[IPsec] PAKE selection: SPSK

["Dan Harkins" <[e-mail]>]

  Hi,

  To kick off the PAKE selection discussion I'd like to tout one of
the candidate protocols. It is based on the dragonfly key exchange
and is described in draft-harkins-ipsecme-spsk-auth. Variants of
dragonfly have been proposed for IEEE 802.11 and as an EAP method.

  The aforementioned draft contains a complete description of how
to add dragonfly to IKE(v2), including the additional payloads
needed, their construction and processing, their format, and how they
fit into IKE(v2). While complete it isn't rigid; things (like the way
this exchange is indicated) can change without affecting security.

  I think the best way to start the discussion is to run through the
enumerated selection criteria from draft-harkins-ipsecme-pake-criteria.

  SEC1: dragonfly is based on a zero knowledge proof. It is resistant
    to passive, active, and (off-line) dictionary attack.
  SEC2: the PFS characteristics of IKE and its resistance to the
    Denning-Sacco attack are unchanged by inclusion of this authentication
    method. Dragonfly generates a shared key that has the properties of
    PFS and it is resistant to the Denning-Sacco attack but the generated
    key is used only for authentication.
  SEC3: the dragonfly exchange happens after IKE(v2)'s Diffie-Hellman
    key exchange and the subsequent payloads, including the ID payloads,
    are protected by a derivative of the resulting key. ID protection is
    not affected.
  SEC4: the protocol does not effect the "crypto agility" of IKE(v2). It
    uses the same Diffie-Hellman group that IKE(v2) uses, uses the same
    prf+() construct, and can use the negotiated MAC in HMAC form. No
    additional primitives (special modes of encryption, for example) are
    needed.
  SEC5: dragonfly has received cryptographic review in its other
    instantiations (IEEE 802.11 and EAP) and is secure. A proof of
    security in the Random Oracle model is underway but has not been
    finished.
  SEC6: the way dragonfly has been added to IKE(v2) is for authentication
    of the exchange in which it resides only. Since dragonfly is key
    generating though, it is possible to use it to parlay the weak secret
    into a long-term credential.
  SEC7: a secret element, based on the password, in the selected group
    is used in the exchange. That element can be stored instead of the
    password to resist exposure of the password in the case of compromise.

  IPR1: dragonfly was first publicly disclosed in January of 2008.
  IPR2: dragonfly was invented by me but I did not patent it nor did
    my employer at the time of invention. It is believed to be patent-free.
  IPR3: no IPR disclosure to the IETF has been made because one is not
    needed.

  MISC1: dragonfly adds one roundtrip to IKEv2 and to IKEv1's Main Mode.
    It turns IKEv1's Aggressive Mode from a three message exchange to
    a four message exchange.
  MISC2: dragonfly requires three "scalar operations", point multiplication
    for ECC or modular exponentiation for MODP. Basically, a "Diffie-
    Hellman and a half".
  MISC3: the nature of the shared secret (password, passphrase, long
    random octet string) does not affect performance.
  MISC4: internationalization can be supported as evidenced by the EAP
    method employing the dragonfly exchange.
  MISC5: dragonfly can use any of the ECP and MODP groups defined in the
    IANA registry for IKE(v2). EC2N groups are disallowed because of
    patent concerns and a desire to make implementation of dragonfly be
    license free. There is no technical reason, though, why EC2N groups
    couldn't be used.
  MISC6: the protocol fits nicely into IKE(v2)'s request/response nature.
    One single request and response is added and additional information
    is piggy-backed on the final request and response in IKE(v2) already.
  MISC7: no additional negotiation is needed: no new groups, no new
    ciphers, no new cipher modes, no key wrapping.
  MISC8: dragonfly does not require trusted third parties nor does it
    require any clock synchronization.
  MISC9: dragonfly requires use of an HMAC instantiation of a hash
    algorithm. The current specification uses this as a "random function".
    Another instantiation of dragonfly uses an HMAC'd hash function in
    "extractor" and "expander" roles, effectively defining two roles for
    the single random function. This can easily be done for IKE(v2) too.
  MISC10: the protocol supports the use of elliptic curve cryptography.
  MISC11: dragonfly is easily implemented. A reference implementation of
    the IEEE 802.11 variant is a Source Forge project named "authsae".
    It requires APIs for ECC and MODP operations found in most modern
    crypto libraries (e.g. OpenSSL).

  Dragonfly is easy to implement, license/royalty/patent free, has
modest overhead, and fits easily into IKE(v2). I believe the current
manner in which it has been added to IKE(v2) is fine but it can be
changed if implementation concerns require it. Dragonfly is secure and
has received cryptographic review. It does not require any additional
group definitions and, aside from an HMAC'd hash algorithm, does not
require additional use of any other cryptographic primitives like
ciphers. It does not require any specific cipher modes (e.g. CBC or ECB)
nor does it prohibit any (e.g. a stream cipher or a block cipher being
used as a stream cipher). It is completely "crypto agile". I believe it is
the best candidate to meet the requirements laid out in the new charter.

  I solicit comments and questions on this proposal.

  regards,

  Dan.