phc-discussions - Re: [PHC] Comments on Argon2 v1.2.1

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Date: Wed, 2 Sep 2015 16:12:02 +0200
From: Dmitry Khovratovich <khovratovich@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Comments on Argon2 v1.2.1

We have corrected the specification (page 5) and updated the test vectors
on github.

Thank you again,
the Argon2 team.

On Wed, Sep 2, 2015 at 11:32 AM, Dmitry Khovratovich <khovratovich@...il.com
> wrote:

> Hi Andrew,
>
> that's great! Thanks for an independent implementation. For your comments:
>
> On Wed, Sep 2, 2015 at 8:33 AM, Andrew Ekstedt <andrew.ekstedt@...il.com>
> wrote:
>
>>
>>
>> When computing the initial hash H_0, the reference implementation calls
>> blake2b_final twice, discarding the result of the first call. This
>> is a problem because blake2b_final is not idempotent; the result of
>> the second call is different from the result of the first.
>>
>> It also makes things difficult for the Go implementation because the
>> BLAKE2b library for Go is careful to make its Final method idempotent.
>>
>>
>> https://github.com/khovratovich/Argon2/blob/d9304a5ecac9fc5b47befd25cbfce6ed026042fe/Source/Core/argon2-core.cpp#L339
>
>
> It is a bug! There should not be this 'tmp' variable and its blake2_final
> call.
>
>
>>
>>
>> ---
>>
>> Section 3.2  of the spec states that when initializing the matrix,
>> B[i][0] and B[i][1] are computed from the Argon2 block permutation G,
>> but the reference implementation uses blake2b_long instead.
>>
>>
>> https://github.com/khovratovich/Argon2/blob/d9304a5ecac9fc5b47befd25cbfce6ed026042fe/Source/Core/argon2-core.cpp#L268
>
>
> The specification is wrong here (the entire text for some weird reason
> comes from version 1.0).
>
>
>
>>
>>
>> ---
>>
>> Section 3.3 describes how to obtain the final output of Argon2:
>>
>>         The hash function is applied iteratively to B_m, each time
>>         outputting the first 32 bytes of the 64-byte hash value, until
>>         the total number of output bytes reaches τ.
>>
>> The reference implementation defines a function blake2b_long to perform
>> this, but deviates from the above description in the following ways:
>>
>> * the input to the first hash is prefixed with
>>   the output length τ (4 bytes, little-endian)
>>
>> * if τ is less than or equal to 64 bytes, blake2b_long computes BLAKE2
>>   with a digest size of τ and returns the entire result. Note that this
>>   is not equal to the τ- truncated 64-byte hash because BLAKE2 includes
>>   the digest size in the hash calculation.
>>
>> * if τ is greater than 64 bytes, the first output blocks are computed as
>>   specified, and the last 33- to 64- byte block is computed as above
>>
>>
>> https://github.com/khovratovich/Argon2/blob/d9304a5ecac9fc5b47befd25cbfce6ed026042fe/Source/Blake2/blake2b-ref.cpp#L396
>>
>>
>>
> Again, the reference implementation is correct here, but specification
> must be changed.
>
> I will update the spec and change the reference implementation (remove the
> tmp variable and its Blake2 call). This will change the test vectors.
>
> Thank you again,
> --
> Dmitry
>



-- 
Best regards,
Dmitry Khovratovich

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