Re: [key4.db] IV size for aes256-CBC

On 03/27/2020 12:21 PM, Louis Abraham wrote:
> Hi Matthew,
>
> Awesome, thanks and sorry for contacting the wrong list!
>
> Since then, I found the answer to the 14 bytes question: 
> https://hg.mozilla.org/projects/nss/rev/fc636973ad06392d11597620b602779b4af312f6#l6.49
> Basically the DER encoding is used instead for compatibility with a 
> bugged implementation.
>
> I tried prepending |b'\x04\x0e'| to DER-encode the IV. However, the 
> value I get makes no sense (and even has an incorrect padding 
> according to pkcs7 <https://tools.ietf.org/html/rfc2315>).
>
>
> Best,
>
> Louis
>
The IV length is still 16 bytes, but only 14 are randomly generated. 
It's because the decoding code had a bug in it that requires the IV to 
look like der encoded data, so the header needed to be added, but the 
whole IV was used (including the 2 byte header) when encrypting/decrypting.

The goal of the AES-256 bit code was  to encode AES-256 while allowing 
older versions of NSS to still decrypt the new keys, since versions of 
NSS may share their databases with other NSS applications running on 
other machines.

bob
>
> Le ven. 27 mars 2020 à 19:57, Matthew N. <[email protected] 
> <mailto:[email protected]>> a écrit :
>
>     Hi Louis,
>
>     The dev-tech-crypto mailing list I'm redirecting this to should be
>     able to get you an answer.
>
>     Thanks,
>     MattN
>
>
>     On Fri, Mar 27, 2020 at 8:51 AM Louis Abraham
>     <[email protected] <mailto:[email protected]>> wrote:
>
>         Hi,
>
>         I'm the main developer of https://github.com/louisabraham/ffpass
>         We are currently trying to accommodate the (not so) recent
>         cryptographic changes in key4.db.
>
>         If I understand correctly, key4.db contains a table metadata.
>         The value item2 defines a cryptographic algorithm in the DER
>         format.
>
>         In the latest version of Firefox, this algorithm is PBES2,
>         using aes256-CBC as the encryption algorithm.
>
>         I'm facing a little problem when trying to execute aes256-CBC
>         because the IV size is only 14 bytes (56 bits) instead of the
>         64 bits defined in the spec.
>
>         Could you please help me to understand?
>
>         Best,
>         Louis
>

-- 
dev-tech-crypto mailing list
[email protected]
https://lists.mozilla.org/listinfo/dev-tech-crypto