Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Standards Track G. Lebovitz
Expires: August 9, 2010 Juniper Networks, Inc.
Internet Architecture Board
IAB
February 05, 2010
A Survey of Authentication Mechanisms
draft-iab-auth-mech-06.txt
Abstract
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
protocol.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on August 9, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
Rescorla, et al. Expires August 9, 2010 [Page 1]
Internet-Draft Authentication Mechanisms February 2010
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Rescorla, et al. Expires August 9, 2010 [Page 2]
Internet-Draft Authentication Mechanisms February 2010
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
Rescorla, et al. Expires August 9, 2010 [Page 3]
Internet-Draft Authentication Mechanisms February 2010
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 . . . . . . . . . . . . . . . . . 43
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 . . . . . . . . . . . . . . 45
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
Rescorla, et al. Expires August 9, 2010 [Page 4]
Internet-Draft Authentication Mechanisms February 2010
Appendix A. IAB Members at the time of this writing . . . . . . . 59
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 59
Rescorla, et al. Expires August 9, 2010 [Page 5]
Internet-Draft Authentication Mechanisms February 2010
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
authentication.
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.
Rescorla, et al. Expires August 9, 2010 [Page 6]
Internet-Draft Authentication Mechanisms February 2010
A Mechanism
(Description)
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
Rescorla, et al. Expires August 9, 2010 [Page 7]
Internet-Draft Authentication Mechanisms February 2010
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
together::
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
password
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
Rescorla, et al. Expires August 9, 2010 [Page 8]
Internet-Draft Authentication Mechanisms February 2010
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
Rescorla, et al. Expires August 9, 2010 [Page 9]
Internet-Draft Authentication Mechanisms February 2010
later, it can be useful when stronger authentication schemes are
employed.
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
later.
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
logins.
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]
program.
An OFFLINE DICTIONARY ATTACK can occur in two ways. First, using
Rescorla, et al. Expires August 9, 2010 [Page 10]
Internet-Draft Authentication Mechanisms February 2010
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
system.
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
Rescorla, et al. Expires August 9, 2010 [Page 11]
Internet-Draft Authentication Mechanisms February 2010
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
DICTIONARY ATTACK.
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
Rescorla, et al. Expires August 9, 2010 [Page 12]
Internet-Draft Authentication Mechanisms February 2010
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
[DTH06]).
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
process.)
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
Rescorla, et al. Expires August 9, 2010 [Page 13]
Internet-Draft Authentication Mechanisms February 2010
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)
RLOGIN
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
Rescorla, et al. Expires August 9, 2010 [Page 14]
Internet-Draft Authentication Mechanisms February 2010
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
systems.
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.
Rescorla, et al. Expires August 9, 2010 [Page 15]
Internet-Draft Authentication Mechanisms February 2010
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:
INCH SEA ANNE LONG AHEM TOUR
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
token.
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
Rescorla, et al. Expires August 9, 2010 [Page 16]
Internet-Draft Authentication Mechanisms February 2010
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
passwords.
Single One-Time-Password used for bootstrapping
S/Key [RFC1760]
Rescorla, et al. Expires August 9, 2010 [Page 17]
Internet-Draft Authentication Mechanisms February 2010
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
Rescorla, et al. Expires August 9, 2010 [Page 18]
Internet-Draft Authentication Mechanisms February 2010
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
WEAK PASSWORD EQUIVALENCE.
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:
STORED = H(PASSWORD || SALT)
RESPONSE = H(STORED || CHALLENGE)
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
Rescorla, et al. Expires August 9, 2010 [Page 19]
Internet-Draft Authentication Mechanisms February 2010
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
server.
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
Rescorla, et al. Expires August 9, 2010 [Page 20]
Internet-Draft Authentication Mechanisms February 2010
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
context).
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
Rescorla, et al. Expires August 9, 2010 [Page 21]
Internet-Draft Authentication Mechanisms February 2010
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
Rescorla, et al. Expires August 9, 2010 [Page 22]
Internet-Draft Authentication Mechanisms February 2010
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
authenticators.
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
encrypted.
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 ->
Ya ->
<- Ys
<- 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.
