Oracle & symmetric-crypto attacks

When it applies

An app protects data with symmetric crypto you can poke — an encrypted session cookie, a "remember me"/state token, a signed-with-a-hash parameter — and it lets you submit modified ciphertext and observe a difference (an error vs. success, a padding error, different output). That observable difference is an oracle you can turn into full decrypt/forge, no key needed.

Why it works

Encryption without authentication is malleable: CBC leaks validity through padding errors, ECB reveals structure and lets you rearrange blocks, and MAC = H(secret‖message) (Merkle–Damgård) can be extended without knowing the secret. The key never leaks — the mode/construction does the work.