Network Working Group Y. Sheffer
Internet-Draft Check Point
Intended status: Standards Track G. Zorn
Expires: August 14, 2009 Network Zen
H. Tschofenig
Nokia Siemens Networks
S. Fluhrer
Cisco
February 10, 2009
An EAP Authentication Method Based on the EKE Protocol
draft-sheffer-emu-eap-eke-01
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Abstract
The Extensible Authentication Protocol (EAP) describes a framework
that allows the use of multiple authentication mechanisms. This
document defines an authentication mechanism for EAP called EAP-EKE,
based on the Encrypted Key Exchange (EKE) protocol. This method
provides mutual authentication through the use of a short, easy to
remember password.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Protocol Overview . . . . . . . . . . . . . . . . . . . . 3
3.2. Message Flows . . . . . . . . . . . . . . . . . . . . . . 4
4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. EAP-EKE Header . . . . . . . . . . . . . . . . . . . . . . 6
4.2. EAP-EKE Payloads . . . . . . . . . . . . . . . . . . . . . 7
4.3. EAP-EKE-ID . . . . . . . . . . . . . . . . . . . . . . . . 7
4.4. EAP-EKE-Commit . . . . . . . . . . . . . . . . . . . . . . 8
4.5. EAP-EKE-Confirm . . . . . . . . . . . . . . . . . . . . . 9
4.6. EAP-EKE-Failure and EAP-EKE-Protected-Failure . . . . . . 10
5. Cryptographic Operations . . . . . . . . . . . . . . . . . . . 11
5.1. Generating Keying Material . . . . . . . . . . . . . . . . 11
5.2. EAP-EKE-Commit/Request . . . . . . . . . . . . . . . . . . 12
5.3. EAP-EKE-Commit/Response . . . . . . . . . . . . . . . . . 12
5.4. EAP-EKE-Confirm/Request . . . . . . . . . . . . . . . . . 13
5.5. EAP-EKE-Confirm/Response . . . . . . . . . . . . . . . . . 14
5.6. MSK and EMSK . . . . . . . . . . . . . . . . . . . . . . . 14
5.7. Diffie-Hellman Groups . . . . . . . . . . . . . . . . . . 14
5.8. Mandatory Algorithms . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7.1. Cryptographic Analysis . . . . . . . . . . . . . . . . . . 20
7.2. Diffie Hellman Group Considerations . . . . . . . . . . . 20
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 23
A.1. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
A.2. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Appendix B. Design Options . . . . . . . . . . . . . . . . . . . 23
B.1. Number of Round Trips . . . . . . . . . . . . . . . . . . 23
B.2. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
The predominant access method for the Internet today is that of a
human using a username and password to authenticate to a computer
enforcing access control. Proof of knowledge of the password
authenticates the human to the computer.
Typically, these passwords are not stored on a user's computer for
security reasons and must be entered each time the human desires
network access. Therefore, the passwords must be ones that can be
repeatedly entered by a human with a low probability of error. They
will likely not possess high entropy and it may be assumed that an
adversary with access to a dictionary will have the ability to guess
a user's password. It is therefore desirable to have a robust
authentication method that is secure even when used with a weak
password in the presence of a strong adversary.
EAP-EKE is an EAP method [RFC3748] that addresses the problem of
password-based authenticated key exchange, using a possibly weak
password for authentication and to derive an authenticated and
cryptographically strong shared secret. This problem was first
described by Bellovin and Merritt in [BM92] and [BM93].
Subsequently, a number of other solution approaches have been
proposed, for example [JAB96], [LUC97], [BMP00], and others.
This proposal is based on the original Encrypted Key Exchange (EKE)
proposal, as described in [BM92]. Some of the variants of the
original EKE has been attacked, see e.g. [PA97], and improvements
have been proposed. None of these subsequent improvements have been
incorporated into the current protocol. However, we have used only
the subset of [BM92] (namely the variant described in Section 3.1)
which has withstood the test of time and is believed secure as of
this writing.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Protocol
3.1. Protocol Overview
EAP is a two-party protocol spoken between an EAP peer and an EAP
server (also known as "authenticator"). An EAP method defines the
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specific authentication protocol being used by EAP. This memo
defines a particular method and therefore defines the messages sent
between the EAP server and the EAP peer for the purpose of
authentication and key derivation.
