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I need to crack a stream cipher with a repeating key.

The length of the key is definitely 16. Each key can be any of the characters numbered 32-126 in ASCII.

The algorithm goes like this:

Let's say you have a plain text:

"Welcome to Q&A for people studying math at any level and professionals in related fields."

Let's say that the password is:

"0123456789abcdef"

Then, to encrypt the plaintext, just XOR them together. If the key isn't long enough, just repeat it. e.g.,

Welcome to Q&A for people studying math at any level and professionals in related fields.

                                 XOR 

0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789ab

I have 2 English messages encrypted with the above algorithm and with the same key. I know about the communicative property of xor and that it can be exploited for the example above. I've read that this is a pretty weak cipher and it has been cracked. However, I have no idea how to do it. So, where can I find a cryptanalysis tool to do it for me?

  • 1
    with repeated key you don't even need a second message, because first message itself acts as multiple key-length messages, so it makes your task easier2012-11-13
  • 0
    This is a version of the Vigènere cipher https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher2015-02-03

4 Answers 4