Block Cipher Modes: ECB vs CBC

Chaining Mode

Plaintext Pattern

Key & IV

Ciphertext Analysis

Total blocks0

Duplicate ciphertext blocks0

Pattern leakage0%

How it works

A block cipher encrypts one fixed-size block at a time under key K. How successive blocks are combined is the mode of operation — the algorithm itself (here a toy keyed hash standing in for AES) never changes.

ECB:  C[i] = E_K( P[i] )

CBC:  C[0] = E_K( P[0] XOR IV )
      C[i] = E_K( P[i] XOR C[i-1] )

Under ECB, identical plaintext blocks always encrypt to identical ciphertext blocks — the block index never enters the computation. Any repeated structure in the source data (a flat background, a repeating byte) shows straight through as a repeated pattern in the ciphertext, even though each individual block is unreadable.

Under CBC, each block is XORed with the previous ciphertext block before encryption, so the input to E_K depends on everything before it in the stream. Two identical plaintext blocks almost never produce the same ciphertext, because they arrive at E_K already mixed with different chaining values. This is why every real protocol (TLS, disk encryption, file formats) forbids ECB for anything but a single unrelated block.

ECB / CBC — switch chaining mode and re-encrypt the same grid.
Pattern — pick a plaintext layout; "Background + subject" is the classic case where ECB visibly leaks the shape of the data underneath the "encryption".
New key / New IV — regenerate the secret key or the initialization vector and watch the ciphertext scramble, while the ECB-vs-CBC pattern behavior itself is unchanged.

Plaintext blocks (front) Ciphertext blocks (back)

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