An brief overview of a TLS in simple language.

The TLS protocol provides 3 properties

• Authentication: is the peer who they say they are. the client authenticates the server. server can optionally authenticate the client.
• Confidentiality: messages sent to the peer are secret and only visible to the peer.
• Integrity: messages sent to peer are tamper proof. If a message is modified then this tamper can be detected.

How are these properties enforced

• Authentication: during the handshake, the server sends a certificate which ties its private key to the public key on the certificate. The certificate is in turn signed by a root CA's certificate, which both the peers need to trust to establich "a chain of trust". The root cert might sign a intermediate cert, which in turn will sign the peer's cert. The "root cert" -> "intermediate cert" -> "peer cert", is the chain and the trust is established by cryptographically verifying the signatures on the cert. The RFC defines x.509 and PKI, which are the components which define certificate infrastructure.
• Confidentiality: during the handshake, the client and server perform a key exchange (kex). There are many kex algorithms (RSA, DHE, EDHE). ECDHE (elliptic curve diffie-hellman ephemeral) is prefered because it is forward-secret. Note the word epehemeral in the title. The responsibility of kex is to allow for each peer to contribute some public key material and together come up with a secret key for the TLS connection. "client public key material" + "server public key material" => "connection secret key". The connection secret key is a strong cryptographic secret, which when used to encrpty messages provides confidentiality.
• Integrity: along with using the "connection secret key" for encryption, we also generate a MAC (message authentication code) for the ciphertext we generate. This tag can be though of as a "signature" on the ciphertext, and guarantees that only a peer which possesses the connection secret key could have generated the encryption/tag. Previously the encryption and authentication were separate steps and algorithms, but today it is good practice to use cipher schemes that offer Authenticated Encryption (authenticaion and encryption).

A full TLS handshake

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A PSK TLS handshake

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