Hash attack playbook. Use when exploiting length extension, MD5/SHA1 collisions, HMAC timing leaks, birthday attacks, or hash-based proof of work in CTF and authorized testing scenarios.

SKILL: Hash Attack Techniques — Expert Cryptanalysis Playbook AI LOAD INSTRUCTION : Expert hash attack techniques for CTF and security assessments. Covers length extension attacks, MD5/SHA1 collision generation, meet in the middle hash attacks, HMAC timing side channels, birthday attacks, and proof of work solving. Base models often incorrectly apply length extension to HMAC or SHA 3, or fail to distinguish between identical prefix and chosen prefix collisions. 0. RELATED ROUTING [rsa attack techniques](../rsa attack techniques/SKILL.md) when hash weaknesses affect RSA signature schemes [symmetric cipher attacks](../symmetric cipher attacks/SKILL.md) when hash is used in key derivation [classical cipher analysis](../classical cipher analysis/SKILL.md) when analyzing hash like constructions in classical ciphers Quick attack selection Scenario Attack Tool H(secret \ \ msg) known, extend message Length extension HashPump, hash extender Need two files with same MD5 Identical prefix collision fastcoll Need specific MD5 prefix match Chosen prefix collision hashclash Byte by byte HMAC comparison Timing attack Custom script Find any collision Birthday attack O(2^(n/2)) Proof of work: find hash with leading zeros Brute force hashcat, Python 1. LENGTH EXTENSION ATTACK 1.1 Vulnerable vs Non Vulnerable Hash Vulnerable Why MD5 Yes Merkle Damgard construction SHA 1 Yes Merkle Damgard construction SHA 256 Yes Merkle Damgard construction SHA 512 Yes Merkle Damgard construction SHA 3 / Keccak No Sponge construction HMAC No Double hashing prevents extension SHA 256 truncated No (if truncated) Missing internal state bits BLAKE2 No Different construction 1.2 Attack Mechanism 1.3 Padding Calculation (MD5/SHA) 1.4 Tool Usage 1.5 Python Implementation 2. MD5 COLLISION ATTACKS 2.1 Identical Prefix Collision (fastcoll) Two messages with same prefix but different content, producing identical MD5. 2.2 Chosen Prefix Collision (hashclash) Two messages with different chosen prefixes, appended with computed suffixes to collide. 2.3 UniColl (Single Block Near Collision) Produces two messages differing in a single byte within one MD5 block, with same hash. 2.4 Collision Applications Application Technique Impact Certificate forgery Chosen prefix Rogue CA certificate (proven in 2008) Binary substitution Identical prefix + conditional Two executables, same MD5, different behavior PDF collision UniColl Two PDFs showing different content Git commit collision Chosen prefix (SHAttered for SHA1) Two commits with same hash CTF: bypass MD5 check fastcoll Two different inputs accepted as same 2.5 CTF MD5 Collision Tricks 3. SHA 1 COLLISION 3.1 SHAttered Attack (2017) First practical SHA 1 collision: two PDF files with same SHA 1. Complexity: ~2^63 SHA 1 computations Cost: ~$110K on GPU clusters (2017 prices) Tool: shattered.io provides the collision PDFs 3.2 SHA 1 Chosen Prefix Collision (2020) Complexity: ~2^63.4 computations Practical for attacking PGP/GnuPG key servers Demonstrates SHA 1 is broken for collision resistance 3.3 Impact 4. BIRTHDAY ATTACK 4.1 Generic Birthday Bound 4.2 Birthday Attack Implementation 5. HMAC TIMING ATTACK 5.1 Vulnerable Comparison 5.2 Attack Strategy 5.3 Constant Time Comparison (Defense) 6. MEET IN THE MIDDLE (HASH) 6.1 Concept Split hash computation into two halves, precompute one, match against the other. 7. HASH PROOF OF WORK 7.1 Common CTF PoW Formats 7.2 GPU Accelerated PoW 8. RAINBOW TABLES & SALTING 8.1 Rainbow Table Attack 8.2 Salt Defeats Rainbow Tables 8.3 Modern Password Hashing Algorithm Salt Iterations Memory Hard Recommended MD5 No 1 No Never SHA 256 No 1 No Never for passwords bcrypt Yes Configurable No Yes scrypt Yes Configurable Yes Yes Argon2 Yes Configurable Yes Best choice PBKDF2 Yes Configurable No Acceptable 9. DECISION TREE 10. TOOLS Tool Purpose Usage HashPump Length extension attack hashpump s MAC d data k secret len a extension hash extender Length extension (multiple algorithms) hash extender data D secret L append E sig MAC fastcoll MD5 identical prefix collision fastcoll p prefix o out1 out2 hashclash MD5 chosen prefix collision hashclash prefix1 prefix2 hashcat Password/hash cracking (GPU) hashcat m MODE a ATTACK hash wordlist John the Ripper Password cracking (CPU/GPU) john wordlist=rockyou.txt hashes.txt CyberChef Quick hash computation and encoding Web based

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