Re: [Crypto-panel] Review of AES-GCM-SIV

[Tibor Jager <[e-mail]>]

Hi all,

Please find my review below:

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Summary: almost ready

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Major concerns:
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none

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Minor comments and recommendations:
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Section 4, first paragraph after the pseudocode:

The length block contains the length of the plaintext and the length of
the additional data (AD), computed as

  bytelen(plaintext)*8  and  bytelen(AD)*8, repectively

Is it possible to encrypt such that the length in *bits* of either the
plaintext or the AD is not a multiple of 8? If yes, then how exactly is
the bytelen() function defined in this case?

For example, let C = Enc(k,m,d,n) be a ciphertext, encrypting plaintext
m with key k, nonce n, and additional data d. Suppose that |d| = 7 bits,
and that bytelen() is implemented such that bytelen(d) = 1 (which may
happen, if bytelen() returns only integers, even tough bytelen(d) = 7/8
would actually be correct). Note that then the encryption algorithm pads
d with one zero, and we have bytelen(d||0) = 1, too. Therefore it holds that

  C = Enc(k,m,d,n) = Enc(k,m,d||0,n)

and the decryption algorithm accepts both d and d||0 as "valid"
additional data for C. (A similar attack may be possible to modify
plaintexts, too.)

Depending on the application, this may pose a security issue, or at
least an implementational difficulty, because it is natural to implement
a bytelen() function in a way such that it returns only integers. (In
particular if a developer only has plaintexts/ADs in mind whose size is
a multiple of 8 bits, even though an application may permit other
plaintext/AD sizes).

Recommendations:
(a) If plaintexts and AD may have bit-lengths which are not a multiple
of 8 (which is somewhat suggested by the statement "plaintexts can have
arbitrary length" in the 1st paragraph of Section 4, and by the fact
that the example
in section 8 talks about the *bit*-length of the plaintext), then the
bytelen() function
should be replaced with a bitlen() function.
(b) Alternatively, if it is implicitly assumed that the lenghts of
plaintext and AD in *bits* is always a multiple of 8 (which may be
reasonable for most applications), then this should be clarified in the
RFC. Then either encryption woukd fail if plaintext or AD do not have
appropriate length, or it should be decribed how plaintext/AD can be
padded to appropriate length.

I think that (a) seems preferable, because it seems simpler, more
general, and appropriate for an encryption scheme based on counter mode
that permits arbitrary-length plaintexts.


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Nitpicking:
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Section 1 "Introduction", 1st paragraph: I suggest to replace
  "...that is easier for practitioners to use correctly."
with
  "that is easier to use correctly."


In Section 4, first paragraph, the text suggests that plaintexts and
additional data of arbitrary length can be encrypted. However, the
description of the decryption procedure in Section 5 rejects ciphertexts
of size larger than 2^36+16 bytes, and Section 6 gives upper bounds on
the plaintext and AD sizes P_MAX and A_MAX.


In Section 4, last paragraph, the result of encryption is the "resulting
ciphertext ... followed by the tag". Thus, in this notation, the tag is
not part of the "ciphertext", but it is separate and sent along with the
ciphertext.
However, at the beginning of Section 5, decryption algorithm receives as
input key, nonce, AD, and a ciphertext, and the ciphertext is split into
the encrypted plaintext and the tag, thus the "ciphertext" contains the
tag here. One could unify this, by always considering the tag as part of
the ciphertext.


Section 8, very very nitpicking: One could mention here that the
plaintext are the bit strings corresponding to the *ASCII encoding* of
"Hello world" and "example".


Section 8, 5th paragraph, again very nitpicking: Some developers may
have difficulties in understanding immediately which numbers are given
in hexadecimal notation, and which in decimal notation. For clarity, one
could write here something like:
"example": 7 characters = 56 bits = 0x38 bits
"Hello world": 11 characters = 88 bits = 0x58 bits


Section 9, 7th paragraph: "Suzuki et al. [multibirthday]", the reference
lists Kazuhiro as first author, so it seems this should be Kazuhiro et al.


I did not check the test vectors.


Regarding Scott's comment on the verbal description of the encryption
and decryption algorithms: I had the same impression, some pseudocode
may be helpful to clarify what is happening here.


Apart from the above minor comments, I think that this is an excellent
RFC, which is very clear, precise, easy to understand, and
well-readable. The large number of test vectors will certainly be
considered very helpful to many implementers. I think it is very useful
to have a nonce misuse-resistant encryption scheme defined in an RFC, in
particular if it is as competitive with weaker solutions regarding
implementational difficulty and computational efficiency as this one.

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Cheers,
Tibor