Hash Attack Techniques

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 when hash weaknesses affect RSA signature schemes
• symmetric-cipher-attacks when hash is used in key derivation
• classical-cipher-analysis when analyzing hash-like constructions in classical ciphers

Quick attack selection

1. LENGTH EXTENSION ATTACK

1.1 Vulnerable vs Non-Vulnerable

1.2 Attack Mechanism

Given:   MAC = H(secret || original_message)
Known:   original_message, len(secret), MAC value
Compute: H(secret || original_message || padding || extension)
         WITHOUT knowing the secret!

How: The MAC value IS the internal hash state after processing
     (secret || original_message || padding).
     Initialize hash with this state, continue hashing extension.

1.3 Padding Calculation (MD5/SHA)

def md5_padding(message_len_bytes):
    """Calculate MD5/SHA padding for given message length."""
    bit_len = message_len_bytes * 8

    # Pad with 0x80 + zeros until length ≡ 56 (mod 64)
    padding = b'\x80'
    padding += b'\x00' * ((55 - message_len_bytes) % 64)

    # Append original length as 64-bit little-endian (MD5)
    # or big-endian (SHA)
    padding += bit_len.to_bytes(8, 'little')  # MD5
    # padding += bit_len.to_bytes(8, 'big')    # SHA

    return padding

1.4 Tool Usage

# HashPump
hashpump -s "known_mac_hex" \
         -d "original_data" \
         -k 16 \            # secret length
         -a "extension_data"

# Output: new_mac, new_data (original + padding + extension)

# hash_extender
hash_extender --data "original" \
              --secret 16 \
              --append "extension" \
              --signature "known_mac_hex" \
              --format md5

1.5 Python Implementation

import struct

def md5_extend(original_mac, original_data_len, secret_len, extension):
    """
    Perform MD5 length extension attack.
    original_mac: hex string of H(secret || original_data)
    """
    # Parse MAC into MD5 internal state (4 × 32-bit words, little-endian)
    h = struct.unpack('<4I', bytes.fromhex(original_mac))

    # Calculate total length after padding
    total_original = secret_len + original_data_len
    padding = md5_padding(total_original)
    forged_len = total_original + len(padding) + len(extension)

    # Continue MD5 from saved state with extension
    # (requires MD5 implementation that accepts initial state)
    from hashlib import md5
    # Most stdlib md5 doesn't expose state setting
    # Use: hlextend library or custom MD5

    import hlextend
    sha = hlextend.new('md5')
    new_hash = sha.extend(extension, original_data, secret_len,
                          original_mac)
    new_data = sha.payload  # includes original + padding + extension

    return new_hash, new_data

2. MD5 COLLISION ATTACKS

2.1 Identical-Prefix Collision (fastcoll)

Two messages with same prefix but different content, producing identical MD5.

# Generate collision pair
fastcoll -p prefix_file -o collision1.bin collision2.bin

# Result: MD5(collision1.bin) == MD5(collision2.bin)
# Files differ in exactly 128 bytes (two MD5 blocks)

2.2 Chosen-Prefix Collision (hashclash)

Two messages with different chosen prefixes, appended with computed suffixes to collide.

# hashclash (Marc Stevens)
./hashclash prefix1.bin prefix2.bin

# Result: MD5(prefix1 || suffix1) == MD5(prefix2 || suffix2)

2.3 UniColl (Single-Block Near-Collision)

Produces two messages differing in a single byte within one MD5 block, with same hash.

Application: forge two PDF/PE files with same MD5
  - File 1: benign content
  - File 2: malicious content
  - Same MD5 hash

2.4 Collision Applications

2.5 CTF MD5 Collision Tricks

// PHP: md5($_GET['a']) == md5($_GET['b']) && $_GET['a'] != $_GET['b']

// Method 1: Array trick (not real collision)
?a[]=1&b[]=2  // md5(array) returns NULL, NULL == NULL

// Method 2: Real collision (fastcoll output, URL-encoded binary)
?a=<collision1_urlencoded>&b=<collision2_urlencoded>

// Method 3: 0e magic hashes (loose comparison ==)
// md5("240610708") = "0e462097431906509019562988736854"
// md5("QNKCDZO")   = "0e830400451993494058024219903391"
// PHP: "0e..." == "0e..." is TRUE (both evaluate to 0 as floats)

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

SHA-1 should NOT be used for:
  ✗ Digital signatures
  ✗ Certificate fingerprints
  ✗ Git commit integrity (migration to SHA-256 in progress)
  ✗ Deduplication based on hash

SHA-1 is still OK for:
  ✓ HMAC-SHA1 (collision resistance not required)
  ✓ HKDF-SHA1 (PRF security suffices)
  ✓ Non-adversarial checksums

4. BIRTHDAY ATTACK

4.1 Generic Birthday Bound

For n-bit hash: expected collisions after ~2^(n/2) hashes

Hash     Bits    Birthday bound
MD5      128     2^64
SHA-1    160     2^80
SHA-256  256     2^128

CTF application: if hash is truncated to k bits,
collision in ~2^(k/2) attempts

4.2 Birthday Attack Implementation

import hashlib
import os

def birthday_attack(hash_func, output_bits, max_attempts=2**28):
    """Find collision for truncated hash."""
    mask = (1 << output_bits) - 1
    seen = {}

    for _ in range(max_attempts):
        msg = os.urandom(16)
        h = int(hash_func(msg).hexdigest(), 16) & mask

        if h in seen and seen[h] != msg:
            return seen[h], msg  # collision!
        seen[h] = msg

    return None

# Example: find collision for first 32 bits of SHA-256
result = birthday_attack(hashlib.sha256, 32)

5. HMAC TIMING ATTACK