# Decrypt a ciphertext knowing of a pattern of key repetition

I have two ciphertexts (I do not know which encryption algorithm was used to create them) which both starts with the same 3-characters long word. This led me to believe that the same key was used for both of them and that I could exploit that to decipher it.

I thought that one-time pad was used there, so I tried to XOR them together to get the XORed plaintexts, but I cannot decipher it so I think some other cipher was used.

What are other probable ciphers that could have been used which are vulnerable to key repetition?

edit: Thanks for the feedback. The characters are ASCII characters, there is no header and it is not a file, just encrypted plain text.

• Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
– Community Bot
Sep 6 '21 at 19:33
• It's also possible the repeated characters (bytes? Windows UCS-2-style 16-bit characters? UTF-8 extended grapheme clusters? "character" is ambiguous) at the start are just a header, and no key has been repeated. Sep 6 '21 at 23:26

Finally, the chance that you get the same 3 bytes (not printable characters) is one in $$2^{24}$$ (about one in 16 million). It seems unlikely that this was generated by a (pseudo)-random function by chance, if this is indeed ciphertext. However, it is possible to deliberately cherry pick ciphertext if someone wanted to confuse you.