Re: [Curdle] AD Review of draft-ietf-curdle-gss-keyex-sha2-05

Eric Rescorla <ekr@rtfm.com> Sat, 07 April 2018 03:03 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 6 Apr 2018 20:03:15 -0700
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To: denis bider <denisbider.ietf@gmail.com>
Cc: draft-ietf-curdle-gss-keyex-sha2@tools.ietf.org, curdle <curdle@ietf.org>
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Subject: Re: [Curdle] AD Review of draft-ietf-curdle-gss-keyex-sha2-05
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On Fri, Apr 6, 2018 at 7:52 PM, denis bider <denisbider.ietf@gmail.com>
wrote:

> Eric:
>
> I'm not an author of this draft, but I can respond with respect to the
> following:
>
>
> > > | gss-group14-sha256-*     | SHOULD/RECOMMENDED
> > > | gss-group15-sha512-*     | MAY/OPTIONAL
> > > | gss-group16-sha512-*     | SHOULD/RECOMMENDED
> >
> > Why are you only specifying SHA-512 with 4096-bit groups.
> > SHA-256 is still reasonable at that size?
>
> There exist NSA recommendations aimed at "organizations that run
> classified or unclassified national security systems (NSS) and vendors that
> build products used in NSS."
>
> https://cryptome.org/2016/01/CNSA-Suite-and-Quantum-Computing-FAQ.pdf
>
> These recommendations cover a usage case for software that implements the
> above algorithms. These recommendations call for the following minimums:
>
> - Diffie Hellman: 3072-bit or larger
>
> - Hashing: SHA-384 or larger
>
> These recommendations are most effectively met by associating group15 and
> group16 with SHA-512.
>
> Otherwise, products that wanted to meet these recommendations would have
> to use much larger and more expensive DH groups in order to meet the
> SHA-384-or-better requirement.
>

My question is why these groups are only specified with SHA-512, and not
SHA-256. I think it's fine to specify them for SHA-512.


