TLS

Transport Layer Security

Overview

• Goal: TLS accomplishes Encryption, Authentication, and Integrity.
• Timing: The TLS handshake is performed at the transport layer after the TCP handshake is successfully completed.

TLS Certificates

• For a website or application to use TLS, it must have a TLS certificate (SSL certificate) installed on its origin server (X.509 standard).
• The certificate contains information about who owns the domain and the server’s public key.

The TLS Handshake

During the handshake, the client and server:

1. Specify which TLS version ($1.0, 1.2, 1.3$, etc.) they will use.
2. Decide which Cipher Suite they will use (specifies algorithms for keys and session).
3. Authenticate the identity of the server using TLS Certificates.
4. Generate session keys for encryption after the handshake is completed (bulk encryption is done using symmetric keys).

Authentication Note: During the handshake, the server digitally signs its messages (transcript and one-time challenge to prevent replay attacks). The client uses the server’s public key to authenticate the server.

Key Exchange Methods

The cipher suite decides which algorithm is used for key exchange.

1. RSA Key Exchange (Deprecated)

• Process: The client makes use of the server’s public key to encrypt the “pre-master secret” and sends it to the server.
• Result: Both server and client derive a set of identical session keys from the pre-master secret.
• Status: Highly deprecated because it cannot provide Perfect Forward Secrecy.

RSA key exchange can provide PFS in theory if the key used for encrypting the pre master secret is ephemeral, but this is not done due to heavy performance loss.

2. Diffie-Hellman (More Secure)

• Benefit: Provides Perfect Forward Secrecy.
• Concept: Allows two people to create a shared secret key without ever transmitting it over the channel and by using ephemeral private values.

The Math:

1. Client and Server agree on two public numbers: a generator ($g$) and a large prime number modulus ($p$).
2. Client and Server use their private numbers ($a$ and $b$) to calculate a public key (private numbers are ephemeral): $A = g^a \pmod p$ $B = g^b \pmod p$
3. The public keys ($A$ and $B$) are exchanged.
4. They calculate the shared secret using each other’s public key: $\text{Shared Secret} = B^a \pmod p = A^b \pmod p$