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> See:   http://en.citizendium.org/wiki/Code_book_attack
>
>   DES and the generation of ciphers that followed it all used a 64-bit
>   block size. To completely break a single key, an attacker would need
>   a code book with 2^64 entries. Even to weaken it significantly takes
>   a code book with 2^32 entries with the same key, 32 gigabytes of data.
>   With any sensible re-keying policy, a code book attack is not a threat.
>   More recent ciphers such as AES use a 128-bit block size,
>   which makes code book attacks utterly impractical.

DES is difficult to implement efficiently in software, but this has
got nothing to do with word size.  It's particular brand of bit
shuffling is just not well-supported by mainstream processors (and the
bit module).