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.


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.

  • $\begingroup$ then why is it that yyyyyyy says in this answer: crypto.stackexchange.com/a/24378/23012 that ecb mode is seen easily broken? since it fails indistinguishability from chosen plain text attacks. $\endgroup$
    – Vincent
    Mar 13, 2015 at 7:37
  • 2
    $\begingroup$ @VincentAdvocaat The difference here is that this question talks about retrieving the key value (if I read it correctly). The ECB distinguishability is an issue, but it is an issue for the ECB mode of operation, not the block cipher itself. $\endgroup$
    – Maarten Bodewes
    Mar 13, 2015 at 10:57

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