9.4 Digital signatures in TLS 1.3
To agree upon the signature algorithms they want to use during their TLS session, Alice and Bob use two TLS 1.3 extensions. The algorithms for verifying digital signatures in certificates – a topic we will cover in detail in Chapter 10, Digital Certificates and Certification Authorities – are transmitted in the signature˙algorithms˙cert extension. The algorithms for digital signatures in CertificateVerify messages are transmitted in the signature˙algorithms extension.
Oftentimes, the set of digital signature algorithms that server Alice implements is different from the one that client Bob implements. The purpose of the signature˙algorithms˙cert extension is to allow Alice and Bob to communicate the algorithms they support and, as a result, determine which algorithms they can use for their TLS session.
If Alice and Bob omit the signature˙algorithms˙cert extension, then the signature algorithms...
