most important advantages of public key authentication are
   that it has no password equivalence and that it can allow
   authentication between parties who have no direct prior arrangement
   together, but who have prior arrangement with some third mutually
   trusted party, and some local configuration by which they will be
   able to accept each other's credentials..

9.1.   Password Equivalence

   With public key authentication, the server knows only the client's
   public key.  It is therefore incapable of forging any kind of
   authentication message from the client.  Similarly, knowledge of the
   public key does not allow an attacker to authenticate to the server.
   Accordingly, public key techniques never store a password equivalent
   on the server.

9.2.   Authentication between Unknown Parties

   One advantage of certificate-based public key authentication systems
   --as opposed to those using pre-arranged public keys--is that it
   allows authentication between parties who have had no prior contact.
   Authentication of servers with which one has had no prior arrangement
   happens all the time in the HTTPS context:  the user wishes to
   connect to a host at a given URL and is able to verify that the
   server certificate matches that URL.

   In addition to strict identity verification, it's possible to use
   certificates to carry authorization information.  This allows a
   central authority to make both authentication and access control
   decisions for distributed servers merely by issuing certificates.
   [BFL96] describes such a system.

   Note that each party does need to do some fairly complex
   configuration and bootstrapping in order to contact a previously
   unknown party in this way.  This work includes:  locating a trusted
   third party, securely downloading and installing that third party's
   certificate and public key (which has complex and nested security
   challenges of its own), determining the protocols and configuration
   to be used for certificate request, retrieval, and revocation
   checking and life-cycle maintenance, generating a public-private key
   pair, crafting a certificate request, sending the certificate
   request, retrieving and installing the granted certificate, and local
   configuration about which connections should employ the certificate.

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9.3.   Key Storage

   The primary security problem with public key authentication protocol
   (assuming the basic protocol is designed correctly) is protecting the
   private keys of the certificate issuer first and foremost, and
   secondarily of the clients.  In server applications and many non-
   mobile client applications, the key is simply stored on disk, often
   encrypted under a password-derived symmetric key.  In applications
   where the user must carry his authentication information around, this
   can be done in essentially two ways:  with a token or by generating
   the key from a password.

9.4.   Tokens

   The general idea of a secure token is relatively simple:  you have a
   tamper-resistant and portable token which carries your private key
   (and probably your certificate).  The token can be interfaced to a
   computer, typically through a portable media interface, like USB
   drive, compact flash, SD, PCexpress, smartcard, etc.  The private key
   is generally protected by a PIN, but of course this PIN is known to
   any computer on which the token is used, since the PIN is sent to the
   token by the computer.  The primary threat to tokens is loss or
   theft.  It's not generally economical to make such tokens completely
   tamper-proof, so a lost token in the hands of a dedicated attacker
   means a lost private key.

   There are two major types of tokens:  those which are pure memory for
   key storage and those which do the cryptography on the token.  The
   first are substantially cheaper but less secure because they give the
   key to the host computer.

9.5.   Password Derived Keys

   It's generally possible to derive a user's private key from a
   relatively short password, simply by using the password to seed a
   cryptographically secure pseudorandom number generator (PRNG) which
   is used to generate the private key.  Unfortunately, this technique
   is susceptible to dictionary attack, since an attacker can dictionary
   search the password space until he finds a password that generates a
   key pair that matches the signature.  Protocols can be designed to
   resist this attack by exchanging the signed client response under the
   server's private key, but many protocols (notably SSL/TLS) do not.
   Accordingly, password derived keys should be viewed as a mechanism
   for using shared keys with public-key-only protocols, not as a fully
   public key system.

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9.6.   Case Study: SMTP over TLS

   SMTP can be combined with TLS as described in [RFC3207].  This
   provides similar protection to that provided when using IPsec.  Since
   TLS certificates typically contain the server's host name, recipient
   authentication may be slightly more obvious, but is still susceptible
   to DNS spoofing attacks.  Protection is provided against replay
   attacks, since the data itself is protected and the packets cannot be

9.7.   List of Mutual Public Key Systems

      SSL/TLS (client auth mode) [RFC5246]
      IPsec IKE [RFC4306]
      S/MIME [RFC3850]

10.  Generic Issues

10.1.  Channel Security Protocols

   Building a full security system into each application protocol is
   extremely expensive in terms of design and implementation effort.
   One common approach is to design a generic channel security protocol
   which provides a generic secure channel abstraction between a pair of
   endpoints.  The endpoints of the channel can be authenticated at
   setup time and then all data flowing between them is automatically
   secured, allowing the application to be mostly agnostic about the
   security properties.  SSL/TLS, SSH, and IPsec all provide this sort
   of functionality.

   TLS [RFC5246] provides a good example of the basic pattern, as shown

        Client                                        Server
        ------                                        ------
        <-----------------  TLS Handshake  ---------------->

        Application message (protected by TLS) ------------>
        <------------ Application message (protected by TLS)

   At the beginning of the TLS session, the client initiates a TCP
   connection to the server (TLS only works over TCP, but DTLS [RFC4347]
   serves a similar function for UDP), but instead of sending
   application data, the client and the server perform a TLS handshake,
   which can authenticate the server and/or the client, and which
   establishes cryptographic keys which are then used to protect all

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   future traffic.  This cryptographically binds any application layer
   traffic to the authentication performed in the handshake.

   A channel security protocol is not itself an authentication
   technology.  Rather, it's built on top of an authentication
   technology, or on top of multiple technologies.  Most such protocols
   support multiple types of authentication.  For instance, TLS can be
   used with X.509 certificates, OpenPGP certificates [RFC5081], shared
   keys [RFC4785], and passwords [RFC5054].

10.1.1.  Limited Authentication Options

   Because a secure channel protocol needs to be able to establish
   cryptographic keys, the authentication options are necessarily
   somewhat limited.  In particular, mechanisms such as passwords in the
   clear (both in the reusable and one-time varieties) may not be
   available.  (See Section 6.1 for one approach to work around this

10.1.2.  Limited Application Integration

   Because the secure channel protocol sits beneath the application
   layer protocol rather than being integrated with it, the level of
   integration between the two protocols is fairly loose.  This is an
   advantage in that the application security protocol need not change
   at all in order to use a channel security protocol.  All that is
   needed is for the implementation to arrange for the channel security
   protocol to run underneath.

