[openssl-dev] Variable length of digest

[Victor Wagner]

On Thu, 24 Dec 2015 11:24:37 +0300
Dmitry Belyavsky <beldmit at gmail.com> wrote:


> 
> If you try to change the output length via the -macopt option of the
> dgst command, you'll see that the text output will be 4 bytes.

It is misfeature of dgst command. You have to use -signopt, not -macopt,
because dgst command doesn't keep EVP_PKEY_CTX during all the MAC
operation.

-macopt is used to configure EVP_PKEY_CTX, which is used during
EVP_DigestSignInit operation and then destroyed, and -signopt - to
configure context, which is used durint EVP_DigestSignFinal.

Forutnately, we don't need to configure GOST-MAC algorithm during init
state. Internal state is always 64 bit, and is truncated only on the
finalization stage.

When using API (such as python ctypescrypto.mac module) and preserving
same EVP_PKEY_CTX for the lifetime of digest, this problem doesn't
occur.