Electronic Codebook (ECB)

The Simplest Mode

AES encrypts 16 bytes at a time. But what if your message is longer?

Block cipher modes define how to handle multiple blocks. ECB is the simplest approach.

How ECB Works

Each block is encrypted independently with the same key.

The Process

1. Split plaintext into 16-byte blocks
2. Encrypt each block separately with AES
3. Concatenate the ciphertext blocks

No block depends on any other. Each one stands alone.

The Fatal Flaw

Identical plaintext blocks produce identical ciphertext blocks.

See the pattern? An attacker can too.

The Penguin Problem

This is ECB’s famous failure. Encrypt an image with ECB mode:

The penguin is still visible. Same-colored regions encrypt to the same ciphertext.

Why This Happens

Images have structure. Large areas share the same pixel values.

• White background blocks all encrypt to the same ciphertext
• Black outline blocks all encrypt to the same ciphertext
• The pattern survives encryption

Any data with repeating patterns leaks information under ECB.

When Is ECB Acceptable?

Almost never. But there are edge cases:

• Encrypting a single block (no pattern possible)
• Random data with no structure
• When you need parallelism and accept the risk

For real-world encryption, use CBC or CTR instead.

Why Learn ECB?

It shows what not to do. Understanding ECB’s weakness helps you appreciate why other modes exist.

The lesson: Encryption isn’t just about scrambling data. It’s about hiding patterns.