HMAC and HKDF

What’s the right way to correct an encryption key from a secret? Well, with TLS 1.3, the right way is to use HKDF. So, let’s look at HMAC and HKDF.

HMAC

HMAC (hash-based message authentication code) supports the usage of a key to hash data. This key is kept secret between Bob and Alice, and can be used to authentication both the data and that the sender still knows the secret. Overall HMAC can be used with a range of different hashing methods, such as MD5, SHA-1, SHA-256 (SHA-2) and SHA-3.

HMAC is a message authentication code (MAC) that can be used to verify the integrity and authentication of a message. It involves hashing the message with a secret key and thus differs from standard hashing, which is purely a one-way function. As with any MAC, it can be used with a standard hash function, such as MD5 or SHA-1, which results in methods such as HMAC-MD5 or HMAC-SHA-1. Also, as with any hashing function, the strength depends on the quality of the hashing function, and the resulting number of hash code bits. Along with this, the number of bits in the secret key is a factor in the strength of the hash. The figure below outlines the operation, where the message to be sent is converted with a secret key, and the hashing function, to an HMAC code. This is then sent with the message. On receipt, the receiver recalculates the HMAC code from the same secret key, and the…