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Re: Determining the encryption used
From: Tim <tim-pentest () sentinelchicken org>
Date: Thu, 11 May 2006 17:46:20 -0400

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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