Encrypting same data with many keys...

Matthew Ghio ghio at temp0107.myriad.ml.org
Tue Aug 12 12:56:10 PDT 1997

Ray Arachelian <sunder at brainlink.com> asked:
> What are the dangers of taking a small block of data - say upto 1K in
> size, then producing many files, each being the same data encrypted by
> other keys?
> Assume we're using 128 bit Blowfish/Idea or better, and discarding
> weak keys.

For a standard block cipher there should be no problem.  For a stream
cipher, you would have the same type of problems as for OTP reuse, but
it would still be secure as long as you never reused a key.  However...

> What if instead of using a private key cypher, we used a public key
> cypher?  Would that make any difference in attack methods?


Having identical plaintexts raised to the same power modulo different
numbers makes the solution much easier.  If you have enough RSA
encryptions of the same number to the same power, you can solve it
outright by the remainder theorem.

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