I am looking at a very old protocol which uses Blowfish to encrypt packets with a constant 8-byte IV in CBC mode.
Assuming I have:
- The ability to arbitrary encrypt known plaintexts
- The IV
I know that this lets me identify which plaintexts begin with an identical block and lets me identify also which plaintexts are exactly identical (since the cipher texts will remain identical until the first divergent block).
Is there any information which IV-reuse might leak that could be used to reduce the brute-force space for a dictionary attack or otherwise learn additional information about the plaintext?
I know that there are several theoretical weaknesses in Blowfish (such as this reflection attack against weak keys) but I am not sure if IV-reuse makes it possible to identify weak keys with less data than the hundreds of gigabytes proposed in those attacks.