Symmetric checksum authentication, hash vulnerabilities, and the HMAC nested mathematical structure.
1. Why plain Hashes cannot Authenticate Messages
2. HMAC Mathematical Formula & Design
Constructing a MAC by simply prepending the key to the message (H(Key || Message)) is vulnerable to Length Extension Attacks in ciphers using the Merkle-Damgård construction (SHA-1, SHA-2). An attacker can append data to the message and compute a valid hash for the extended message without knowing the key.
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