AES-GCM vs AES-CBC: Why Authenticated Encryption (AEAD) is Non-Negotiable

What is Authenticated Encryption with Associated Data (AEAD)?

Traditional symmetric encryption (such as AES in CBC mode) guarantees confidentiality — an eavesdropper cannot read the ciphertext. However, it does not guarantee integrity or authenticity.

An attacker can modify ciphertext blocks in transit, and the decrypting party will decrypt the manipulated bytes without realizing tampering has occurred.

The Fatal Flaw of AES-CBC: Padding Oracle Attacks

AES-CBC requires plaintext to be padded to 16-byte block boundaries (typically using PKCS#7). When a server attempts to decrypt manipulated ciphertext and returns distinct error messages for padding errors versus application errors, an attacker can exploit this padding oracle to decrypt arbitrary ciphertexts without ever knowing the key!

Why AES-GCM is the Modern Standard

Galois/Counter Mode (GCM) combines counter-mode stream encryption with a universal hash function over a binary Galois field:

1. It produces a 128-bit authentication tag.
2. If even a single bit of the ciphertext or IV is altered, decryption immediately fails before any plaintext is processed.
3. It requires no padding, eliminating padding oracle attacks entirely.

Test authenticated in-browser encryption using our AES-256-GCM Encryptor & Decryptor.