Master of Science (MS)
Substitution ciphers normally use prefix free codes there is no code word which is the prefix of some other code word. Prefix free codes are used for encryption because it makes the decryption process easier at the receiver's end.
In this project, we study the feasibility of substitution ciphers with nonprefix codes. The advantage of using nonprefix codes is that extracting statistical information is more difficult. However, the ciphertext is nontrivial to decrypt.
We present a dynamic programming technique for decryption and verify that the plaintext can be recovered. This shows that substitution ciphers with nonprefix codes are feasible. Finally, we view the cipher from the attacker's perspective and experimentally study various attacks. We show that a limited attack is possible in the case of known plaintext. However, the ciphertextonly attack appears to be very challenging, which is in stark contrast to substitution ciphers with prefix free codes.
Muralidhar, Rashmi Bangalore, "Substitution Cipher with NonPrefix Codes" (2011). Master's Projects. 176.