From: Tim (tim-pentest@sentinelchicken.org)
Date: Thu May 11 2006 - 17:46:20 EDT
> Is it possible to determine the encryption used by
> "looking" at the encrypted results or lenght ?
>
> I know that with Base64 it's pretty easy because of
> the "==" at the end.
This can be very difficult. Ciphertext is supposed to look completely
random, so a secure algorithm will give away little. Length can be
helpful though. If the encryption was done with a block cipher in ECB
or CBC modes, then all texts will have lengths which are a multiple of
the block size. There are many ciphers which have the same block size
now though, and most new ones are using 128 bit (16-byte) blocks.
Common ciphers which use 64-bit blocks are DES and Blowfish.
If the length across many instances is not a multiple of any large block
size, then it may be a block cipher used in OFB/CFB modes, or a stream
cipher. That, or something home-grown and likely weak.
HTH,
tim
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