   The disadvantage is that the application protocol tends to have
   limited visibility into what the channel security protocol is doing.
   IPsec provides an extreme example of this:  because much of the stack
   typically lives in the kernel, the application cannot even portably
   specify security properties or determine which properties apply to a
   given class of traffic association (there are APIs for this such as
   PF_KEY [RFC2367] but they are not universally deployed).  Even with
   more tightly coupled protocols such as SSH or TLS, the applications
   are typically limited to setting general policy and interrogating the
   state of the association.  They cannot, for instance, control the
   protection properties of individual PDUs.

10.1.3.  List of Channel Security Protocols

      IPsec [RFC4301]
      SSH [RFC4251]
      SSL/TLS [RFC5246]

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      DTLS [RFC4347]

10.2.  Authentication Frameworks

   Another popular approach is to use a pluggable authentication
   framework.  The general idea behind a pluggable application framework
   is that you would like the application protocol actively involved in
   the authentication (unlike with a channel security protocol) but that
   you want to avoid specifying all of the details.  Typically, the
   protocol framework doesn't provide any authentication features per se
   but instead allows you to negotiate the authentication mechanisms you
   wish to use.  SASL [RFC4422], for instance, allows the negotiation of
   plaintext passwords, CRAM-MD5 (a digest-based challenge/response
   mechanism), and TLS among other mechanisms.  Another example is in
   IKEv2 where the EAP framework is used to allow various forms of user
   authentication, e.g.  EAP-MD5, OTP (like SecurID) or Generic Token
   Card.  GSS-API is another example of an authneticaion framework.

   Authentication frameworks are appealing to security mechanism
   developers since they enable mechanisms to be supported by multiple
   protocols by writing a single specification.  In general, it is
   easier to provide support for a mechanism with a framework than to
   integrate a security mechanism within each protocol which might use

   Generic authentication mechanisms are attractive to application
   protocol designers because when properly used, they allow protocol
   designers to treat mechanism-specific details in an abstract manner.

   While frameworks still require protocol designers to determine the
   threats and required security services (e.g. need for authentication/
   integrity/confidentiality/replay protection, protection against
   active attacks, etc.) as well as naming of the conversation
   endpoints, details of individual mechanisms can be abstracted.  For
   example, it is not necessary for a protocol designer to concern
   themselves about how to locate a Kerberos KDC, or what information
   the latest revision of example.com's proprietary authentication token
   requires; these issues are handled by the framework.

   While frameworks inherently provide abstraction benefits for protocol
   designers, the detail hiding is generally imperfect, especially from
   the perspective of implementers.  For instance, if the framework
   provides mechanisms with a wide variety of security levels, designers
   and implementors need to be conscious of what security is provided
   with each level.  This is often difficult to get right.

   Some applications such as DNSSEC focus on providing a service to the
   Internet at large, that is inter-domain services.  For these

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   applications, where interoperability of authentication between
   parties who have no prior association is critical, having an
   authentication mechanism that is "mandatory-to-implement," as opposed
   to a pluggable authentication framework, is likely to be the right
   approach.  Other examples include BGP's authentication mechanism
   TCP-AO, and DKIM.  In these types of situations, an authentication
   framework is likely to add significant complexity.  If there is not a
   compelling reason to use an authentication framework in such
   Internet-wide, inter-domain protocols, then it should not be used.

   For applications that are often used in intra-domain contexts, i.e.
   within a single organization, and where end-users are authenticating
   within the application, frameworks may be more appropriate.  This is
   especially true when authentication is used in a context where
   parties have prior associations that they use to establish
   credentials.  With respect to intra-domain authentication, we have
   seen considerable diversity in the credential types that are used.
   Some organizations adopt PKI and smartcards, some use token cards,
   others use passwords or OTP systems.  Authentication frameworks
   enable that diversity to be supported within a single architecture.
   For example, SSH is typically used for a party who has established a
   credential to access some service.  Similarly, applications like
   IMAP, XMPP, and LDAP are used within a context of a prior
   relationship.  It is desirable that products from one vendor
   interoperate with products from another vendor.  However, it is more
   important within an intra-domain deployment that products (like an
   SSH system) accessing related resources (like an LDAP server) be able
   to use the same authentication mechanisms.  That is, the example.com
   administrators are more concerned that whatever SSH implementation
   they choose can support an authentication mechanism that is also
   supported in the IMAP implementation they choose, than they are
   having all SSH implementations share an authentication mechanism.
   Authentication frameworks aim to allow protocol implementers to
   develop applications that support this deployment goal.

   For protocols in the class (intra-domain), a framework may be used if
   it is available.  If a framework is chosen, each protocol must define
   a mandatory-to-implement authentication mechanism.  However, the
   framework will permit vendors to implement multiple authentication
   mechanisms so that those deploying implementations may choose the
   same mechanism across protocols.  In such cases, designers should use
   an existing framework like EAP, SASL, or GSS-API as opposed to
   attempting to create something from scratch.  These frameworks have
   taken much (re-)work to get to their current states, with more work
   ongoing.  Attempting to replicate these efforts from scratch is not
   recommended, and strongly discouraged.

   However, interoperability difficulty has emerged where many disparate

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   authentication mechanisms all use the same credentials.  Therefore,
   consideration must be given to limiting the number of specified
   mechanisms for any one class in an authentication framework, and,
   again, at least one "mandatory-to-implement" mechanism must be
   specified.  See more on this in section Section 10.2.2.

10.2.1.   Downgrade Attacks

   One of the most serious problem with generic authentication
   mechanisms is their susceptibility to DOWNGRADE ATTACK, in which the
   attacker interferes with the negotiation to force the parties to
   negotiate a weaker mechanism than they otherwise would.  This issue
   is generally worse with frameworks which do not provide channel
   security because the weakest provided mechanism is often quite weak.
   Consider a set of peers, each of which supports both challenge/
   response and simple passwords.  An attacker can force them into using
   a simple password and then capture that password.

   The standard countermeasure to downgrade attack is to authenticate a
   message digest of the offered mechanisms, as is done in the
   handshakes of both IKE and TLS.  However, this is not possible if a
   simple password mechanism is supported (as is the case in many
   frameworks), and policy enables it to be negotiated, because the
   attacker can simply capture the password in flight.

   Note that if the client can establish an authenticated, integrity
   protected channel to the server (as is done in SSH), then the client
   authentication mechanism can be negotiated without fear of downgrade.
   Some protection against downgrade attacks can also be provided by
   having an endpoint cache the other endpoint's offers and complain if
   less secure mechanisms than were previously offered suddenly becomes
   available.  This approach obviously bears the risk of false positives
   under simple misconfiguration.