Rescorla, et al. Expires August 9, 2010 [Page 23]
Internet-Draft Authentication Mechanisms February 2010
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
Rescorla, et al. Expires August 9, 2010 [Page 24]
Internet-Draft Authentication Mechanisms February 2010
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
[RFC5386]
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) ->
Rescorla, et al. Expires August 9, 2010 [Page 25]
Internet-Draft Authentication Mechanisms February 2010
<- 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
Rescorla, et al. Expires August 9, 2010 [Page 26]
Internet-Draft Authentication Mechanisms February 2010
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
name.
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
Rescorla, et al. Expires August 9, 2010 [Page 27]
Internet-Draft Authentication Mechanisms February 2010
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.
8.1.1.1. 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
Rescorla, et al. Expires August 9, 2010 [Page 28]
Internet-Draft Authentication Mechanisms February 2010
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
access.
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.
8.1.1.2. 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:
=MjFkNWQyOGRjYjlmM2IwMmJjMzk0NGFhODg0YTQ4YTcK?page=foo.html
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.
Rescorla, et al. Expires August 9, 2010 [Page 29]
Internet-Draft Authentication Mechanisms February 2010
8.1.1.3. Cookies
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.
8.1.1.4. 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
Rescorla, et al. Expires August 9, 2010 [Page 30]
Internet-Draft Authentication Mechanisms February 2010
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.
Rescorla, et al. Expires August 9, 2010 [Page 31]
Internet-Draft Authentication Mechanisms February 2010
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.
Rescorla, et al. Expires August 9, 2010 [Page 32]
Internet-Draft Authentication Mechanisms February 2010
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
replayed.
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
below.
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
Rescorla, et al. Expires August 9, 2010 [Page 33]
Internet-Draft Authentication Mechanisms February 2010
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
limitation.)
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]
Rescorla, et al. Expires August 9, 2010 [Page 34]
Internet-Draft Authentication Mechanisms February 2010
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. MD5 Challenge, 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
it.
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 implementors. 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 DNS Sec focus on providing a service to the
Rescorla, et al. Expires August 9, 2010 [Page 35]
Internet-Draft Authentication Mechanisms February 2010
Internet at large, that is inter-domain services. For these
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 for
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 (as opposed to
unattended machines) are authenticating within the application,
frameworks may be more appropriate. This is espectially 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. Authenticatoin
frameworks aim to allow protocol implementors to develop applications
that support this deployment goal.
For protocols in the class (intra-domain, end-user interfacing), 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
Rescorla, et al. Expires August 9, 2010 [Page 36]
Internet-Draft Authentication Mechanisms February 2010
discouraged.
However, interoperability difficulty has emerged where many disparate
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 authenticatoin 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
detection.
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
Rescorla, et al. Expires August 9, 2010 [Page 37]
Internet-Draft Authentication Mechanisms February 2010
introduce interoperability problems as well as additional complexity.
Trouble arises when we have many disparate authentication mechanisms
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 [I-D.funk-eap-ttls-v0] 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 have been defined, including EAP-SIM [RFC4186],
EAP-AKA [RFC4187], EAP-PSK [RFC4764], EAP-PAX [RFC4746], EAP-SAKE
[RFC4763], and 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
Rescorla, et al. Expires August 9, 2010 [Page 38]
Internet-Draft Authentication Mechanisms February 2010
widely supported, and implementations offering mechansims 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 at some
point the development of additional standard EAP mechanisms may
provide a mechanism suitable for 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, Kerberos supports multiple credential
types, and customers typically don't think of that as a problem, but
rather a benefit, because they don't have to support each credential
type with each application individually. However, if Kerberos were
to support multiple different ways authenticating with the same
credential type, so that vendors could all claim compliance without
actually interoperating, one could expect customers to be frustrated
with interoperability issues. While neither Kerberos nor (D)TLS are
authentication frameworks, each now supports multiple credential
types. This seems quite powerful (and desirable) for customers.
This becomes even more powerful industry wide when one "mandatory-to-
implement" mechanism is specified, ensuring interoperability out-of-
the-box.
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
Rescorla, et al. Expires August 9, 2010 [Page 39]
Internet-Draft Authentication Mechanisms February 2010
appropriate mechanisms if proven necessary over time. Experience has
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
together.