3.2. Message Flows
EAP-EKE defines three message exchanges: an Identity exchange, a
Commit exchange and a Confirm exchange. A successful authentication
is shown in Figure 1.
The peer and server use the EAP-EKE Identity exchange to learn each
other's identities and to agree upon a ciphersuite to use in the
subsequent exchanges. In the Commit exchange the peer and server
exchange information to generate a shared key and also to bind each
other to a particular guess of the password. In the Confirm exchange
the peer and server prove liveness and knowledge of the password by
generating and verifying verification data.
+--------+ +--------+
| | EAP-EKE-ID/Request | |
| EAP |<------------------------------------| EAP |
| peer | | server |
| (P) | EAP-EKE-ID/Response | (S) |
| |------------------------------------>| |
| | | |
| | EAP-EKE-Commit/Request | |
| |<------------------------------------| |
| | | |
| | EAP-EKE-Commit/Response | |
| |------------------------------------>| |
| | | |
| | EAP-EKE-Confirm/Request | |
| |<------------------------------------| |
| | | |
| | EAP-EKE-Confirm/Response | |
| |------------------------------------>| |
| | | |
| | EAP-Success | |
| |<------------------------------------| |
+--------+ +--------+
Figure 1: A Successful EAP-EKE Exchange
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Schematically, the original exchange as described in [BM92] (and with
the roles reversed) is:
Server Peer
------ ----
E(Password, Y_S)) ->
<- E(Password, Y_P), E(SharedSecret, Nonce_P)
E(SharedSecret, Nonce_S | Nonce_P) ->
<- E(SharedSecret, Nonce_S)
Where:
o Y_S and Y_P are the server's (and respectively, the peer's)
ephemeral public key, i.e. g^x (mod p), g^y (mod p).
o Nonce_S, Nonce_P are random strings generated by the server and
the peer as cryptographic challenges.
o SharedSecret is the secret created by the Diffie-Hellman
algorithm, namely g^(x*y) (mod p).
The current protocol extends the basic cryptographic protocol, and
the regular successful exchange becomes:
EAP-EKE-ID/Request: S -> P
ID_S, CryptoProposals
EAP-EKE-ID/Response: S <- P
ID_P, CryptoSelection
EAP-EKE-Commit/Request: S -> P
E(Password, Y_S))
EAP-EKE-Commit/Response: S <- P
E(Password, Y_P), E(Ke, Nonce_P)
EAP-EKE-Confirm/Request: S -> P
E(Ke, Nonce_S | Nonce_P), Auth_S
EAP-EKE-Confirm/Response: S <- P
E(Ke, Nonce_S), Auth_P
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As shown in the exchange above, the following information elements
are added to the original protocol: identity values for both protocol
parties (ID_S, ID_P), negotiation of cryptographic protocols, and
signature fields to protect the integrity of the negotiated
parameters (Auth_S, Auth_P). In addition the shared secret is not
used directly. Note that a few details have been omitted for
clarity.
4. Packet Formats
4.1. EAP-EKE Header
The EAP-EKE header consists of the standard EAP header (see Section 4
of [RFC3748]), followed an EAP-EKE exchange type. The header has the
following structure:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | EKE-Exch | Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: EAP-EKE Header
The Code, Identifier, Length, and Type fields are all part of the EAP
header, and defined in [RFC3748]. The Type field in the EAP header
MUST be the value allocated by IANA for EAP-EKE version 1.
The EKE-Exch field identifies the type of EAP-EKE payload
encapsulated in the Data field. This document defines the following
values for the EKE-Exch field:
o 0x00: Reserved
o 0x01: EAP-EKE-ID exchange
o 0x02: EAP-EKE-Commit exchange
o 0x03: EAP-EKE-Confirm exchange
o 0x04: EAP-EKE-Failure message
o 0x05: EAP-EKE-Protected-Failure message
Further values of this EKE-Exch field are available via IANA
registration.
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4.2. EAP-EKE Payloads
EAP-EKE payloads all contain the EAP-EKE header and encoded
information, which differs for the different exchanges.
