Decrypt using BcryptDecrypt

[Mandar Apte]

Hello Werner,

          Thank you very much for the response.

The way you have shown in the email chain below, I had done same thing in
my code as well. Also, I am passing the data of block length size only to
gcry_cipher_encrypt and gcry_cipher_decrypt APIs.
Now, my goal is to check, if the AES256 encryption/decryption is same for
libgcrypt and Bcrypt library. Thats the reason I am trying to decrypt the
data, which was encrypted using Libgcrypt APIs, using Bcrypt APIs on
windows.

I am pretty sure if I use windows version of Libgcrypt my problem wont be
there at all.

I think I myself have to handle the padding while encrypting using
Libgcrypt library APIs.

Since, I have to handle padding in my code, is there any APIs in libgcrypt
with which I ensure that I am padding the data in standard way? Are there
any APIs in Libgcrypt using which I can get padded data along with my plain
text data which I can encrypt using gcry_cipher_encrypt?


Thank you in advance.
Best Regards,
Mandar



On Fri, 5 Jun 2020, 2:05 pm Werner Koch, <wk at gnupg.org> wrote:

> On Tue,  2 Jun 2020 16:57, Mandar Apte said:
> > On windows I am using Bcrypt library which also supports AES 256 in CBC
> > mode.
>
> FWIW, Libgcrypt runs very well on Windows.
>
> > Hence, I wanted to check, if the Libgcrypt APIs are doing padding
> > internally since I am not passing any such instruction to the Libgcrypt
> > library explicitly?
>
> No, Libgcrypt does not do any padding and it expects complete blocks.
> gcry_cipher_get_algo_blklen() tells you the block length of the cipher
> algorithm.
>
> There is a flag to enable ciphertext stealing (GCRY_CIPHER_CBC_CTS) but
> in this case you need to pass the entire plaintext/ciphertext to the
> encrypt/decrypt function; there is no way to do this incremental.
>
> For the standard padding as used in CMS (S/MIME), you need to handle the
> padding in your code; here is a snippet
>
>  if (last_block_is_incomplete)
>    {
>       int i,
>       int npad = blklen - (buflen % blklen);
>
>       p = buffer;
>       for (n=buflen, i=0; n < bufsize && i < npad; n++, i++)
>         p[n] = npad;
>       gcry_cipher_encrypt (chd, buffer, n, buffer, n);
>     }
>
>
>
> Shalom-Salam,
>
>    Werner
>
>
> --
> Die Gedanken sind frei.  Ausnahmen regelt ein Bundesgesetz.
>
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