Re: Review of draft-ecc-09

Marko Kreen <markokr@gmail.com> Tue, 21 February 2012 09:49 UTC

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Date: Tue, 21 Feb 2012 11:49:01 +0200
From: Marko Kreen <markokr@gmail.com>
To: Andrey Jivsov <openpgp@brainhub.org>
Cc: ietf-openpgp@imc.org
Subject: Re: Review of draft-ecc-09
Message-ID: <20120221094901.GA27705@gmail.com>
References: <20120218215115.GA18978@gmail.com> <4F4346B7.1040804@brainhub.org>
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On Mon, Feb 20, 2012 at 11:24:39PM -0800, Andrey Jivsov wrote:
> On 02/18/2012 01:51 PM, Marko Kreen wrote:
> >[ I updated the review with diff from -09.  Thanks for taking
> >   my comments on the ref section into account. ]
> Sorry, I was busy in recent days and this is the first time I take
> close look at your feedback. The credit for -09 should go to Sean
> Turner, IETF Security Area director.

Heh.  I noticed few old references there, then the new draft
fixed those without me sending anything.

Note - the links in text are not updated (see [nits] on draft page).

> >>    The point is encoded in MPI format.  The content of the MPI is the
> >>    following:
> >>
> >>         B = B0 || x || y
> >>
> >>    where x and y are coordinates of the point P = (x, y), each encoded
> >>    in big endian format and zero-padded to the underlying field size.
> >*Then*, they are also padded to byte boundary.  As this is not mentioned
> >anywhere, it caused me some confusion, because I assumed they already
> >are on byte boundary, perhaps even power-of-two.
> >
> >As it only matters to P-521 keys, the bad assumtions work fine on
> >P-256 and P-384 keys.  (Basically I assumed P-521 uses 512-bit values...)
> 
> Note, that what you are suggesting would not work in practice, if I
> understand your initial confusion correctly. Note that both x and y
> are taken mod a 521 bit prime. They can be exactly 521 bits, but in
> 50% of cases, 520 bits. Let's say that either x or y is 521 and
> another one is 520 bits. How would we know which one of x and y is
> the longest one?

The text already states "zero-padded to the underlying field".
So we already are talking about bits in field, not bits
in actual bignum.

The missing '*then*' part is about rounding field bits.

> The algorithm is the following: given the MPI with the point,
> * get its size in full bytes ( len=(MPI_bits+7)/8 )
> * len = len-1 ( to remove the B0 byte )
> * len = len/2 -- the 'len' is the size of x and y in bytes.

I think it's better to show how to calculate proper size
from field size, not from MPI size.  Because MPI is "untrusted",
we need to validate it.

Maybe just add:

   where x and y are coordinates of the point P = (x, y), each encoded
   in big endian format and zero-padded to the underlying field size,
   then zero-padded to multiple of 8 if the field size is not multiple of 8.

> >Missing detail: In addition to size check, what other validation must
> >be done when parsing a point?  This applies to when reading a public key,
> >but especially when reading incoming ECDH/ECDSA message.  I suggest
> >adding:  "You must check that the point is on curve." here.
> 
> I will take it as a suggestion and think how to integrate it, given
> that this is a general check for ECC and is not specific to OpenPGP.
> This valid concern may be satisfied through sources in References
> section.

It's security detail, and it's simple to mention.

The References are rather verbose, so it's easy to miss
the important details.

> >>8. EC DH Algorithm (ECDH)
> >>    The key wrapping method is based on [RFC3394].  KDF produces the
> >>    AES key that is used as KEK as specified in [RFC3394].  Refer to
> >>    section 13 for the details regarding the choice of the KEK
> >>    algorithm, which MUST be one of three AES algorithms.
> >This is only place which says that KEK *MUST* be AES-only.  (Ignoring 12.2)
> >This is in conflict with section 12.1 and 13, where non-AES is not
> >disallowed.  I'm not disagreeing with it, just I think it would be good
> >to clarify why.
> >
> >It might be good idea to disallow cipher_ids<  AES128, but why disallow
> >Camellia and Twofish?  I could imagine that because the algorithm
> >is not per-message but comes from key is the reason - you may not
> >know at key generation time all the people who want to send you messages
> >and what features their software has.  But I think it would be
> >good to put the reason in the doc.
> >
> >In any case, 12.1 and 13 should be synced with the requirement.
> 
> I also see the need for clarity here.
> 
> In reference to compatibility, if the code understands ECC, the
> document assumes that it must supports AES. This is something
> OpenPGP RFC 4880 cannot generally rely upon. One of main criteria of
> this document is simplicity through limited choices, as I recall,
> something that Ian G advocated. It's much easier to write "use AES"
> than to define "strong enough" cipher.

cipher_num > 7?