4.3. EAP-EKE-ID
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NumProposals | Reserved | Proposal ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... Proposal | IDType | Identity ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: EAP-EKE-ID Payload
The EAP-EKE-ID payload contains the following fields:
NumProposals:
The NumProposals field contains the number of proposals
subsequently contained in the Proposal field. In the EAP-EKE-ID/
Request the NumProposals field MUST NOT be set to zero (0) and in
the EAP-EKE-ID/Response message the NumProposals field MUST be set
to one (1). The offered proposals in the Request are listed
contiguously in priority order, most preferable first. The
selected proposal in the Response MUST be fully identical with one
of the offered proposals.
Proposal:
Each proposal consists of four one-octet fields, in this order:
Group Description:
This field's value is taken from the IANA registry for Diffie-
Hellman groups defined in Section 6.4.
Encryption:
This field's value is taken from the IANA registry for
encryption algorithms defined in Section 6.1.
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PRF:
This field's value is taken from the IANA registry for pseudo
random functions defined in Section 6.2.
MAC:
This field's value is taken from the IANA registry for keyed
message digest algorithms defined in Section 6.3 used to
provide integrity protection.
IDType
Denotes the Identity type. This is taken from the IANA registry
defined in Section 6. The server and the peer MAY use different
identity types.
The Identity field is always printable, and its meaning depends on
the values of the Code and IDType fields.
o EAP-EKE-ID/Request: server ID
o EAP-EKE-ID/Response: peer ID
The server SHOULD assert its own identity (e.g. its host name), or it
MAY use the peer's identity if it knows it before the protocol
starts.
The length of the Identity is computed from the Length field in the
EAP header.
4.4. EAP-EKE-Commit
In this exchange both parties send their encrypted ephemeral public
key, and the peer also includes a Challenge.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DHComponent ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Commit_P ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 4: EAP-EKE-Commit Payload
DHComponent:
This field contains the password-encrypted Diffie-Hellman public
key, see Section 5.2.
Commit_P:
This field only appears in the response, and contains the
encrypted challenge value sent by the peer. See Section 5.3.
4.5. EAP-EKE-Confirm
In this exchange both parties complete the authentication by
generating a shared temporary key, authenticating the entire
protocol, and generating key material for the EAP consumer protocol.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Confirm_? ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Auth_? ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: EAP-EKE-Confirm Payload
Confirm_?:
This field contains the encrypted response to the other peer's
challenge, see Section 5.4 and Section 5.5.
Auth_?
This field signs the Identity and the negotiated fields, to
prevent downgrade attacks. See Section 5.4 and Section 5.5.
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4.6. EAP-EKE-Failure and EAP-EKE-Protected-Failure
The EAP-EKE-Failure message format is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Failure-Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
EAP-EKE-Failure Payload
The EAP-EKE-Protected-Failure payload format is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Failure-Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
... MAC ...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
EAP-EKE-Protected-Failure Payload
The MAC field contains the keyed message digest computed with the MAC
algorithm selected during the initial exchange computed over the
Failure-Code using the MAC key (see Section 5 on how this key is
derived). A protected failure response can only be returned once the
MAC key has been derived.
Currently the following Failure-Code values are defined:
o 0x00000000: Reserved
o 0x00000001: No Error
o 0x00000002: Protocol Error
o 0x00000003: Password Not Found
o 0x00000004: Authentication Failure
o 0x00000005: Authorization Failure
o 0x00000006: No Proposal Chosen
Additional values of this field are available via IANA registration.
"No Error" is used for failure acknowledgement, see below. "Protocol
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Error" indicates a failure to parse or understand a protocol message
or one of its payloads. "Password Not Found" indicates a password
for a particular user could not be located, making authentication
impossible. "Authentication Failure" indicates failure in the
cryptographic computation most likely caused by an incorrect
password, or an inappropriate identity type. "Authorization Failure"
indicates that while the password being used is correct, the user is
not authorized to connect. "No Proposal Chosen" indicates that the
peer is unwilling to select any of the cryptographic proposals
offered by the server.
When the peer encounters an error situation, it MUST respond with
either EAP-EKE-Failure or EAP-EKE-Protected-Failure, depending on
whether it believes a common MAC key has been agreed upon. The
server MUST send an EAP-Failure message to end the exchange.