   Finally, downgrade prevention can be achieved by users of generic
   security profiling the mechanisms they offer to ensure that they are
   all adequately strong--at least strong enough to provide downgrade

10.2.2.   Multiple Equivalent Mechanisms

   The ease of adding new security mechanisms to generic authentication
   layers enables the development of multiple mechanisms with similar
   characteristics or even multiple mechanisms supporting the same
   authentication technology.  This diversity has the potential to
   introduce interoperability problems as well as additional complexity.

   Trouble arises when we have many disparate authentication mechanisms

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   using the same credentials.  One particularly egregious example is
   token card support in EAP, where we have a variety of EAP mechanisms
   supporting RSA SecurID:

   a.  Use of EAP Generic Token Card (GTC) defined in [RFC3748], along
   with a tunneling mechanism such as PEAPv0 [MS-PEAP], PEAPv1, EAP-
   TTLSv0 [RFC5281] or [RFC5216].

   b.  Use of EAP-RSA along with a tunneling mechanism such as PEAPv0.

   c.  Use of EAP Protected One Time Password (POTP) [RFC4793].

   Given this level of diversity, it is common today for popular EAP
   peer and server implementations from different vendors to be unable
   to negotiate a common EAP method for SecurID support.  In practice,
   the fact than none of these mechanisms are designated as "mandatory-
   to-implement" has made it very difficult for customers to put
   together multi-vendor deployments with any hope of interoperability
   -- yet the non-interoperable vendors can each claim that they
   implement "standards" by supporting an IETF RFC or Internet-Draft.

   Another example occurs with pre-shared key mechanisms.  [RFC3748]
   defined EAP-MD5; since this mechanism did not support key generation
   it did not satisfy the security requirements outlined in [RFC4017]
   for use on wireless networks.  In order to address this weakness,
   additional mechanisms supporting key generation have subsequently
   been defined and published as Informational RFCs, including EAP-SIM
   [RFC4186], EAP-AKA [RFC4187], EAP-PSK [RFC4764], EAP-PAX [RFC4746],
   EAP-SAKE [RFC4763].  To address the lack of a standardized mechanism,
   the IETF EMU WG has produced a standards-track pre-shared key method
   known as EAP-GPSK [RFC5433].

   Often the proliferation of mechanisms is driven by the need to
   support widely deployed authentication technologies, particularly
   those embodied in hardware which enable "what you have"
   authentication.  Aside from manufacturing and distribution costs,
   deployment of these mechanisms may involve training or backend
   integration costs which can only be recouped after a considerable
   period of use.

   However, when a limited set of standardized mechanisms is defined,
   specification for protocol authors and deployment for network
   operators becomes far more successful.  Whenever feasible, limiting a
   set of standardized mechanisms is recommended, and should be
   encouraged.  At the very least, specifying a "mandatory-to-implement"
   is a must.

   For example, today EAP authentication within RADIUS [RFC3579] is now

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   widely supported, and implementations offering mechanisms satisfying
   the security requirements outlined in [RFC4017] are common in such
   implementations as FreeRADIUS.  As a result, greenfield client or
   server deployments rarely have a need for use of EAP-MD5, and the
   development of standardized pre-shared key mechanism may eventually
   enable replacement of EAP-MD5 as the mandatory-to-implement EAP
   authentication mechanism.

   The real point here it to ensure that multiple authentication
   mechanisms aren't trying to authenticate the same credential type,
   rather than to arbitrarily limit the number of authentication
   mechanisms.  For example, while neither Kerberos nor (D)TLS are
   authentication frameworks, each now supports multiple credential
   types.  This is both powerful and desireable for customers, since it
   means that they don't have to support each credential type with each
   application individually.

   However, having multiple different ways to authenticate with the same
   credential type, enabling vendors to claim compliance without
   interoperating, would be likely to result in customer frustration.
   This can be avoided by standardizing a "mandatory-to-implement"
   mechanism for each credential type, ensuring interoperability out-of-

   In order to encourage interoperability and the reduction of
   complexity, it is recommended that the IETF standardize only a small
   number of authentication mechanisms within a pluggable authentication
   framework.  Proliferation of mechanisms should be limited to no more
   than one for any given class within the framework.  Recalling section
   10.2, having a small number of mechanisms and clearly stating this
   minimal set in the protocol specification is particularly important
   when all implementations on the Internet will need to use the same
   mechanisms for authentication in order to interoperate.  Examples of
   such Internet wide protocols have included DKIM, DNSSEC, and
   infrastructure protocols like BGP.  Support for a standardized
   password-based (includes pin + OTP) mechanism is highly recommended
   for protocols where end-users (as opposed to unattended machines)
   will be involved.

   When working to limit the number of mechanisms, designers should take
   care not to break the architecture of an existing framework.  For
   example, for SASL, it goes against the architecture to have
   mechanism-specific information such as specific mechanism
   restrictions in a protocol.  Care must also be taken that such
   restrictions do not lead to mechanism-specific details making their
   way directly into protocols.  Such layering violations make it harder
   to revise mechanisms in the future or to change the set of
   appropriate mechanisms if proven necessary over time.  Experience has

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   demonstrated that we are likely to need to change the set of
   mechanisms over time, as new technologies or new requirements emerge.

10.2.3.  Channel Bindings

   Many applications desire channel security but tighter integration of
   authentication with the application than is typically provided by
   channel security protocols.  A common approach is to run the
   application protocol on top of a channel security protocol (most
   commonly TLS) but to use an authentication framework (most commonly
   SASL) for client (and sometimes server) authentication.  As described
   in Section 6 this is potentially subject to man in the middle attack.
   As described in Section 6.1 and Section 8 if the server can be
   authenticated by the channel security protocol, then a MITM attack is
   not possible.

   If the server cannot be so authenticated, then the authentication
   performed by the framework must be cryptographically bound to the
   cryptographic context formed by the channel security protocol (this
   is often called a CHANNEL BINDING) so that the authentication
   framework will fail if a MITM attack is underway.

   For example, consider the figure below.  If TLS is used without
   client or server authentication to provide only privacy (via
   encryption) and per-packet authentication and integrity (meaning I
   know that the packet came from the same person with whom I started
   this connection, and that it hasn't been changed along the way), it
   is still suceptible to a MitM attack.  A challenge-response mechanism
   may be used inside the TLS to add end-point authentication.  If so,
   one must include something from the TLS exchange in the challenge-
   response exchange in order to actually protect against a MitM attack.
   I.e. the channel and the authentication exchange must be bound

   Channel =           ==========================
                           |               |
               Client      |               |     Server
                           V               V

    Authentication     --------------------------
    Exchange =

                          Channel Binding Example

   For instance, if the authentication framework is using a challenge-

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   response mechanism, the response computation could include a
   characteristic value from channel security protocol, thus forcing the
   response given to the attacker and the response expected by the
   server to be different.  Note that care must be used in selecting the
   characteristic value to ensure that the attacker cannot force the
   values to be the same for both connections.
   [I-D.williams-ipsec-channel-binding] and
   [I-D.williams-sshv2-channel-bindings] describe selection of values
   for TLS, IPsec, and SSH.

