Take the 2-minute tour ×
Cryptography Stack Exchange is a question and answer site for software developers, mathematicians and others interested in cryptography. It's 100% free, no registration required.

I'm currently dealing with multiple blowfish-encrypted files that share the same key. All are encrypted using ECB mode judging from their appearance. I don't know what the key is but I know 64 byte (or 6 blocks) of consecutive plaintext. The key length used is most certainly 448 bit.

Now, I just recently started to read up on this topic and I haven't been involved into cryptography before either. However I stumbled across this post on this site and it talks about precomputed tables to bruteforce the used key. I understand that the post is talking about a 40bit key so it eventually means that there is no way to use this in my situation.

But if it is, are there any tools to use for this scenario? I'm sadly lacking the skills to write them. I just want to be able to decrypt and re-encrypt the files.

Thank you very much for any reply.

share|improve this question

1 Answer 1

No, the most you can do is to compare ciphertext blocks for equality and link those blocks with identical plaintext. That ECB is used does not hurt the security of Blowfish by itself. Block ciphers should not be vulnerable to known plaintext attacks, and there seems to be no known attack on Blowfish in this regard.

To state it a bit more formally: that ECB is used or that plaintext is known does not make the cipher itself distinguishable from a random permutation; the repetition of ciphertext blocks only show that it likely is a permutation.

Obviously, brute forcing such a large key is out of the question. That there are attacks on 40 bit keys doesn't translate to keys over 64 bits let alone keys of more than 11 times the size.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.