When the server encounters an error situation, it MUST respond with
either EAP-EKE-Failure or EAP-EKE-Protected-Failure, depending on
whether it believes a common MAC key has been agreed upon. The peer
MUST send back either EAP-EKE-Failure or EAP-EKE-Protected-Failure
(corresponding to the server's selection), containing a "No Error"
failure code. Then the server MUST send an EAP-Failure message to
end the exchange.
5. Cryptographic Operations
5.1. Generating Keying Material
Keying material will always be derived as the output of the
negotiated prf algorithm. Since the amount of keying material needed
may be greater than the size of the output of the prf algorithm, we
will use the prf iteratively. We will use the terminology prf+ to
describe the function that outputs a pseudo-random stream based on
the inputs to a prf as follows: (where | indicates concatenation)
prf+ (K,S) = T1 | T2 | T3 | T4 | ...
where:
T1 = prf(K, S | 0x01)
T2 = prf(K, T1 | S | 0x02)
T3 = prf(K, T2 | S | 0x03)
T4 = prf(K, T3 | S | 0x04)
continuing as needed to compute all required keys. The keys are
taken from the output string without regard to boundaries (e.g., if
the required keys are a 256-bit Advanced Encryption Standard (AES)
key and a 160-bit HMAC key, and the prf function generates 160 bits,
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the AES key will come from T1 and the beginning of T2, while the HMAC
key will come from the rest of T2 and the beginning of T3).
The constant concatenated to the end of each string feeding the prf
is a single octet. prf+ in this document is not defined beyond 255
times the size of the prf output.
5.2. EAP-EKE-Commit/Request
The server computes
Y_S = g^x mod N,
where 'x' is a randomly chosen number in the range 2 .. N-1. The
randomly chosen number is the private key, and the calculated field
is the corresponding public key. The calculated value MUST NOT be
zero modulo N. If the peer receives a bad value for this field, it
MUST take action to disconnect or disable the link. Each of the
peers MUST use a fresh, random value for this field on each run of
the protocol.
Note: If Elliptic Curve Diffie-Hellman is used, the corresponding
additive group operations are to be understood.
The server transmits
DHComponent_S = E(prf+(password, "EAP-EKE Password"), Y_S),
encrypted using the algorithm negotiated during the previous
exchange. If required by the encryption algorithm/mode, the
encrypted field is preceded by an Initialization Vector (IV), whose
length depends on the algorithm. The IV value MUST be unpredictable.
When using block ciphers, it may be necessary to pad Y_S on the
right, to fit the encryption algorithm's block size. In such cases,
random padding MUST be used, and this randomness is critical to the
security of the protocol. Randomness recommendations can be found in
[RFC4086]. When decrypting this field, the real length of Y_S is
determined according to the negotiated Diffie Hellman group.
If the password needs to be stored on the server, it is RECOMMENDED
to store the randomized password value, i.e. prf+(password, ...), as
a password-equivalent, rather than the cleartext password.
5.3. EAP-EKE-Commit/Response
The peer computes
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Y_P = g^y mod N
and sends
DHComponent_P = E(prf+(password, "EAP-EKE Password"), Y_P)
If the password is non-ASCII, it SHOULD be normalized by the peer
before the EAP-EKE message is constructed. The normalization method
is SASLprep, [RFC4013]. Note that the password is not null-
terminated.
Both sides calculate
SharedSecret = g^(x*y) mod N
The encryption key is computed:
Ke = prf+(SharedSecret, "EAP-EKE Ke" | ID_S | ID_P)
The MAC key is computed:
MAC = prf+(SharedSecret, "EAP-EKE MAC" | ID_S | ID_P)
And the peer generates
Challenge_P = E(Ke, Nonce_P),
where Nonce_P is a randomly generated binary string. Nonce_P has
length equal to the block size of E for block ciphers, or 32 octets
if E is a stream cipher.
5.4. EAP-EKE-Confirm/Request
The server sends:
Commit_S = E(Ke, Nonce_P | Nonce_S),
where Nonce_S is a randomly generated string, similar to Nonce_P.
