Network Working Group Y. Sheffer
Internet-Draft Independent
Intended status: Informational S. Fluhrer
Expires: September 10, 2011 Cisco
March 9, 2011
HUSH: Using HUmanly memorable SHared secrets with IKEv2
draft-sheffer-ipsecme-hush-02
Abstract
This document defines a new mode for IKEv2, where both peers can
authenticate using a short, humanly memorable shared secret. This
mode is based on the EKE protocol.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 4
4.1. The IKE_SA_INIT Exchange . . . . . . . . . . . . . . . . . 5
4.2. The IKE_HUSH Exchange . . . . . . . . . . . . . . . . . . 5
4.2.1. Message #1 . . . . . . . . . . . . . . . . . . . . . . 5
4.2.2. Message #2 . . . . . . . . . . . . . . . . . . . . . . 6
4.2.3. Message #3 . . . . . . . . . . . . . . . . . . . . . . 7
4.2.4. Message #4 . . . . . . . . . . . . . . . . . . . . . . 7
5. Protocol Formats . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Encrypt Payload . . . . . . . . . . . . . . . . . . . . . 7
5.2. Protect Payload . . . . . . . . . . . . . . . . . . . . . 8
6. Cryptographic Details . . . . . . . . . . . . . . . . . . . . 9
6.1. Diffie-Hellman Groups . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 12
A.1. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
A.2. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
There is strong interest in a simple method for bootstrapping an IKE
[RFC4306] security association between two peers, requiring neither
PKI nor AAA infrastructure. Although IKEv2 supports EAP-based
authentication in part to provide for this capability, it has been
claimed that the use of an extra authentication layer/protocol adds
little benefit and increases complexity.
This protocol integrates the well known EKE protocol [BM92] into
IKEv2, to provide password-based authentication. Some of the
benefits of this protocol are:
o EKE is a well known protocol, which has had multiple deep
cryptographic analyses applied to it.
o EKE provides the benefit of a well known, clear IPR status.
This protocol is not intended for use in enterprise-scale remote
access. As a result, only the basic authentication capability is
provided. Some capabilities typically associated with the use of
passwords for remote access include: password change and expiry,
password recovery, and enforcement of password strength policy.
In this preliminary version of the protocol many issues are not yet
covered, such as:
o Integration with other IKE elements, e.g. optional notifications,
Session Resumption...
o Error handling.
o Generation of a high-quality PSK, so that the password doesn't
need to be used for each authentication. Secure signalling of PSK
possession.
o Security analysis.
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. Overview
This protocol attempts to preserve the general structure of IKEv2,
minimizing the number of new constructs and round trips, while
retaining IKE's security guarantees, including identity protection.
The resulting protocol only adds one round trip to the shared secret
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authentication mode of IKEv2.
At a high level, the message exchange is as follows:
Initiator Responder
--------- ---------
HDR, SAi1, KEi, Ni, N(HUSH)
<- HDR, SAr1, KEr, Nr, N(HUSH)
HDR, SK{IDi, [IDr,], SAi2, TSi, TSr, Encrypt{Yi}} ->
<- HDR, SK{IDr, Encrypt{Yr}, Protect{Nr2}}
HDR, SK{AUTH, Protect{Ni2|Nr2}}
<- HDR, SK{AUTH, SAr2, TSi, TSr, Protect{Ni2}}
The changes to IKEv2 are summarized in the following list:
o The regular IKE_SA_INIT exchange is followed by a new 2-round trip
exchange, IKE_HUSH.
o Negotiation of the new mode using notifications in IKE_SA_INIT.
o Negotiation of cryptographic algorithms: the encryption algorithm,
the integrity protection algorithm and the pseudo-random function
are the ones negotiated for the IKE SA itself, while a new Diffie-
Hellman group is selected for the HUSH exchange, by extending the
SAi1/SAr1 negotiation.
o A new encrypted payload type, denoted Encrypt{}, for exchanging an
ephemeral public key encrypted by the password.
o A new encrypted and integrity-protected payload type, denoted
Protect{}, for exchanging nonces encrypted by the EKE shared
secret.
o The IKE AUTH payloads provide cryptographic binding of the IKE
shared secret with the password-based authentication.
4. Protocol Sequence
The protocol consists of a slightly extended IKE_SA_INIT exchange,
followed by the 4-message IKE_HUSH exchange.
