HMAC-MD5

What it is

HMAC stands for “Hash-based Message Authentication Code.” It’s a way to verify both the integrity and authenticity of a message using a secret key and a hash function—in this case, MD5 :)

How it works (step-by-step)

• You start with your secret key (e.g. your password). If it’s longer than MD5’s block size (64 bytes), you hash it first; if it’s shorter, you pad it with zero bytes.
• Create two derived keys:
  • ipad = the key XOR’d with 0x36 for every byte
  • opad = the key XOR’d with 0x5C for every byte
• Compute the inner hash:
  1. Concatenate ipad || message
  2. MD5(…) that blob → call this inner_digest
• Compute the outer hash:
  1. Concatenate opad || inner_digest
  2. MD5(…) that blob → that’s your final HMAC-MD5 digest :)

Why use it

• It protects the message: if even one bit flips in transit, the digest won’t match when the receiver recomputes it.
• It authenticates the sender: only someone with the secret key could generate the correct digest for a given challenge message.

Technical nitty-gritty

• MD5 processes data in 512-bit (64 byte) chunks, producing a 128-bit (16 byte) hash.
• HMAC-MD5 outputs a 128-bit MAC, usually represented as 32 hex characters.
• The double-hash structure (inner then outer) stops certain length-extension and collision attacks that plain MD5 would be vulnerable to.

Security considerations

• MD5 itself has known collision weaknesses—two different messages can be crafted to have the same MD5 hash.
• However, HMAC-MD5 is still considered safe against collision attacks in the context of authentication, because an attacker would need to know the secret key to forge a valid MAC.
• That said, modern systems often prefer stronger hashes (HMAC-SHA-256 or better) for future-proofing and compliance :3