Alex Bligh wrote:
>
>
> --On 10 February 2006 19:30 +0000 Ben Laurie wrote:
>
>> Also, FWIW, hash algorithms are perfectly usable as encryption
>> algorithms.

>
> Pardon? One of the features of most hashes is that they are not invertible,
> and another features is that the range is far smaller than the domain. I
> agree that most encryption algorithms are usable as hash algorithms (as
> without the decrypt key, a good encryption algorithm is not invertible) if
> a subset of the output range is taken (i.e. a limited number of bits), but
> encryption algorithms with a range of less size than their domain are not
> in general helpful, so the converse as far as I know is not true.
>
> But we'd all be grateful for the SHA-256 "decryption" algorithm if you
> have one!


I didn't say you could decrypt a hash function, I said you could use one
for encryption.

The construction is known as "Chaffing and Winnowing", was invented by
Ron Rivest, is hugely inefficient and was designed to show the
foolishness of crypto export laws.

http://theory.lcs.mit.edu/~rivest/chaffing.txt

Cheers,

Ben.


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