   Note that this technique cannot be used with non-cryptographic
   mechanisms such as simple passwords or one-time passwords.  If these
   mechanisms are to be used in environments where MITM attacks are a
   concern, then the server must be authenticated by the channel
   security protocol.

10.2.4.   Excessive Layering

   Many of the legacy authentication mechanisms that users and
   administrators wish to support are themselves generic frameworks of
   one kind or another.  In general, when two security frameworks are
   run together with one as a mode of the other, it becomes very
   difficult to make assertions about the security properties of the
   composed system.  Among the issues are:

   o  The state machines can become interlinked, causing confusion at
      one layer about the state of the other layer.  For instance, TLS
      has a simple two round trip exchange, but [I-D.nir-tls-eap]
      extends that with a generic "EAPMsg" that may occur an arbitrary
      number of times without transitions in the TLS state machine.
   o  Understanding the composed system becomes difficult.  Experts in
      one security protocol often are not experts in all, and unless the
      encapsulation boundaries are very carefully drawn, analyzing the
      composed protocol may require an unavailable level of general
   o  Any proofs of security that may be available for one of the
      systems almost certainly depend on knowledge of the available
      cryptographic mechanisms, but if one of those mechanisms is a
      framework, then those proofs no longer apply.

   These issues have been encountered within the Extensible
   Authentication Protocol (EAP), defined in [RFC3748] Where EAP runs
   over link layers that support authentication mechanisms other than
   EAP (such as PPP or IEEE 802.16), it may be necessary to first
   negotiate use of EAP, and then within EAP, to negotiate the specific
   EAP mechanism to be used.  This may introduce security
   vulnerabilities.  For example, since neither PPP authentication

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   negotiation nor EAP mechanism are secured, it is necessary for both
   PPP and EAP authentication policy to be pre-provisioned on the EAP
   peer and server in order to prevent bidding down attacks.

   Since EAP supports a wide range of security mechanisms, support for
   multiple link layer authentication mechanisms is generally
   unnecessary, and in general greenfield link layer designs supporting
   EAP are best advised to forgo other approaches.

   The issue of multiple negotiation layers is also encountered within
   EAP methods.  While some EAP methods (such as EAP-TLS [RFC5216] only
   support a single authentication mechanism, other such as EAP-FAST
   [RFC4851] and [I-D.funk-eap-ttls-v0] act as "tunneling methods",
   providing for negotiation of an "inner EAP method".  As noted in
   [RFC3748] Section 7.4, unless the inner and outer authentication
   mechanisms are cryptographically bound, tunneling methods are
   vulnerable to a man-in-the-middle attack.

   In accordance with the principle of having as few mechanisms as
   possible, applications should try to avoid having multiple
   negotiation layers.  If that is not possible, applications should
   profile a single negotiation layer.  If application Foo is to be used
   with framework Bar which supports authentication methods Alpha and
   Bravo, itself supports framework Baz, which supports authentication
   methods Alpha, Bravo, and Charlie, Foo should indicate whether Alpha
   and Bravo are to be supported via Bar or Baz.

10.2.5.   List of Authentication Frameworks

      GSS-API [RFC2743]
      SASL [RFC4422]
      EAP [RFC3748]

11.   Sharing Authentication Information

   In many cases, users will use the same authentication data for a
   large number of services.  For instance, users may expect to use the
   same username/password pair for TELNET, IMAP, and FTP.  In such
   cases, it is generally desirable for all such services to share a
   single set of authentication data.  For instance, TELNET, IMAP, and
   FTP typically all share the same password database.

11.1.   Authentication Services

   This problem is made more difficult if the services which must share
   authentication data reside on different machines.  This problem is
   typically solved (when it is solved, as opposed to simply ignored) by

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   having some unique system which has the credentials.  Such a machine
   may either provide authentication as service (as in Kerberos) or
   simply provide credentials to authorized machines (YP, NIS).  In
   either case, this protocol needs to be secured.

11.2.   Single Sign-On

   A related problem is that it's undesirable to have users manually
   authenticate each time some service wants authentication.  First,
   it's inconvenient for the users.  Second the cognitive load
   associated with frequent authentication seems likely to lead to
   careless use of credentials, enabling attacks such as phishing.
   Rather, they want to authenticate once and have software take care of
   the rest.  This capability is called SINGLE SIGN-ON.

   If all authentication will be performed by one program, this can be
   fixed simply by having the program cache the user's credentials.  If
   credentials need to be shared across multiple services then it's
   necessary to have some way to pass them from the program which first
   authenticates to others (or to have some central credential manager).
   As a special case, consider the case where mutually suspicious
   systems all want to allow a user to authenticate with a single set of
   credentials.  If certificate-based authentication is being used, the
   protocols are straightforward since all relying parties can have the
   same verifier.  In the case where passwords are being used, the
   typical solution is to have some third party authentication service
   which authenticates the user and then vouches for the user to the
   services.  Microsoft Passport is one such provider.

11.3.   Case Study: RADIUS

   RADIUS, defined in [RFC2865], is a protocol for Authentication,
   Authorization and Accounting (AAA), commonly implemented in Network
   Access Servers (NASes).  NAS devices are often constrained in terms
   of their CPU power, memory, or non-volatile storage.  As a result, it
   may be difficult for them to implement a variety of authentication
   mechanisms.  Also, given that access networks may contain hundreds or
   even thousands of NAS devices, management concerns may lead to
   implementation of a centralized authentication scheme.  As a result,
   NAS devices may not perform authentication directly, instead
   delegating this to one or more authentication servers.

   When utilizing AAA servers for authentication, NAS devices act as
   "pass-through" devices, forwarding authentication exchanges between
   the user and the AAA server.  Such an arrangement implicitly assumes
   the AAA server acts as a trusted third party, and that communication
   between the NAS and AAA server is authenticated and integrity and
   replay protected.

