On Tue, 12 Aug 1997, Ray Arachelian wrote:
A known plaintext attack won't help you to break the keys unless you have one of the eight keys, but will having many keys that encrypt the same data significanltly weaken the security of that tiny chunk of data?
And no, I don't mean, there's N keys so the odds of brute forcing the data is now N times easier. Assume we're using 128 bit Blowfish/Idea or better, and discarding weak keys. Are there any differential or other cryptanalysis methods to use the eight resulting cyphertexts to get at the data other than brute forcing it if you don't know any of the keys?
What if instead of using a private key cypher, we used a public key cypher? Would that make any difference in attack methods?
The only thing I can think of is if you use something like CFB mode, and the IV is also the same at the beginning, the first 8 bytes will leave a hole - I don't remember exactly, but I was burned by exactly this when I saw 8 bytes of plaintext after resetting the IV in an app that xors some encrypted blocks of data to do something else. A PK to encode the conventional key works better since you can do a long or complex conventional key and other material such as an IV once, and then bury that several times. --- reply to tzeruch - at - ceddec - dot - com ---