> Suite B and other government
> standards require AES, so most application will implement it to
> comply, leaving us with the question why bother at all about the
> non-AES case?
> 
> I am leaning toward fixing KEK at AES. There is no backward
> compatibility issue here. So, the action item for me is to make this
> choice clear.

Fine by me.

> >>    For convenience, the synopsis of the encoding method is given
> >>    below, however, this section, [NIST SP800-56A], and [RFC3394] are
> >>    the normative sources of the definition.
> >>
> >>        Obtain authenticated recipient public key R
> >>        Generate ephemeral key pair {v, V=vG}
> >>        Compute shared point S = vR;
> >>        m = symm_alg_ID || session key || checksum || pkcs5_padding;
> >>        curve_OID_len = (byte)len(curve_OID);
> >>        Param = curve_OID_len || curve_OID || public_key_alg_ID || 03 ||
> >>            01 || KDF_hash_ID || AES_alg_ID for AESKeyWrap ||
> >>           "Anonymous Sender    " || recipient_fingerprint;
> >>        Z_len = key size for AES_alg_ID to be used with AESKeyWrap
> >>        Compute Z = KDF( S, Z_len, Param );
> >>        Compute C = AESKeyWrap( Z, m ) as per [RFC3394]
> >>        VB = convert point V to octet string
> >>        Output (MPI(VB) || len(C) || C).
> >>
> >>    The decryption is the inverse of the method given.  Note that the
> >>    recipient obtains the shared secret by calculating
> >>
> >>        S = rV = rvG, where (r,R) is the recipient's key pair.
> >>
> >>    Consistent with section 5.13 Sym. Encrypted Integrity Protected
> >>    Data Packet (Tag 18) of [RFC4880], the MDC SHOULD be used anytime
> >>    symmetric key is protected by ECDH.
> >Missing detail:  How to generate v?  What requirements it has?
> >I suggest expanding the second step with:
> >
> >      Generate ephemeral key pair {v, V} where V=vG and v is random number
> >      in range 0<  v<  n  (n - curve modulus)
> This detail is also related to a question of external reference, but
> I am glad you raised this question. FIPS 186-3 defines two methods
> in B.4 on p.61. The idea is this:
> B.4.1: if you do "v = v' mod order", the input v' must be 64 bits
> longer than "order".
> B.4.2: or you can repeatedly getting random bits until "v' < order"
> (probably too wasteful for the entropy)

I think the "How" can be left to references.  I just would like
to see the requirement "0 < v < n" mentioned here.

RFC6090 Appendix A seems even better reference for this.

> >Final note: the section 8 was quite successful at describing ECDH,
> >how hard would it be to have same level of description of ECDSA here?
> >At least I would like to see packet format here, even if the
> >algorithm is not described.
> 
> Marko, did you realize that it happens that the signature format for
> ECDSA is identical to DSA?

Well, FIPS 186-3 tells that actual ECDSA is described in ANS X9.62..

> The (r,s) signature pair consists of
> (true) MPIs in both cases. The ECDSA key is defined in section 9.
> Overall, ECDSA for OpenPGP is much simpler because it has no
> complexities like key wrapping. With this in mind, what would you
> like to see added to properly implement ECDSA.

At minimum, it should mention that

  Algorithm-Specific Fields for ECDSA signatures are:
     - MPI of ECDSA value r.
     - MPI of ECDSA value s.

then the spec makes clear the mapping between packet layout
and algorithm.

I would like to see short algorithm description also here,
with symbols consistent with rest of the document.

But it's proably OK having good reference in the form "OpenPGP variant
of ECDSA is described here: [REF]".  And if the REF describes 4
different EC signature variants, then point directly to section.

Eg: "How to pad/shorten H(m) for use in ECDSA" is another
step that needs clear "in OpenPGP we do it like this"
description.  (shorten: trunc-left/trunc-right/mod.  And how short?)

-- 
marko