HUSH: Using HUmanly memorable SHared secrets with IKEv2
draft-sheffer-ipsecme-hush-00

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.

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Table of Contents

1.  Introduction
2.  Terminology
3.  Overview
4.  Protocol Sequence
    4.1.  The IKE_SA_INIT Exchange
    4.2.  The IKE_HUSH Exchange
        4.2.1.  Message #1
        4.2.2.  Message #2
        4.2.3.  Message #3
        4.2.4.  Message #4
5.  Protocol Formats
    5.1.  Encrypt Payload
    5.2.  Protect Payload
6.  Cryptographic Details
    6.1.  Diffie-Hellman Groups
7.  IANA Considerations
8.  Security Considerations
9.  Acknowledgements
10.  References
    10.1.  Normative References
    10.2.  Informative References
Appendix A.  Change Log
    A.1.  -00
§  Authors' Addresses

1.  Introduction

There is strong interest in a simple method for bootstrapping an IKE [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) 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] (Bellovin, S. and M. Merritt, “Encrypted Key Exchange: Password-Based Protocols Secure Against Dictionary Attacks,” May 1992.) into IKEv2, to provide password-based authentication. Some of the benefits of this protocol are:

• EKE is a well known protocol, which has had multiple deep cryptographic analyses applied to it.
• 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:

• Integration with other IKE elements, e.g. optional notifications, Session Resumption...
• Error handling.
• Generation of a high-quality PSK, so that the password doesn't need to be used for each authentication. Secure signalling of PSK possession.
• 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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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 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:

• The regular IKE_SA_INIT exchange is followed by a new 2-round trip exchange, IKE_HUSH.
• Negotiation of the new mode using notifications in IKE_SA_INIT.
• 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.
• A new encrypted payload type, denoted Encrypt{}, for exchanging an ephemeral public key encrypted by the password.
• A new encrypted and integrity-protected payload type, denoted Protect{}, for exchanging nonces encrypted by the EKE shared secret.
• 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.

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 (Diffie-Hellman Groups). 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 Payload)),

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] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). 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 of the protocol. Randomness recommendations can be found in [RFC4086] (Eastlake, D., Schiller, J., and S. Crocker, “Randomness Requirements for Security,” June 2005.).

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] (Zeilenga, K., “SASLprep: Stringprep Profile for User Names and Passwords,” February 2005.). 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 [I‑D.krawczyk‑hkdf] (Krawczyk, H. and P. Eronen, “HMAC-based Extract-and-Expand Key Derivation Function (HKDF),” January 2010.).

The responder computes the encryption key:

Ke2 = prf+(EkeSharedSecret, "HUSH encryption" | IDi | IDr)

and the authetication (integrity protection) key:

Ka2 = prf+(EkeSharedSecret, "HUSH 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.

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:

                        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)            ~
   ~                                                               ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

• Data Length is a 2 octet length of the encrypted data, exclusive of padding. This field itself is unencrypted.
• 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 (Encrypt Payload)) with the addition of an integrity-protection ICV field. The payload is denoted by Protect(enc-key, integ-key, data) and defined as follows:

                        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 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 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.

7.  IANA Considerations

TBD: one notification, one transform type, two payloads.

8.  Security Considerations

Will be added.

9.  Acknowledgements

Much of this protocol is derived from [I‑D.sheffer‑emu‑eap‑eke] (Sheffer, Y., Zorn, G., Tschofenig, H., and S. Fluhrer, “An EAP Authentication Method Based on the EKE Protocol,” April 2010.), and authors (and reviewers) of that draft are acknowledged.

10.  References

10.1. Normative References

10.2. Informative References

Appendix A.  Change Log

Note to RFC Editor: please remove this section before publication.

A.1.  -00

Initial version, a very rough draft.

Authors' Addresses