It computes:
Ka = prf+(SharedSecret, "EAP-EKE Ka" | ID_S | ID_P | Nonce_P |
Nonce_S)
And sends:
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Auth_S = prf(Ka, "EAP-EKE server" | EAP-EKE-ID/Request | EAP-EKE-
ID/Response | EAP-EKE-Commit/Request | EAP-EKE-Commit/Response),
where the literal string is encoded using ASCII with no zero
terminator. The messages are included in full, starting with the EAP
header, and including any possible future extensions.
5.5. EAP-EKE-Confirm/Response
The peer computes Ka, and sends:
Commit_P = E(Ke, Nonce_S)
Auth_P = prf(Ka, "EAP-EKE peer" | EAP-EKE-ID/Request | EAP-EKE-ID/
Response | EAP-EKE-Commit/Request | EAP-EKE-Commit/Response)
5.6. MSK and EMSK
Following the last message of the protocol, both sides compute and
export the shared keys, each 512 bits in length:
MSK = prf+(SharedSecret, "EAP-EKE MSK" | ID_S | ID_P | Nonce_P |
Nonce_S)
EMSK = prf+(SharedSecret, "EAP-EKE EMSK" | ID_S | ID_P | Nonce_P |
Nonce_S)
5.7. Diffie-Hellman Groups
Many of the commonly used Diffie Hellman groups are inappropriate for
use in EKE. Most of these groups use a generator which is not a
primitive element of the group. As a result, an attacker running a
dictionary attack would be able to learn at least 1 bit of
information for each decrypted password guess.
Any MODP Diffie Hellman group defined for use in this protocol MUST
have the following properties, to ensure that it does not leak a
usable amount of information about the password:
1. The generator is a primitive element of the group.
2. The most significant 64 bits of the prime number are 1.
3. The group's order p is a "safe prime", i.e. (p-1)/2 is also
prime.
The last requirement is related to the strength of the Diffie Hellman
algorithm, rather than the password encryption. It also makes it
easy to verify that the generator is primitive.
We have defined the following groups:
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o DHGROUP_EKE_14 is defined by: the prime number of Group 14
[RFC3526], with the generator 11 (decimal).
o Additional groups will be defined by future versions of this
document.
5.8. Mandatory Algorithms
To facilitate interoperability, the following algorithms are
mandatory to implement:
o ENCR_AES128_CBC (encryption algorithm)
o PRF_HMAC_SHA1 (pseudo random function and keyed message digest)
o DHGROUP_EKE_14 (DH-group)
6. IANA Considerations
This document allocates an EAP method type, for "EAP-EKE Version 1".
This document requires IANA to create the registries described in the
subsequent sections. Values can be added or modified in these
registries per Specification Required [RFC5226].
6.1. Encryption Algorithm Registry
This section defines an IANA registry for encryption algorithms:
+-----------------+---------+----------------------------------+
| Name | Value | Definition |
+-----------------+---------+----------------------------------+
| Reserved | 0 | |
| ENCR_AES128_CBC | 1 | AES with a 128-bit key, CBC mode |
| | 2-127 | Available for allocation via IANA|
| | 128-255 | Reserved for private use |
+-----------------+---------+----------------------------------+
6.2. Pseudo Random Function Registry
This section defines an IANA registry for pseudo random function
algorithms:
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+---------------+---------+-------------------------------------+
| Name | Value | Definition |
+---------------+---------+-------------------------------------+
| Reserved | 0 | |
| PRF_HMAC_SHA1 | 1 | HMAC SHA-1, as defined in [RFC2104] |
| | 2-127 | Available for allocation via IANA |
| | 128-255 | Reserved for private use |
+---------------+---------+-------------------------------------+
6.3. Keyed Message Digest Registry
This section defines an IANA registry for keyed message digest
algorithms:
+---------------+---------+-------------------------------------+
| Name | Value | Definition |
+---------------+---------+-------------------------------------+
| Reserved | 0 | |
| PRF_HMAC_SHA1 | 1 | HMAC SHA-1, as defined in [RFC2104] |
| | 2-127 | Available for allocation via IANA |
| | 128-255 | Reserved for private use |
+---------------+---------+-------------------------------------+
6.4. Diffie-Hellman Group Registry
This section defines an IANA registry for Diffie-Hellman groups:
+----------------+---------+-------------------------------------------+
| Name | Value | Definition |
+----------------+---------+-------------------------------------------+
| Reserved | 0 | |
| DHGROUP_EKE_14 | 1 | 2048-bit MODP Group defined in this |
| | | document |
| | 2-127 | Available for allocation via IANA |
| | 128-255 | Reserved for private use |
+----------------+---------+-------------------------------------------+
