symmetric-cipher-attacks

A playbook for attacking symmetric encryption in CTF and authorized testing, focused on exploiting block-cipher mode weaknesses and stream-cipher mistakes rather than breaking the underlying cipher. It begins with a quick selection table linking observable behavior to the likely weakness and attack: identical ciphertext blocks for identical plaintext point to ECB (cut-and-paste or byte-at-a-time), a distinguishable padding error points to a CBC padding oracle, malleable ciphertext without integrity checks points to CBC bit flipping, XOR/stream key reuse points to a two-time pad, predictable PRNG output points to LCG/LFSR state recovery, and double encryption points to meet-in-the-middle.

The CBC padding oracle section explains the P_i = D_K(C_i) XOR C_{i-1} relationship and gives a full byte-at-a-time decryption routine that manipulates the previous ciphertext block, including the false-positive check for the last byte, plus PadBuster usage. CBC bit flipping shows how flipping a byte in one ciphertext block flips the corresponding plaintext byte in the next while corrupting the current one, with a worked admin=0 to admin=1 example. ECB coverage includes duplicate-block detection, cut-and-paste block reordering to forge a privileged token, and byte-at-a-time decryption of an appended secret against an encryption oracle. Stream-cipher coverage starts with known-plaintext and key reuse (two-time pad) where XORing two ciphertexts cancels the key, and the selection table also flags LFSR/LCG state recovery and meet-in-the-middle against 2DES-style double encryption.

Use it to identify which mode weakness a target exhibits and to carry out the matching attack, often decrypting or forging data without the key. Related routing points to rsa-attack-techniques when the symmetric key is RSA-protected, hash-attack-techniques for HMAC/hash authentication, and lattice-crypto-attacks for LCG/LFSR recovery via lattices. An advanced BLOCK_CIPHER_ATTACKS.md companion is referenced for full attack scripts, a step-by-step byte-at-a-time ECB walkthrough, PadBuster and custom padding-oracle scripts, and LCG/LFSR recovery code.