This is because their MAC keys are directly derived from the encryption keys. Note that AEAD ciphers that combine both authentication and encryption will not need MAC keys but will need two other keys instead: client_write_IV and server_write_IV. The symmetric ciphers chosen in the handshake will dictate how long these keys we generate need to be. This is mostly by respect of good practices. Interestingly, two keys are generated for every purpose: one key per side. So now that we have a fixed length value, we can derive 4 keys from it:Īs you can probably guess, MAC keys are for the authentication and integrity with whatever MAC algorithm you chose in the cipher suite, write keys are for the symmetric encryption. The Master SecretĪ master secret is always 48 bytes. But let's move on, and we will see later what exactly is that PRF.
PRF stands for Pseudo-random function, basically some concrete construction that emulates a random oracle: given an input will produce an output computationally indistinguishable from a truly random sequence. This is to bound the soon-to-be master key to this session. The two random values ClientHello.random and ServerHello.random, sometimes called "nonces", are randomly generated and sent during the ClientHello of each parties. master_secret = PRF(pre_master_secret, "master secret",
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Here the RFC tells us how to compute it from the pre-master secret after having removed the leading zeros bytes. The fixed-length value we'll call master secret. This is the reason behind a pre master secret. To make things simpler, we would want a fixed-length value to derive the keys for any cipher suite we would want to use. Its length varies depending on the algorithm and the parameters used during the key exchange. The pre-master key is the value you directly obtain from the key exchange (e.g. This blogpost is about what happens between this key exchange and the encryption/authentication of data. The handshake can currently use 5 different algorithms to do the key exchange: RSA, Diffie-Hellman, Elliptic Curve Diffie-Hellman and the ephemeral versions of the last two algorithms. But as you may know, if you've read RFCs before, it is not easy to parse (plus they have some sort of double spaces non-sense).īefore we can encrypt/MAC everything with keys to secure our connection, we need to go over a key exchange called the Handshake to safely agree on a set of keys for both parties to use. TLS, Pre-Master Secrets and Master Secrets posted March 2016Įverything you want to know about TLS 1.2 is in RFC 5246.
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