6.5. Identity Type Registry
In addition, an identity type registry is defined:
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+-----------+---------+---------------------------------------------+
| Name | Value | Definition |
+-----------+---------+---------------------------------------------+
| Reserved | 0 | |
| ID_OPAQUE | 1 | A printable string whose format is |
| | | undefined |
| ID_NAI | 2 | A Network Access Identifier, as defined in |
| | | [RFC4282] (mandatory to implement) |
| ID_IPv4 | 3 | An IPv4 address, in binary format |
| ID_IPv6 | 4 | An IPv6 address, in binary format |
| ID_FQDN | 5 | A fully qualified domain name (mandatory to |
| | | implement) |
| | 6-127 | Available for allocation via IANA |
| | 128-255 | Reserved for private use |
+-----------+---------+---------------------------------------------+
6.6. Failure-Code Registry
This section defines an IANA registry for the Failure-Code registry,
a 32-bit long code. Initial values are defined in Section 4.6. All
values up to 0xff000000 are available for allocation via IANA. The
remaining values up to 0xffffffff are available for private use.
7. Security Considerations
Any protocol that claims to solve the problem of password-
authenticated key exchange must be resistant to active, passive and
dictionary attack and have the quality of forward secrecy. These
characteristics are discussed further in the following paragraphs.
Resistance to Passive Attack A passive attacker is one that merely
relays messages back and forth between the peer and server,
faithfully, and without modification. The contents of the
messages are available for inspection, but that is all. To
achieve resistance to passive attack, such an attacker must not be
able to obtain any information about the password or anything
about the resulting shared secret from watching repeated runs of
the protocol. Even if a passive attacker is able to learn the
password, she will not be able to determine any information about
the resulting secret shared by the peer and server.
Resistance to Active Attack An active attacker is able to modify,
add, delete, and replay messages sent between protocol
participants. For this protocol to be resistant to active attack,
the attacker must not be able to obtain any information about the
password or the shared secret by using any of its capabilities.
In addition, the attacker must not be able to fool a protocol
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participant into thinking that the protocol completed
successfully. It is always possible for an active attacker to
deny delivery of a message critical in completing the exchange.
This is no different than dropping all messages and is not an
attack against the protocol.
Resistance to Dictionary Attack For this protocol to be resistant to
dictionary attack any advantage an adversary can gain must be
directly related to the number of interactions she makes with an
honest protocol participant and not through computation. The
adversary will not be able to obtain any information about the
password except whether a single guess from a single protocol run
is correct or incorrect.
Forward Secrecy Compromise of the password must not provide any
information about the secrets generated by earlier runs of the
protocol.
[RFC3748] requires that documents describing new EAP methods clearly
articulate the security properties of the method. In addition, for
use with wireless LANs, [RFC4017] mandates and recommends several of
these. The claims are:
1. Mechanism: password.
2. Claims:
* Mutual authentication: the peer and server both authenticate
each other by proving possession of a shared password. This
is REQUIRED by [RFC4017].
* Forward secrecy: compromise of the password does not reveal
the secret keys (MSK and EMSK) from earlier runs of the
protocol.
* Replay protection: an attacker is unable to replay messages
from a previous exchange either to learn the password or a key
derived by the exchange. Similarly the attacker is unable to
induce either the peer or server to believe the exchange has
successfully completed when it hasn't.
* Key derivation: a shared secret is derived by performing a
group operation in a finite cyclic group (e.g. exponentiation)
using secret data contributed by both the peer and server. An
MSK and EMSK are derived from that shared secret. This is
REQUIRED by [RFC4017].
* Dictionary attack resistance: an attacker can only make one
password guess per active attack, and the protocol is designed
so that the attacker does not gain any confirmation of her
guess by observing the decrypted Y_x value (see below). The
advantage she can gain is through interaction not through
computation. This is REQUIRED by [RFC4017].
