Individual skill

crypto-oracle-attacks

Break weak symmetric crypto in real apps — decrypt or forge encrypted tokens/cookies without the key via padding oracles (CBC), ECB block shuffling, and hash-length-extension of homemade MACs. Load when the app hands you ciphertext you can tamper with and resend: an encrypted cookie/token, a "state"/"data" blob, an IV+ciphertext, a `H(secret‖msg)` signature, base64 that changes on edit.

skills/cryptography/crypto-oracle-attacks/SKILL.md

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Last checked: Nov 7, 2026

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Repository stars (not skill ratings): 27
Repository last updated: Oct 14, 2026
License: MIT

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# 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.

## Method