Introduction

The most naive mode of operation is called Electronic Cookboook (ECB) mode. It divides the message into blocks ${\mu }_1,{\mu }_2,...,{\mu }_q$ with length $l_b$, according to whatever block cipher is used, and then separately encrypts each block with the block cipher's encryption algorithm and the same key $k$. The final ciphertext is produced by concatenating the ciphertexts of each block.

Decryption is just the opposite - it divides the ciphertext into blocks ${\sigma }_1,{\sigma }_2,...,{\sigma }_q$ and decrypts each one separately. The original message is recovered by concatenating the decryptions of every ciphertext block.

Security of ECB Mode

The ECB Mode is very simple so it comes as no surprise that it is not very secure.

In particular, it is not CPA-secure, since it is entirely deterministic. Moreover, it is not even semantically secure because if a block is repeated in the plaintext, then the corresponding ciphertext block will also be repeated in the ciphertext which reveals a lot of information about the underlying message.