* Session independence: this protocol is resistant to active and
passive attacks and does not enable compromise of subsequent
or prior MSKs or EMSKs from either passive or active attacks.
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* Denial of Service resistance: it is possible for an attacker
to cause a server to allocate state and consume CPU. Such an
attack is gated, though, by the requirement that the attacker
first obtain connectivity through a lower-layer protocol (e.g.
802.11 authentication followed by 802.11 association, or 802.3
"link-up") and respond to two EAP messages--the EAP-ID/
Request and the EAP-EKE-ID/Request.
* Man-in-the-Middle Attack resistance: this exchange is
resistant to active attack, which is a requirement for
launching a man-in-the-middle attack. This is REQUIRED by
[RFC4017].
* Shared state equivalence: upon completion of EAP-EKE the peer
and server both agree on MSK, EMSK. The peer has
authenticated the server based on the Server_ID and the server
has authenticated the peer based on the Peer_ID. This is due
to the fact that Peer_ID, Server_ID, and the generated shared
secret are all combined to make the authentication element
which must be shared between the peer and server for the
exchange to complete. This is REQUIRED by [RFC4017].
* Fragmentation: this protocol does not define a technique for
fragmentation and reassembly.
* Resistance to "Denning-Sacco" attack: learning keys
distributed from an earlier run of the protocol, such as the
MSK or EMSK, will not help an adversary learn the password.
3. Key strength: the strength of the resulting key depends on the
finite cyclic group chosen. Sufficient key strength is REQUIRED
by [RFC4017].
4. Key hierarchy: MSKs and EMSKs are derived from the secret values
generated during the protocol run, using a negotiated pseudo-
random function.
5. Vulnerabilities (note that none of these are REQUIRED by
[RFC4017]):
* Protected ciphersuite negotiation: the ciphersuite proposal
made by the server is not protected from tampering by an
active attacker. However if a proposal was modified by an
active attacker it would result in a failure to confirm the
message sent by the other party, since the proposal is bound
by each side into its Confirm message, and the protocol would
fail as a result.
* Confidentiality: none of the messages sent in this protocol
are encrypted.
* Integrity protection: all messages in this protocol are
integrity protected.
* Channel binding: this protocol does not enable the exchange of
integrity-protected channel information that can be compared
with values communicated via out-of-band mechanisms.
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* Fast reconnect: this protocol does not provide a fast
reconnect capability.
* Cryptographic binding: this protocol is not a tunneled EAP
method and therefore has no cryptographic information to bind.
* Identity protection: the EAP-EKE-ID exchange is not protected.
An attacker will see the server's identity in the EAP-EKE-ID/
Request and see the peer's identity in EAP-EKE-ID/ Response.
7.1. Cryptographic Analysis
When analyzing the Commit exchange, it should be noted that the base
security assumptions are different from "normal" cryptology.
Normally, we assume that the key has strong security properties, and
that the data may have little. Here, we assume that the key has weak
security properties (the attacker may have a list of possible keys),
and hence we need to ensure that the data has strong properties
(indistinguishable from random). This difference may mean that
conventional wisdom in cryptology might not apply in this case. This
also imposes severe constraints on the protocol, e.g. the mandatory
use of random padding, and the need to define specific finite groups.
7.2. Diffie Hellman Group Considerations
It is fundamental to the dictionary attack resistance that the Diffie
Hellman public values Y_S and Y_P are indistinguishable from a random
string. If this condition is not met, then a passive attacker can do
trial-decryption of the encrypted DHComponent_P, DHComponent_S values
based on a password guess, and if they decrypt to a value which in
not a valid public value, they know that the password guess was
incorrect.
For MODP groups, Section 5.7 gives conditions on the group to make
sure that this criterion is met. For other groups (for example,
Elliptic Curve groups), some other means of ensuring this must be
employed. The standard way of expressing Elliptic Curve public
values does not meet this criterion, as a valid Elliptic Curve X
coordinate can be distinguished from a random string with probability
approximately 0.5.
A future version of this document might introduce a group
representation, and/or a slight modification of the password
encryption scheme, so that Elliptic Curve groups can be accommodated.
