md5(sha1($pass)) Hash

What is md5(sha1($pass))?

The cryptographic hash format `md5(sha1($pass))` represents a nested hash function where the plaintext password `$pass` is first processed by the SHA-1 algorithm, and the resulting 160-bit (20-byte) hash is then fed into MD5, producing a final 128-bit (16-byte) digest. This hybrid construction emerged in the late 1990s and early 2000s as a pragmatic workaround to mitigate the known vulnerabilities of SHA-1 alone, particularly its susceptibility to collision attacks and reduced security margins due to its 160-bit output length. While SHA-1 was originally designed for digital signatures and data integrity, its adoption in password hashing was later superseded by more secure alternatives like bcrypt or Argon2. The primary use-case for this format was in legacy systems where SHA-1 was already deployed but required an additional layer of obfuscation to resist early brute-force and rainbow table attacks. However, the concatenation of two flawed hash functions—SHA-1 (compromised by length-extension attacks and practical collision resistance) and MD5 (broken by collision attacks in 2005)—creates a compounded security liability, as the output remains vulnerable to both individual weaknesses.

The vulnerability of `md5(sha1($pass))` stems from fundamental cryptographic flaws exacerbated by implementation oversights and computational optimizations. First, the absence of a cryptographic salt renders the hash susceptible to precomputed attacks, as identical passwords produce identical outputs, enabling efficient rainbow table lookups. Second, the nested design does not distribute computational effort evenly: MD5’s 128-bit output truncates the effective entropy of SHA-1’s 160-bit hash, reducing collision resistance to approximately 2²⁸ operations (per the birthday paradox), while SHA-1’s pre-image resistance is already degraded by length-extension attacks. Third, modern hardware acceleration—particularly GPU/FPGA-based brute-force tools like Hashcat—exploits the parallelizable nature of both MD5 and SHA-1, achieving cracking speeds of millions of hashes per second. Additionally, the format’s reliance on legacy algorithms (SHA-1 deprecated by NIST in 2011, MD5 in 2012) makes it vulnerable to quantum algorithm advancements (e.g., Grover’s algorithm), which reduce pre-image resistance to ~2⁶⁴ operations. The compounded entropy loss and lack of adaptive resistance to computational advancements render this scheme effectively broken for modern security requirements.

Identifying md5(sha1($pass)) Hashes

Identifying a hash blindly is often difficult because multiple algorithms can output strings of the exact same length. For example, a standard 32-character hexadecimal string could be MD5, NTLM, MD4, or LM. To definitively identify md5(sha1($pass)), security researchers analyze the hash against a strict regular expression (regex) signature (shown above) to find structural matches.

How to Crack md5(sha1($pass))

If you need to recover the plaintext password for a md5(sha1($pass)) hash during a penetration test or forensics investigation, you must supply the correct module ID to your cracking software. Use the exact commands provided at the top of this page to target the hash successfully.