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   As described in "The Network Access Identifier" [RFC4282] and
   "Chargeable User Identity" [RFC4372], there are circumstances in
   which the user desires to keeps its identity confidential both to a
   potential attacker that may be snooping on the conversation between
   the user and the NAS, as well as to the NAS itself.  In these
   circumstances, only the AAA server may authenticate the identity of
   the user, and the NAS may only be provided with a "temporary
   identity" sufficient for authorization and billing purposes.  "

11.4.   Case Study: Kerberos

   Kerberos [RFC4210] is a popular authentication/single sign-on
   service.  Kerberos is based on the Needham-Schroeder authentication
   protocol.  The authentication server role is played by a Key
   Distribution Center (KDC).  When a client first signs on the client
   proves its identity to the KDC, usually by means of a password shared
   with the KDC.  Kerberos is unusual in that the authentication service
   is provided to the client rather than the server.  When a client
   wishes to communicate with a server, it first contacts the KDC and
   acquires a TICKET.  That ticket contains a new symmetric key
   encrypted for both the client and server.  The client can transmit
   the ticket to the server and use it both to prove its identity and
   establish a secure channel.

11.5.   List of Authentication Server Systems

      Kerberos [RFC4120]
      RADIUS [RFC2865]
      Diameter [RFC3588]

12.   Guidance for Protocol Designers

   Adding authentication to protocols is difficult and is made even more
   difficult by the large number of options.  This section attempts to
   provide some guidance to protocol designers.  No single document can
   tell you how to build a secure system, but the following guidelines
   provide generally good advice.  If you feel you need to violate one
   of these rules of thumb, make sure you know why you're doing it.

12.1.   Know what you're trying to do

   The first thing to do is figure out what the security problem you're
   trying to solve is.  Questions to ask include:

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12.1.1.   What's my threat model?

   Sorting out the threat model (see [RFC3552]) is always the first step
   in deciding what sorts of security mechanisms to use.  In the case of
   authentication you must consider, at minimum:

   1.  What will be the result of various forms of attack?
   2.  Does the threat model include active attack?  (Hint:  it should.)
   3.  Do I need protection for my data or just the authentication?
       (Hint:  probably you do)
   4.  How valuable is the data being secured?  Are exhaustive
       computational attacks practical?
   5.  How competent are my users going to be?

12.1.2.   How many users will this system have?

   In general, the difficulty of managing a system scales with (or
   greater than) the number of users.  This means that mechanisms which
   are practical with a small number of users may simply have too much
   overhead with a large number of users.  For example, many token-based
   solutions charge by the token, which may be a prohibitive expense if
   there are many users.

   The complexity compared to scale of users will also be affected
   depending on whether all the users are "managed" or are "unmanaged".
   Managed users are those whose machines are owned, operated and
   managed by the organization deploying the system.  Unmanaged users
   are those whose machines are not own, operated or managed by the
   deploying organization.  System administrators tend to have far more
   control over managed machines, e.g. a company issued laptop or
   desktop machine, as compared to unmanaged machines, e.g. someone's
   home computer or personal smart phone.  The complexity may further
   decrease for managed machines when a central management system is
   available for making configuration changes quickly and easily to all
   devices in the system.  A central management system is unlikely for a
   solution with unmanaged machines.

12.1.3.   What's my protocol architecture?

   In some systems (e.g.  POP, IMAP, TELNET), clients connect directly
   to the server.  In others (e.g.  HTTP, SIP, RSVP, BGP),
   authentication may need to be established over multiple hops when the
   entities have no independent authentication.  Each case requires a
   different strategy.  See Section 13.2.2 for more discussion on this

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12.1.4.   Do I need to share authentication data?

   If authentication data needs to be shared, especially between
   multiple servers, it's generally worth considering some sort of
   authentication server or using certificates.

12.2.   Use as few mechanisms as you can

   In the best case, each system would only have one or a small number
   of forms of authentication.  The more methods of authentication a
   system allows, the more things there are to go wrong, and the more
   difficult it is to gain solid interoperability.  Unfortunately, this
   is not always possible.  In general, there are two reasons why
   systems allow more than one authentication mechanism.  The first is
   that you're retrofitting a system which already has a large number of
   authentication mechanisms which cannot be displaced.  The second is
   that users have widely different environments which for some reason
   cannot use the same authentication mechanism conveniently (e.g. some
   users have tokens and some do not).  Even for the same user, some of
   the user's machines could accept a token, e.g. a laptop, while others
   may not, e.g. a smart phone.

   Naturally, designers need to take such considerations into account
   but they should take reasonable steps to minimize the number of
   mechanisms.  Designers should take special care to minimize the
   number of mechanisms that use the same underlying keying material in
   different ways.  For instance, a system that provides a challenge/
   response mechanism and a public key based mechanism is a reasonable
   design, one that provides three different challenge/response
   mechanisms using the same passwords/keys presents serious complexity
   challenges and should be avoided if possible.  Again, this is not
   always possible in systems with legacy authentication mechanisms but
   should be avoided in new designs.

   This doesn't mean that designers should not use security frameworks
   where multiple mechanisms are appropriate, but it does mean that they
   should be avoided unless there's a good a priori case for diversity
   in authentication mechanisms.  Where generic security frameworks are
   used, designers need to carefully analyze the threats relevant to
   each mechanism in the context of the specific application layer
   protocol environment.  In order to mimimize the attack surface,
   individual deployments would be wise to specify policies which
   disallow mechanisms which are unnecessary in their environment, even
   if they are specified in the protocol.  For instance, if users are
   expected to use challenge response, then optimally clients would be
   configured not even to perform plaintext passwords, as this
   represents a security threat no matter what the server configuration

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12.3.   Avoid simple passwords

   It's widely known that simple plaintext passwords are unsafe, but
   what's less widely known is that merely providing such a scheme can
   weaken systems even if stronger mechanisms are present.  The
   difficulty is that simple passwords almost never provide the user
   with any form of server authentication.  Consider the case where a
   system uses a negotiation framework that allows passwords.  A
   downgrade attack can force the user to reveal his password even if
   both client and server support stronger mechanisms.

   Even when an authenticated and encrypted channel to the server is
   available, the use of cleartext passwords places strong requirements
   on the protection provided by encryption, in part because the same
   plaintext is transmitted repeatedly.  [RFC3579] and [CHVV03] describe
   examples of such situations.

   Accordingly, designers should avoid deploying simple password
   mechanisms if at all possible, not just provide stronger mechanisms.