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4.1. The IKE_SA_INIT Exchange
During this exchange, the initiator sends an empty HUSH_SUPPORTED
notification. If the responder understands this protocol and wishes
to use it, it sends back another empty HUSH_SUPPORTED notification.
In addition, this protocol defines a new transform type for the IKE
protocol, called "EKE D-H Group". Possible transforms are the EKE
groups defined in Section 6.1. This transform type is negotiated
between the initiator and responder with the usual SAi1, SAr1
payloads. If the initiator suspects that the responder does not
support this protocol, it SHOULD also include a proposal that omits
this transform, to allow the negotiation to revert to regular IKE.
During successful negotiation, an EKE D-H Group MUST be negotiated if
(and only if) the responder indicates support for this protocol.
4.2. The IKE_HUSH Exchange
This exchange consists of two message pairs, and includes all
payloads normally contained in the IKE_AUTH exchange. These latter
payloads are not described in this subsection, in order to focus on
the new HUSH payloads.
4.2.1. Message #1
The initiator computes
Yi = g^x mod N,
where x is a randomly chosen number in the range 2 .. N-1, as
defined by the negotiated Diffie-Hellman group. The randomly chosen
number is the private key, and the calculated value is the
corresponding public key. Each of the peers MUST use a fresh, random
value for x on each run of the protocol.
Note: If Elliptic Curve Diffie-Hellman is used in a future version of
this protocol, the corresponding additive group operations are to be
understood.
The initiator generates the Encrypt payload (Section 5.1),
Encrypt(prf+(password, "HUSH Password"), Yi),
where the literal string is encoded using ASCII with no zero
terminator. The prf+ notation is as defined in [RFC4306]. When
using block ciphers, it may be necessary to pad Yi 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
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of the protocol. Randomness recommendations can be found in
[RFC4086].
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.
If the password is non-ASCII, it SHOULD be normalized by the sender
before the message is constructed. The normalization method is
SASLprep, [RFC4013]. Note that the password is not null-terminated.
4.2.2. Message #2
Similarly to Message #1, the responder picks a random private key,
generates an ephemeral public key Yr, encrypts it by the expanded
password and includes the resulting Encrypt payload in the message:
Encrypt(prf+(password, "HUSH Password"), Yr),
The responder now calculates
EkeSharedSecret = prf(0+, g^(x*y) mod N)
where the first argument to "prf" is a string of zero octets whose
length is the output size of the base hash algorithm, e.g. 20 octets
for HMAC-SHA1; the result is of the same length. This extra
application of the pseudo-random function is the "extraction step" of
[RFC5869].
The responder computes the encryption and authentication (integrity
protection) keys:
Ke2, Ka2 = prf+(EkeSharedSecret, "HUSH encryption and
authentication" | IDi | IDr)
Now the responder can generate the Protect payload included in the
message:
Protect(Ke2, Ka2, Nr2),
where Nr2 is a randomly generated binary string (nonce). Nr2 has
length equal to the block size of the negotiated encryption algorithm
for block ciphers, or 32 octets if this algorithm is a stream cipher.
The responder sends this value as an Encrypt payload.
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4.2.3. Message #3
The initiator computes the EkeSharedSecret, Ke2 and Ka2 values as
above.
It then picks a random nonce Ni2, of the same format as Nr2,
concatenates the two nonces, and generates
Protect(Ke2, Ka2, Ni2 | Nr2),
In addition, it computes
AUTH = prf(prf(Shared Secret, Ni2 | Nr2 | IDi | IDr),
<InitiatorSignedOctets>)
where the Shared Secret is the regular IKE shared secret, created by
the IKE_SA_INIT exchange.
4.2.4. Message #4
The responder verifies Nr2 and the received AUTH payload, and MUST
terminate the protocol if either of them fails to verify. The
responder generates
Protect(Ke2, Ka2, Ni2)
and
AUTH = prf(prf(Shared Secret, Ni2 | Nr2 | IDi | IDr),
<ResponderSignedOctets>)
The initiator MUST verify the Ni2 and AUTH values when receiving
Message #4.
5. Protocol Formats
5.1. Encrypt Payload
This payload contains encrypted, but non-integrity protected, data.
