Encryption Model, E Channel Decryption Model, D Introduction to Cryptography Ek DK Intruder p L A I N T E XT P L A I N T E X T CIPHERTEXT Active Intruder Passive Intruder FIG. THE ENCRYPTION MODEL (For a Symmetric-Key Cipher)
The messages to be encrypted ,known as Plaintext , are transformed by a key.
The output of encryption process, known as Cipher text , is the transmitted through the channel .
We assume that the intruder hears & accurately copies down the complete Cipher text. However, unlike the intended receiver, the intruder does not know what the decryption key is and so can’t decrypt the Cipher text easily.
The combination of coding & ciphering, if used properly, can produce a cryptosystem that is far more difficult to break than a cryptosystem that uses either.
To prevent from the attacks, the codebook needs to be changed frequently. But changing & securely communicating codebooks is difficult challenge. So, codes are not well adapted in modern communication systems.