TLS (Transport Layer Security)

Secure Communication between applications

Properties

• Authenticated
• Integrity Protected
• Confidential
• Secure against replay and deletion

Building Blocks

• Strong block ciphers (AES, Twofish, ...)
• Authenticated encryption mpdes (GCM, CCM)
• Diffie-Hellman Key agreement
• Groups that yield small keys and fast DH operations (ECC)
• Public key authentication with certificates
• Authentication trough signatures
• Cryptographic hash functions for key generation (HKDF) (e.g. AES session keys)

A cipher suite determinces the set of cryptographic algrogithms to be used.

Layer

• TLS works on top of TCP
• Can be implemented in user space (directly in the application software)
• Does not have to worry about lost/retransmitted data

Protocol stack

TLS 1.3 Record Protocol

TLS Record Protocol

Defines the TLS packet format; all data that are using TLS are transported within TLS Records

Handshake Protocol

Used to establish TLS sessions

Change Cipher Spec Protocol (deprecated)

Dummy messages only. In TLS 1.2, it indicated switching to the newly negotiated secure communication relationship and keys

Alert Protocol

Indicates warnings/errors (e.g., certificate expired)

Application Data

Data meaningful to applications (not for TLS)

TLS message formats

Phases

Handshake

Authentication and establishment of cryptographic algorithms and key material

Data Exchange

Exchange protected data

Connection teardown

Disconnect safely