Channel = ==========================
TLS
| |
Client | | Server
V V
Authentication --------------------------
Exchange =
Challenge/Response
Channel Binding Example
Rescorla, et al. Expires August 9, 2010 [Page 40]
Internet-Draft Authentication Mechanisms February 2010
For instance, if the authentication framework is using a challenge-
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.altman-tls-channel-bindings],
[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
expertise.
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
Rescorla, et al. Expires August 9, 2010 [Page 41]
Internet-Draft Authentication Mechanisms February 2010
vulnerabilities. For example, since neither PPP authentication
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.
Rescorla, et al. Expires August 9, 2010 [Page 42]
Internet-Draft Authentication Mechanisms February 2010
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
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
Rescorla, et al. Expires August 9, 2010 [Page 43]
Internet-Draft Authentication Mechanisms February 2010
"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.
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.
Rescorla, et al. Expires August 9, 2010 [Page 44]
Internet-Draft Authentication Mechanisms February 2010
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:
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
Rescorla, et al. Expires August 9, 2010 [Page 45]
Internet-Draft Authentication Mechanisms February 2010
different strategy. See Section 13.2.2 for more discussion on this
topic.
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
Rescorla, et al. Expires August 9, 2010 [Page 46]
Internet-Draft Authentication Mechanisms February 2010
configured not even to perform plaintext passwords, as this
represents a security threat no matter what the server configuration
is.
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
Rescorla, et al. Expires August 9, 2010 [Page 47]
Internet-Draft Authentication Mechanisms February 2010
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
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
architectures.
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
Rescorla, et al. Expires August 9, 2010 [Page 48]
Internet-Draft Authentication Mechanisms February 2010
only one or two best mechanisms for each scenario. We describe them
here.
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
Rescorla, et al. Expires August 9, 2010 [Page 49]
Internet-Draft Authentication Mechanisms February 2010
include:
a. Provisioning of trust anchors. In a number of scenarios, such as
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.
Rescorla, et al. Expires August 9, 2010 [Page 50]
Internet-Draft Authentication Mechanisms February 2010
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.
13.2.2.1. 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.
13.2.2.2. 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
Rescorla, et al. Expires August 9, 2010 [Page 51]
Internet-Draft Authentication Mechanisms February 2010
in secure messages. Recursive encapsulation can be used to provide
authentication to proxies.
13.2.2.3. 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.
Rescorla, et al. Expires August 9, 2010 [Page 52]
Internet-Draft Authentication Mechanisms February 2010
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
[SP800-63]
"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
'96.".
[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
Rescorla, et al. Expires August 9, 2010 [Page 53]
Internet-Draft Authentication Mechanisms February 2010
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.
[I-D.altman-tls-channel-bindings]
Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", draft-altman-tls-channel-bindings-07 (work in
progress), October 2009.
[I-D.funk-eap-ttls-v0]
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.
[I-D.newman-auth-scram]
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.
[I-D.nir-tls-eap]
Nir, Y., Sheffer, Y., Tschofenig, H., and P. Gutmann, "TLS
using EAP Authentication", draft-nir-tls-eap-06 (work in
Rescorla, et al. Expires August 9, 2010 [Page 54]
Internet-Draft Authentication Mechanisms February 2010
progress), April 2009.
[I-D.williams-ipsec-channel-binding]
Williams, N., "End-Point Channel Bindings for IPsec Using
IKEv2 and Public Keys",
draft-williams-ipsec-channel-binding-01 (work in
progress), April 2008.
[I-D.williams-sshv2-channel-bindings]
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",
Rescorla, et al. Expires August 9, 2010 [Page 55]
Internet-Draft Authentication Mechanisms February 2010
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.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
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.
Rescorla, et al. Expires August 9, 2010 [Page 56]
Internet-Draft Authentication Mechanisms February 2010
[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.
Rescorla, et al. Expires August 9, 2010 [Page 57]
Internet-Draft Authentication Mechanisms February 2010
[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.
Rescorla, et al. Expires August 9, 2010 [Page 58]
Internet-Draft Authentication Mechanisms February 2010
[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.
[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
Rescorla, et al. Expires August 9, 2010 [Page 59]
Internet-Draft Authentication Mechanisms February 2010
Authors' Addresses
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Email: ekr@rtfm.com
Gregory Lebovitz
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089-1206
USA
Email: gregory.ietf@gmail.com
Internet Architecture Board
IAB
Rescorla, et al. Expires August 9, 2010 [Page 60]
