ECB

A weak block cipher mode that encrypts identical blocks identically, leaking patterns.

ECB is the simplest block cipher mode, dividing plaintext into blocks and encrypting each one independently with the same key. Its critical weakness is that identical plaintext blocks produce identical ciphertext blocks, so structural patterns in the data remain visible, famously illustrated by an encrypted bitmap whose image is still recognizable. Because it provides no diffusion across blocks and no integrity, it is considered insecure for most data. The exam contrast is modes like CBC or GCM that chain blocks or add randomization to hide patterns.