Homework Number Two - Part Two

Part Two: Vigenere and all that

1. The file cask.vgr contains about 10K of Vigenere enrypted English text. Decrypt it.
2. Using the statistical model from class, compute the expected value of the IOC between two Vigenere encrypted streams of text which both have length m, one of them encrypted with the key "bcd" and the other with the key "cdb".

3. A student proposed the following variation on the kind of ciphersystem we have looked at. Suppose that we fix a very long and random-looking string X which is well-known (say it is generated by a simple mathematical formula). Given a plaintext P first encrypt P using X as a one-time pad to get a ciphertext Y, and then encrypt Y again using (for example) the Vigenere cipher to get a ciphertext Z.

   The recipient (who knows the Vigenere key) does Vigenere decrytion on Z to get Y, then subtracts Y to get X.

   Intuitively this should be more secure than Vigenere because Y is highly random-looking. Is it any more secure than just using Vigenere? Would the idea work better with another cryptosystem in place of Vigenere?