-Ekr


> On Fri, Apr 6, 2018 at 6:24 PM, Eric Rescorla <ekr@rtfm.com> wrote:
>
>> Rich version of this review at:
>> https://mozphab-ietf.devsvcdev.mozaws.net/D4544
>>
>> This document has a huge amount of duplicated material which makes it
>> very hard to read. Please refactor so that the common material is in
>> one place.
>>
>>
>>
>>
>> COMMENTS
>> >      Due to security concerns with SHA-1 [RFC6194] and with MODP groups
>> >      with less than 2048 bits [NIST-SP-800-131Ar1]  we propose the use
>> of
>> >      the SHA-2 based hashes with DH group14, group15, group16, group17
>> and
>> >      group18 [RFC3526].  Additionally we add support for key exchange
>> >      based on Elliptic Curve Diffie Hellman with NIST P-256, P-384 and
>> >      P-521 as well as X25519 and X448 curves.  Following the rationale
>> of
>>
>> "the X25519..."
>>
>>
>> >          +--------------------------+--------------------------------+
>> >          | Key Exchange Method Name | Implementation Recommendations |
>> >          +--------------------------+--------------------------------+
>> >          | gss-group14-sha256-*     | SHOULD/RECOMMENDED             |
>> >          | gss-group15-sha512-*     | MAY/OPTIONAL                   |
>> >          | gss-group16-sha512-*     | SHOULD/RECOMMENDED             |
>>
>> Why are you only specifying SHA-512 with 4096-bit groups. SHA-256 is
>> still reasonable at that size?
>>
>>
>> >
>> >      Each of these methods specifies GSS-API-authenticated
>> Diffie-Hellman
>> >      key exchange as described in Section 2.1 of [RFC4462]  with SHA-256
>> >      as HASH, and the group defined in Section 8.2 of [RFC4253] The
>> method
>> >      name for each method is the concatenation of the string "gss-
>> >      group14-sha256-" with the Base64 encoding of the MD5 hash [RFC1321]
>>
>> Why is this MD5? Is there some legacy reason for this? It's not
>> necessarily bad but it's odd to modern eyes.
>>
>>
>> >      Each of these methods specifies GSS-API-authenticated
>> Diffie-Hellman
>> >      key exchange as described in Section 2.1 of [RFC4462]  with SHA-512
>> >      as HASH, and the group defined in Section 7 of [RFC3526] The method
>> >      name for each method is the concatenation of the string "gss-
>> >      group18-sha512-" with the Base64 encoding of the MD5 hash of the
>> >      ASN.1 DER encoding of the underlying GSS-API mechanism's OID.
>>
>> These all seem to be boilerplate. is there a way to refactor into a
>> single paragraph with a table that describes the substitutions?
>>
>>
>> >      ASN.1 DER encoding of the underlying GSS-API mechanism's OID.
>> >
>> >   5.  New Elliptic Curve Diffie-Hellman Key Exchange methods
>> >
>> >      In [RFC5656] new SSH key exchange algorithms based on Elliptic
>> Curve
>> >      Cryptography are introduced.  We reuse much of section 4 to
>> implement
>>
>> s/implement/define/
>>
>>
>> >      This section defers to [RFC7546] as the source of information on
>> GSS-
>> >      API context establishment operations, Section 3 being the most
>> >      relevant.  All Security Considerations described in [RFC7546] apply
>> >      here too.
>> >
>> >      The Client:
>>
>> This section should be refactored to put all the EC mechanics (which
>> are symmetrical) in one place.
>>
>>
>> >            and then y coordinate.  The coordinate coversion MUST
>> preserve
>> >            leading zero octets.  Thus for nistp521 curve the encoded x
>> >            coordinate will always have a length of 66 octets while the
>> Q_C
>> >            representation will be 133 octets long.  This is the
>> >            uncompressed representation specified in Section 4.3.6 of
>> >            [ANSI-X9-62-2005].
>>
>> I have two questions about this:  1. Why are you specifying the
>> detailed computation of the public key? This seems like you could
>> defer it to another spec. 2. Why are you specifying uncompressed
>> representations for NIST curves? We did this in TLS because people
>> already supported them, but in general they are worse. Is there a
>> reason here?
>>
>>
>> >            by 31 zero octets for curve255519 and as an octect of value
>> >            0x05 followed by 55 zero octets.
>> >
>> >            Calculating Q_C as the result of the call to X25519 or X448
>> >            function, respectively for curve25519 and curve448 key
>> >            exchange, with parameters d_C and g.
>>
>> This material all seems to be in RFC 7748 S 6.1 and 6.2.
>>
>>
>> >
>> >         For NIST curves, the server verifies that the q_C is not a point
>> >         at infinity, that both coordinates are in the interval [0, p -
>> 1],
>> >         where p is the prime associated with the curve of the selected
>> key
>> >         exchange and that the point lies on the curve (satisfies the
>> curve
>> >         equation).
>>
>> You should probably cite to the X9.62 or SP-800 for this procedure.
>>
>>
>> >         For curve25519, the server verifies that the the high-order bit
>> of
>> >         the last octet is not set - this prevents distinguishing attacks
>> >         between implementations that use Montgomery ladder
>> implementation
>> >         of the curve and ones that use generic elliptic-curve libraries.
>> >         If the bit is set, the key exchange SHOULD fail.  For curve448
>> any
>> >         bit can be set.
>>
>> I'm not following what this is supposed to do. If you are worried
>> about this, why don't you just mask off the top bit.
>>
>>
>> >            For NIST curves, the peers perform point multiplication using
>> >            d_U and q_V to get point P.
>> >
>> >            For NIST curves, peers verify that P is not a point at
>> >            infinity.  If P is a point at infinity, the key exchange MUST
>> >            fail.
>>
>> Why is this text here? It describes the client's behavior.
>>
>>
>> >            and q_V.  The result of the function is the shared secret.
>> >
>> >            For curve25519 and curve448, if all the octets of the shared
>> >            secret are zero octets, the key exchange MUST fail.
>> >
>> >         H = hash(V_C || V_S || I_C || I_S || K_S || Q_C || Q_S || K).
>>
>> This kind of just comes out of nowhere. You probably want some
>> prefatory text.
>>
>>
>> >
>> >      7.  C verifies that the key Q_S is valid the same way it is done in
>> >      step 3.  If the key is not valid the key exchange MUST fail.
>> >
>> >      8.  C computes the shared secret K and H and verifies that it is
>> >      valid the same way it is done in step 5.  It then calls
>>
>> This check only applies to CFRG curves.
>>
>>
>> >          string    server public host key and certificates (K_S)
>> >
>> >      Since this key exchange method does not require the host key to be
>> >      used for any encryption operations, this message is OPTIONAL.  If
>> the
>> >      "null" host key algorithm described in Section 5 of [RFC4462] is
>> >      used, this message MUST NOT be sent.
>>
>> I am assuming in this situation there is some other form of
>> authentication?
>>
>>
>> >          string    I_C, payload of the client's SSH_MSG_KEXINIT
>> >          string    I_S, payload of the server's SSH_MSG_KEXINIT
>> >          string    K_S, server's public host key
>> >          string    Q_C, client's ephemeral public key octet string
>> >          string    Q_S, server's ephemeral public key octet string
>> >          mpint     K,   shared secret
>>
>> The actual equation is way up above this in the document, which is
>> presumably not great.
>>
>>
>> >      Each key exchange method is implicitly registered by this document.
>> >      The IESG is considered to be the owner of all these key exchange
>> >      methods; this does NOT imply that the IESG is considered to be the
>> >      owner of the underlying GSS-API mechanism.
>> >
>> >   5.2.1.  gss-nistp256-sha256-*
>>
>> Again, can you refactor this section so it's not so duplicative.
>>
>>
>> >      the target the user intended..  Some mechanisms implementations
>> (like
>> >      commonly used krb5 libraries) may use insecure DNS resolution to
>> >      canonicalize the target name; in these cases spoofing a DNS
>> response
>> >      that points to an attacker-controlled machine may results in the
>> user
>> >      silently delegating credentials to the attacker, who can then
>> >      impersonate the user at will.
>>
>> Is this something new in this document?
>>
>>
>> _______________________________________________
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>>
>>
>