Electronic codebook (ECB) mode encrypts a long message by splitting it into fixed-size blocks and encrypting each block independently. Because block-cipher encryption is deterministic, identical plaintext blocks produce identical ciphertext blocks. section 4.3
That repeated structure is the trap: ciphertext can reveal patterns in the original message. ECB may conceal individual block contents, but it does not conceal repetition across blocks, making it unsuitable for safely encrypting long structured data. section 4.3
The ECB penguin
An image encrypted with ECB can still reveal the penguin’s outline because repeated image regions become repeated ciphertext blocks. The visible pattern demonstrates why deterministic block-by-block encryption leaks information.