12.4.   Avoid inventing new frameworks

   Despite the large number of mechanisms we've discussed, this document
   describes only a small number of the available authentication
   mechanisms.  There are very few situations in which designers cannot
   use some preexisting mechanism.  This is vastly preferable to
   designing their own version of one of the standard mechanisms.  In
   particular, designers should avoid designing their own authentication
   frameworks or channel security systems.  If you want an
   authentication framework, use SASL or GSS-API or (if you're in a
   network access context) EAP.  If you want a channel security system,
   use IPsec, TLS, or DTLS.  Note that none of these systems can be
   blindly dropped into an existing system and provide adequate
   security.  Care must be taken to analyze the protocol being secured
   and determine the correct interaction model.  [RFC5406] provides
   guidance on this topic for IPsec.

12.5.   Use the strongest mechanisms you can

   Having the strongest security you can propose is generally a good
   plan.  It's particularly good advice here, since passwords in the
   clear, one-time passwords, challenge-response and zero-knowledge
   password proofs all require the user to have the same kind of
   credential:  a password.  (Note that some OTP schemes such as SecurID
   require a token.)  When designing a new system, the ability to
   provide a familiar interface to a user is valuable, minimizing
   additional work for client and server implementors is not.  NIST
   Spec.  Pub. 800-63 [SP800-63] provides good guidance about the

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   minimum requirements for various applications.

12.6.   Consider providing message integrity

   Although most of the authentication mechanisms we've described are
   themselves resistant to active attacks, many are subject to hijacking
   after authentication has completed.  If your threat model includes
   active attack (it should), you should strongly consider providing
   message integrity for all of your protocol messages in order to
   prevent hijacking.

   Message integrity provides a cryptographic indication in each message
   that, when validated, confirms that the message has not been altered
   since being sent from a trusted host.  When applied correctly, it is
   often possible to use one cryptographic operation to achieve both the
   message integrity and the message authentication.  For example, doing
   a digest authentication operation over a suitably large portion of
   the message (or payload, or packet, depending on the protocol in
   question) using a connection key known by both parties provides both
   message integrity and message authentication.  IPsec, TLS, DTLS and
   SSH are all examples of protocols that provide message integrity and
   message authentication in one operation.

13.   Scenarios

   Despite the proliferation of authentication mechanisms, there are
   generally one or two optimal mechanisms for each scenario.  We
   attempt to describe those mechanisms here.  This section is divided
   into two parts, attacking the problem from different angles.  In the
   first, we consider the various kinds of capabilities entities might
   have and the best mechanisms to use with those capabilities.  In the
   second part we discuss a number of different protocol architectures
   and the potential mechanisms which can be used with those

13.1.   Capability Considerations

   There are three primary authentication scenarios:  (1) Neither side
   has a public/private key pair. (2) One side has an authenticated key
   pair (either via a certificate or prior arrangement). (3) Both sides
   have authenticated key pairs

   Despite the proliferation of authentication mechanisms, there are
   only one or two best mechanisms for each scenario.  We describe them

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13.1.1.   Neither side has a public/private key pair

   Three basic strategies are suitable for the situation where neither
   side has a key pair:  challenge/response, one-time passwords, and
   ZKPPs.  The only situation in which OTP systems are superior to
   challenge/response systems is when adapting a legacy system in which
   it is difficult to change the client software.  If the client
   software can be changed, challenge/response offers roughly equivalent
   security with significantly less management complexity.  ZKPP proofs
   are technically superior however, in at least two cases (SACRED and
   IPS), IETF WGs have chosen not to require ZKPPs due to IPR concerns.

   These considerations make challenge/response the best choice for this
   scenario.  If at all possible, it should be performed under cover of
   an anonymous key exchange, as described in Section 6.  With this
   adaptation, an attacker needs to mount an active attack in order to
   dictionary search the password space.

13.1.2.   One side has an authenticated key pair

   If the server has a key pair which the client can authenticate, then
   several alternatives are available for password authentication.

   Simple username/password encrypted under the server's public key is
   the preferred authentication mechanism.  Rather than encrypting
   directly under the server's public key, the standard practice here is
   to use the server's key to establish a secure channel and then pass
   the password over that channel.  Challenge/response is in fact weaker
   in this case because it is is password equivalent.

   In this situation, using a single OTP only to authenticate the client
   to the server during a first connection, as a bootstrap, is quite
   useful.  Once authenticated, and a secure channel is established, the
   client can pass some long term credential to the server, like its
   self-generated key pair's public key.  Then after, the public key is
   used for the client authentication proof.  This is how some mobile
   devices "register" and bootstrap secure connections to servers (e.g.
   how the handset registers to it's home mail server).

   Assuming that an authenticated server key pair is available, the use
   of a continuous list of OTPs and ZKPP systems offer significant
   additional management complexity for marginal security benefit.

   However, the difficulties involved in establishment of an
   authenticated server key pair may be substantial.  These issues

   a.  Provisioning of trust anchors.  In a number of scenarios, such as

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   establishment of network access from an unprovisioned host, trust
   anchors may not be pre-populated or utilization of pre-populated
   trust anchors may introduce security vulnerabilities.  In such
   circumstances, either trust anchors need to be verified after the
   fact, leaving the client vulnerable to active attack, or out-of-band
   provisioning mechanisms need to be provided.

   b.  Certificate validation.  In practice, the deployment of
   Certificate Revocation Lists (CRLs) or Online Certificate Status
   Protocol (OCSP) may present practical difficulties.

   c.  Man-in-the-Middle attacks.  In order to avoid trust anchor
   provisioning or certificate validation, "leap of faith" approaches
   such as that used within SSH may be appealing.  However, such an
   approach assumes that an attacker cannot gain sufficient access to
   disrupt the initial authentication attempt which establishes trust in
   the server public key pair.  In some scenarios (e.g. client
   authenticating to a server in a restricted environment), this
   assumption may be valid; in other scenarios (wireless network
   authentication), it may not be.

13.1.3.   Both sides have authenticated key pairs

   If both sides have key pairs, the optimal mechanism is mutual public
   key authentication.

13.2.   Architectural Considerations

   In this section, we consider 3 different network architectures and
   the authentication mechanisms that are most suitable for each.

13.2.1.   Simple Connection

   The simplest authentication scenario is where the peers are connected
   by some interactive connection.  Mercifully, this situation is quite
   common in such protocols as IMAP, TELNET, etc.  In this simple case,
   mostly any authentication mechanism can be employed and so the choice
   depends on other factors, such as what credentials are available and
   the degree of security required.

13.2.2.   Proxied Client/Server

   It's quite common for client/server communication to be propagated
   through some gateway, as happens with HTTP.  This situation has two
   potential authentication problems.

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   1.  How does the client authenticate to the proxy so that the proxy
       knows to serve it?
   2.  How does the client authenticate to the server with the proxy in
       the way?