Unfortunately the simpler term "Encrypted Payload" is used by IKEv2
for a payload that contains encrypted and integrity-protected data.
Compared to the IKE Encrypted Payload, this payload does not contain
other embedded payloads. The payload is denoted Encrypt(key, data),
and defined thus:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| (length is block size for encryption algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Length | ~
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| Encrypted Data ~
~ ~
~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ | ~
+-+-+-+-+-+-+-+-+ Random Padding (0-255 octets) ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Encrypt Payload
o Data Length is a 2 octet length of the encrypted data, exclusive
of padding. This field itself is unencrypted.
o Random Padding MUST indeed be random and unpredictable if it is
included. This randomness is critical to the security of the
protocol.
5.2. Protect Payload
This payload contains encrypted and integrity protected data.
This payload is identical to the Encrypt Payload (Section 5.1) with
the addition of an integrity-protection ICV field. The payload is
denoted by Protect(enc-key, integ-key, data) and defined as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| (length is block size for encryption algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Length | ~
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| Encrypted Data ~
~ ~
~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ | ~
+-+-+-+-+-+-+-+-+ Random Padding (0-255 octets) ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integrity Check Value (ICV) |
| (length depends on integrity algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Protect Payload
o The Integrity Check Value is computed over the Initialization
Vector, Data Length, Encrypted Data and Random Padding (if any).
The length of the field is determined by the negotiated integrity
algorithm.
6. Cryptographic Details
6.1. 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
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algorithm, rather than the password encryption. It also makes it
easy to verify that the generator is primitive.
We have defined the following groups. The Value column is used when
negotiating the group. Additional groups may be defined through IANA
allocation. Future non-MODP groups require a document to define
their interaction with this protocol.
+----------------+--------+---------+-------------------------------+
| Name | Length | Value | Description |
+----------------+--------+---------+-------------------------------+
| Reserved | | 0 | |
| DHGROUP_EKE_2 | 1024 | 1 | The prime number of Group 2 |
| | | | [RFC4306], with the generator |
| | | | 5 (decimal) |
| DHGROUP_EKE_5 | 1536 | 2 | The prime number of Group 5 |
| | | | [RFC3526], g=31 |
| DHGROUP_EKE_14 | 2048 | 3 | The prime number of Group 14 |
| | | | [RFC3526], g=11 |
| DHGROUP_EKE_15 | 3072 | 4 | The prime number of Group 15 |
| | | | [RFC3526], g=5 |
| DHGROUP_EKE_16 | 4096 | 5 | The prime number of Group 16 |
| | | | [RFC3526], g=5 |
| Available for | | 6-127 | |
| allocation via | | | |
| IANA | | | |
| Reserved for | | 128-255 | |
| private use | | | |
+----------------+--------+---------+-------------------------------+
7. IANA Considerations
TBD: one notification, one transform type, two payloads, a new
exchange. Also a new DH group registry.
8. Security Considerations
Will be added.
9. Acknowledgements
Much of this protocol is derived from [I-D.sheffer-emu-eap-eke], and
authors (and reviewers) of that draft are acknowledged.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
10.2. 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.
[BMP00] Boyko, V., MacKenzie, P., and S. Patel, "Provably Secure
Password Authenticated Key Exchange Using Diffie-Hellman",
Advances in Cryptology, EUROCRYPT 2000 , 2000.
[I-D.sheffer-emu-eap-eke]
Sheffer, Y., Zorn, G., Tschofenig, H., and S. Fluhrer, "An
EAP Authentication Method Based on the EKE Protocol",
draft-sheffer-emu-eap-eke-09 (work in progress),
October 2010.
[PA97] Patel, S., "Number Theoretic Attacks On Secure Password
Schemes", Proceedings of the 1997 IEEE Symposium on
Security and Privacy , 1997.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, February 2005.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
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Key Derivation Function (HKDF)", RFC 5869, May 2010.
Appendix A. Change Log
Note to RFC Editor: please remove this section before publication.
A.1. -01
Reissued, changed the derivation of the payload encryption and
authentication keys.
A.2. -00
Initial version, a very rough draft.
Authors' Addresses
Yaron Sheffer
Independent
Email: yaronf.ietf@gmail.com
Scott Fluhrer
Cisco Systems.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Email: sfluhrer@cisco.com
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