[BR02] presents several alternative solutions for this problem.
8. Acknowledgements
Much of this document was unashamedly picked from
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[I-D.harkins-emu-eap-pwd] and [I-D.ietf-pppext-eap-srp-03], and we
would like to acknowledge the authors of these documents: Dan
Harkins, Glen Zorn, James Carlson, Bernard Aboba and Henry Haverinen.
We would like to thank David Jacobson and Steve Bellovin for their
useful comments.
9. References
9.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, February 2005.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
9.2. Informative References
[BM92] Bellovin, S. and M. Merritt, "Encrypted Key Exchange:
Password-Based Protocols Secure Against Dictionary
Attacks", Proc. IEEE Symp. on Research in Security and
Privacy , May 1992.
[BM93] Bellovin, S. and M. Merritt, "Augmented Encrypted Key
Exchange: A Password-Based Protocol Secure against
Dictionary Attacks and Password File Compromise", Proc.
1st ACM Conference on Computer and Communication
Security , 1993.
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[BMP00] Boyko, V., MacKenzie, P., and S. Patel, "Provably Secure
Password Authenticated Key Exchange Using Diffie-Hellman",
Advances in Cryptology, EUROCRYPT 2000 , 2000.
[BR02] Black, J. and P. Rogaway, "Ciphers with Arbitrary Finite
Domains", Proc. of the RSA Cryptographer's Track (RSA CT
'02), LNCS 2271 , 2002.
[I-D.harkins-emu-eap-pwd]
Harkins, D. and G. Zorn, "EAP Authentication Using Only A
Password", draft-harkins-emu-eap-pwd-03 (work in
progress), November 2008.
[I-D.ietf-pppext-eap-srp-03]
Carlson, J., Aboba, B., and H. Haverinen, "EAP SRP-SHA1
Authentication Protocol", draft-ietf-pppext-eap-srp-03
(work in progress), July 2001.
[JAB96] Jablon, D., "Strong Password-Only Authenticated Key
Exchange", ACM Computer Communications Review Volume 1,
Issue 5, October 1996.
[LUC97] Lucks, S., "Open Key Exchange: How to Defeat Dictionary
Attacks Without Encrypting Public Keys", Proc. of the
Security Protocols Workshop LNCS 1361, 1997.
[PA97] Patel, S., "Number Theoretic Attacks On Secure Password
Schemes", Proceedings of the 1997 IEEE Symposium on
Security and Privacy , 1997.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC5114] Lepinski, M. and S. Kent, "Additional Diffie-Hellman
Groups for Use with IETF Standards", RFC 5114,
January 2008.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
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Appendix A. Change Log
A.1. -01
Revised following comments raised on the CFRG mailing list. In
particular, added security considerations and a new DH group
definition, to resolve the vulnerability in case the group's
generator is not a primitive element.
A.2. -00
Initial version.
Appendix B. Design Options
B.1. Number of Round Trips
We have looked at three options: 2 round trips, 3 round trips, and a
normal run of 2 round trips with an optional third. Some of the
decision factors include:
o Performance (latency).
o Crypto-agility, the ability to negotiate cryptographic algorithms.
Ideally this applies to both the symmetric and asymmetric
algorithms.
o Complexity of the protocol state machine, when some messages are
optional.
o Dependence on a higher-level protocol sending the peer's identity
before EAP-EKE starts, so that the correct password can be used.
The initial version of this protocol has 3 round trips, primarily for
simplicity.
B.2. Fragmentation
While similar documents ( [I-D.harkins-emu-eap-pwd]) provide
fragmentation support at the level of the EAP method, we have decided
that such support is unnecessary given the expected size of messages
in EAP-EKE.
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Authors' Addresses
Yaron Sheffer
Check Point Software Technologies Ltd.
5 Hasolelim St.
Tel Aviv 67897
Israel
Email: yaronf@checkpoint.com
Glen Zorn
Network Zen
1310 East Thomas Street
#306
Seattle, Washington 98102
USA
Phone: +1 (206) 377-9035
Email: gwz@net-zen.net
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Scott Fluhrer
Cisco Systems.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Email: sfluhrer@cisco.com
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