   The problem of authenticating to the proxy looks essentially like the
   ordinary client/server authentication problem (except in the case
   where there are multiple proxies in which case authenticating to
   anything other than the first hop proxy looks rather like problem 2.)

   The problem of authenticating through the proxy is rather more
   difficult.  The obstacle is that either client nor server may not
   trust the proxy and they do not want to involve it in their
   authentication.  They therefore need to provide an authentication
   method (preferably with message integrity) that doesn't require
   trusting the proxy.  This rules out simple passwords and makes one-
   time passwords extremely questionable.  There are three basic
   strategies available.   Tunnel

   If the client and the server establish a tunnel through the proxy
   then they can behave as if this was an ordinary client/server
   transaction.  Although this rather obviates the point of having a
   proxy, it's still a popular strategy and is used with HTTPS
   [RFC2817], [RFC2818].  Since the proxy is untrusted, the application
   protocol must either be run over a secure channel or hardened against
   active attacks.   Challenge/Response

   A shared symmetric key between client and server can be used for
   authentication even in the face of a proxy by using standard
   challenge/response methods (with appropriate protocol modifications
   to distinguish between protocol data units (PDUs) directed towards
   the proxy and those directed towards endpoints.)  These methods
   should include integrity protection for the individual PDUs.

   On a small scale, this technique works (it's what's used in HTTP when
   HTTPS is not used) but it quickly becomes unwieldy.  If there are a
   large chain of proxies each of which wishes to authenticate the
   client, server, other proxies or all three, an enormous number of
   pairwise keys need to be established and maintained.  In a protocol
   where long proxy chains are expected, symmetric key based
   authentication is probably impractical.

   A variant of this technique is to use a message-based system with
   symmetric keying such as S/MIME.  All PDUs can then be encapsulated

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   in secure messages.  Recursive encapsulation can be used to provide

   authentication to proxies.   Digital Signatures

   The final approach is to use public-key based digital signatures.
   Each endpoint signs each message (possibly with some set of nonces to
   prevent replay attack).  The disadvantage of this approach is that it
   requires a PKI.  The advantage is that it doesn't require pairwise
   keys.  Each proxy in the chain can validate the client and the server
   based solely on their signatures.

13.2.3.   Store and Forward

   A number of important IETF protocols, most importantly, e-mail, are
   of the store and forward messaging variety.  Such protocols have
   roughly the same security options as proxied protocols except that
   tunneling is no longer possible.  Additionally, since store and
   forward protocols are non-interactive, many of the usual challenge/
   response techniques for preventing replay attack no longer work and
   so care must be taken to either make one's system idempotent or
   introduce a specific anti-replay mechanism.  The standard technique
   for store-and-forward situations is message security a la S/MIME.

13.2.4.   Multicast

   A number of IETF protocols have the property that multicast or
   broadcast message integrity needs to be provided.  For example,
   routing and DNS both require the ability for a single sender to
   broadcast authenticated and integrity protected messages to a large
   number of receivers.  There are two relevant cases:  In the first,
   all members of the group are trusted and so it's feasible to have
   some group key which is used for authenticating all transmissions.
   This group key may be manually configured or established via some
   protocol such as GSAKMP [RFC4535].

   In the second case, individual group members are anticipated to
   possibly forge messages.  With such systems, it's not really
   practical to use symmetric key systems because the sender would need
   to agree on a key with each recipient (there may not even be a return
   channel).  The only really practical approach in these multicast
   situations is for the sender to digitally sign each transmission with
   its private key.

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14.  Acknowledgements

   Eric Rescorla was the original author and editor of this document,
   for versions -00 through -05.  Gregory Lebovitz assumed the editor
   role starting in draft -06.  Early versions of this document were
   reviewed by Fred Baker, Lisa Dusseault, Ted Hardie, and Mike St.
   Johns.  Thanks to Jeffrey Altman, Sam Hartman, Paul Hoffman, John
   Linn, and Nico Williams for their reviews and comments.  Bernard
   Aboba contributed extensive sections of this document, performed
   several reviews, and kept the meticulous issue tracker, which
   received plenty of contributions.

   Though published in 2010, the vast majority of the work done on this
   document occurred under the IAB teams of 2003-2006.  Your efforts are
   remembered and appreciated.

15.  References

15.1.  Normative References

15.2.  Informative References

              "National Institute of Standards and Technology,
              "Electronic Authentication Guideline: Recommendations of
              the National Institute of Standards and Technology", SP
              800-63", 2004.

   [AKA]      "Technical Specification Group Services and System
              Aspects; 3G Security; Security Architecture (Release 5)
              3GPP TS 33.102 V5.1.0.".

   [BFL96]    Blaze, M., Feigenbaum, J., and J. Lacy, ""Decentralized
              trust management", IEEE Symposium on Security and Privacy

   [Bell89]   Bellovin, S., ""Security Problems in the TCP/IP Protocol
              Suite", Computer Communications Review", 1989.

   [Bell92]   Bellovin, S. and M. Merritt, ""Encrypted Key Exchange:
              Password-based protocols secure against dictionary
              attacks", Proceedings of the IEEE Symposium on Research in
              Security and Privacy '92.", 1992.

   [Bell94]   Bellovin, S. and M. Merritt, ""Augmented Encrypted Key
              Exchange: a Password-Based Protocol Secure Against
              Dictionary Attacks and Password File Compromise". AT&T

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              Bell Laboratories Technical Report", 1994.

   [CHVV03]   Canvel, B., Hiltgen, A., Vaudenay, S., and M. Vuagnoux,
              ""Password Interception in a SSL/TLS Channel", Advances in
              Cryptology CRYPTO 2003.", 2003.

   [DTH06]    Dhamija, R., Tygar, J., and M. Hearst, ""Why Phishing
              Works", CHI 2006", 2006.

   [Crack]    Muffet, A., "CRACK v 5.0a".

   [Wil68]    Wilkes, M., ""Time-Sharing Computer Systems", American
              Elsevier New York.".

   [Worm88]   Spafford, E., ""The Internet Worm Program: An Analysis",".

   [Jab96]    Jablon, D., ""Strong Password-Only Authenticated Key
              Exchange", Computer Communication Review", 1996.

   [KP01]     Kaufman, C. and R. Perlman, ""PDM: A New Strong Password-
              Based Protocol", Proceedings of the 10th USENIX Security
              Symposium '01", 2001.

   [Klein90]  Klein, D., ""Foiling the Cracker: A Survey of Improvements
              to Password Security"", 1990.

              Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", draft-altman-tls-channel-bindings-07 (work in
              progress), October 2009.

