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

1. Fingerprint the token. Decode (base64/hex); measure length. A multiple of 8/16 bytes → a block cipher (CBC/ECB). Flip one byte and resend: a distinct "padding"/"decryption" error = CBC padding oracle; identical ciphertext blocks for identical plaintext = ECB.
2. CBC padding oracle — when the server reveals valid vs. invalid padding: recover plaintext byte by byte, and encrypt arbitrary plaintext (forge a token) by controlling the preceding block. padbuster <url> <ciphertext> <blocksize> -cookies ..., or a Python oracle loop. This is the classic "decrypt/forge the admin cookie" attack.
3. ECB weaknesses — identical plaintext blocks → identical ciphertext:
   • Cut-and-paste: rearrange whole blocks to splice a privileged value (e.g. move an aligned role=admin block into place).
   • Byte-at-a-time decryption: if your input is prepended to a secret and ECB-encrypted, recover the secret one byte per aligned block.
4. Hash length extension — a signature of the form H(secret‖data) (MD5/SHA1/SHA2) with known data and secret length: append your own data and compute a valid new MAC with hashpump (hashpump -s <sig> -d <data> -a <append> -k <keylen>), bypassing the "integrity" check.
5. Weak/predictable tokens — if it turns out not to be real crypto: ECB pattern, a static IV, XOR with a fixed keystream, or a guessable PRNG seed (timestamp) → derive/forge directly.
6. Forge and use — mint the token that says what you want (admin, another user) and replay it; validate the access gained with reporting-triage-validation.

Gotchas

• Authenticated encryption (AES-GCM, encrypt-then-HMAC) kills these — a padding oracle needs an unauthenticated CBC error signal; if tampering just fails uniformly, move on.
• The oracle can be subtle — a timing difference, a 500 vs 403, or a different redirect counts; diff responses carefully before concluding "not vulnerable".
• Block alignment is everything for ECB cut-and-paste and byte-at-a-time — pad your input to land targets on block boundaries.
• HMAC (not H(secret‖msg)) is not extendable — length extension only hits raw Merkle–Damgård concatenation MACs.

Verify success

A token you decrypted or forged that the application accepts as another/privileged user (or recovered plaintext of a value you couldn't read), reproduced from a clean session.

References

Vaudenay padding-oracle; padbuster; the Cryptopals set (mechanisms); hashpump. Related: web-auth-jwt, crypto-rsa-attacks, web-idor.