              Funk, P. and S. Blake-Wilson, "EAP Tunneled TLS
              Authentication Protocol Version 0 (EAP-TTLSv0)",
              draft-funk-eap-ttls-v0-05 (work in progress), April 2008.

              Menon-Sen, A., Melnikov, A., Newman, C., and N. Williams,
              "Salted Challenge Response (SCRAM) SASL Mechanism",
              draft-newman-auth-scram-13 (work in progress), May 2009.

   [MS-PEAP]  Microsoft Corporation, "MS-PEAP: Protected Extensible
              Authentication Protocol (PEAP) Specification",
              January 2010.

              Nir, Y., Sheffer, Y., Tschofenig, H., and P. Gutmann, "TLS
              using EAP Authentication", draft-nir-tls-eap-06 (work in

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              progress), April 2009.

              Williams, N., "End-Point Channel Bindings for IPsec Using
              IKEv2 and Public Keys",
              draft-williams-ipsec-channel-binding-01 (work in
              progress), April 2008.

              Williams, N., "Channel Binding Identifiers for Secure
              Shell Channel", draft-williams-sshv2-channel-bindings-00
              (work in progress), November 2007.

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, May 1983.

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.

   [RFC1760]  Haller, N., "The S/KEY One-Time Password System",
              RFC 1760, February 1995.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2195]  Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
              AUTHorize Extension for Simple Challenge/Response",
              RFC 2195, September 1997.

   [RFC2289]  Haller, N., Metz, C., Nesser, P., and M. Straw, "A One-
              Time Password System", RFC 2289, February 1998.

   [RFC2367]  McDonald, D., Metz, C., and B. Phan, "PF_KEY Key
              Management API, Version 2", RFC 2367, July 1998.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2660]  Rescorla, E. and A. Schiffman, "The Secure HyperText
              Transfer Protocol", RFC 2660, August 1999.

   [RFC2695]  Chiu, A., "Authentication Mechanisms for ONC RPC",

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              RFC 2695, September 1999.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC2808]  Nystrom, M., "The SecurID(r) SASL Mechanism", RFC 2808,
              April 2000.

   [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
              HTTP/1.1", RFC 2817, May 2000.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC2945]  Wu, T., "The SRP Authentication and Key Exchange System",
              RFC 2945, September 2000.

   [RFC2964]  Moore, K. and N. Freed, "Use of HTTP State Management",
              BCP 44, RFC 2964, October 2000.

   [RFC2965]  Kristol, D. and L. Montulli, "HTTP State Management
              Mechanism", RFC 2965, October 2000.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3290]  Bernet, Y., Blake, S., Grossman, D., and A. Smith, "An
              Informal Management Model for Diffserv Routers", RFC 3290,
              May 2002.

              4rev1", RFC 3501, March 2003.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

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   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC3850]  Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Certificate Handling",
              RFC 3850, July 2004.

   [RFC4017]  Stanley, D., Walker, J., and B. Aboba, "Extensible
              Authentication Protocol (EAP) Method Requirements for
              Wireless LANs", RFC 4017, March 2005.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC4186]  Haverinen, H. and J. Salowey, "Extensible Authentication
              Protocol Method for Global System for Mobile
              Communications (GSM) Subscriber Identity Modules (EAP-
              SIM)", RFC 4186, January 2006.

   [RFC4187]  Arkko, J. and H. Haverinen, "Extensible Authentication
              Protocol Method for 3rd Generation Authentication and Key
              Agreement (EAP-AKA)", RFC 4187, January 2006.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210, September 2005.

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

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   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [RFC4372]  Adrangi, F., Lior, A., Korhonen, J., and J. Loughney,
              "Chargeable User Identity", RFC 4372, January 2006.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP: Group Secure Association Key Management
              Protocol", RFC 4535, June 2006.

   [RFC4746]  Clancy, T. and W. Arbaugh, "Extensible Authentication
              Protocol (EAP) Password Authenticated Exchange", RFC 4746,
              November 2006.

   [RFC4763]  Vanderveen, M. and H. Soliman, "Extensible Authentication
              Protocol Method for Shared-secret Authentication and Key
              Establishment (EAP-SAKE)", RFC 4763, November 2006.

   [RFC4764]  Bersani, F. and H. Tschofenig, "The EAP-PSK Protocol: A
              Pre-Shared Key Extensible Authentication Protocol (EAP)
              Method", RFC 4764, January 2007.

   [RFC4785]  Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
              Ciphersuites with NULL Encryption for Transport Layer
              Security (TLS)", RFC 4785, January 2007.

   [RFC4793]  Nystroem, M., "The EAP Protected One-Time Password
              Protocol (EAP-POTP)", RFC 4793, February 2007.

   [RFC4851]  Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "The
              Flexible Authentication via Secure Tunneling Extensible
              Authentication Protocol Method (EAP-FAST)", RFC 4851,
              May 2007.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              RFC 4949, August 2007.

   [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol for TLS
              Authentication", RFC 5054, November 2007.

   [RFC5081]  Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
              Layer Security (TLS) Authentication", RFC 5081,
              November 2007.

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   [RFC4248]  Hoffman, P., "The telnet URI Scheme", RFC 4248,
              October 2005.

   [RFC4266]  Hoffman, P., "The gopher URI Scheme", RFC 4266,
              November 2005.

   [RFC5216]  Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
              Authentication Protocol", RFC 5216, March 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5281]  Funk, P. and S. Blake-Wilson, "Extensible Authentication
              Protocol Tunneled Transport Layer Security Authenticated
              Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5386]  Williams, N. and M. Richardson, "Better-Than-Nothing
              Security: An Unauthenticated Mode of IPsec", RFC 5386,
              November 2008.

   [RFC5406]  Bellovin, S., "Guidelines for Specifying the Use of IPsec
              Version 2", BCP 146, RFC 5406, February 2009.

   [RFC5433]  Clancy, T. and H. Tschofenig, "Extensible Authentication
              Protocol - Generalized Pre-Shared Key (EAP-GPSK) Method",
              RFC 5433, February 2009.

Appendix A.  IAB Members at the time of this writing

   Marcelo Bagnulo
   Gonzalo Camarillo
   Stuart Cheshire
   Vijay Gill
   Russ Housley
   John Klensin
   Olaf Kolkman
   Gregory Lebovitz
   Andrew Malis
   Danny McPherson
   David Oran
   Jon Peterson
   Dave Thaler

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Authors' Addresses

   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303

   Email:  ekr@rtfm.com

   Gregory Lebovitz
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089-1206

   Email:  gregory.ietf@gmail.com

   Internet Architecture Board

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