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TLS Cryptography In-Depth
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Equipped with the mathematical background on public-key cryptography, we can now explore how it is applied in TLS 1.3.
In the previous chapter, we learned that client Bob and server Alice exchange messages during the TLS handshake phase to establish all parameters necessary for deriving the TLS secrets and, subsequently, the TLS keys.
Technically, these cryptographic parameters are transmitted in the key˙share
extension of the ClientHello
, ServerHello
and, if needed, HelloRetryRequest
messages.
In a ClientHello
message, the key˙share
extension – more precisely, its extension˙data
field – contains a value called KeyShareClientHello
defined as follows:
struct {
KeyShareEntry client_shares<0..2^16-1>;
} KeyShareClientHello;
The client˙shares
variable is a list of key shares offered by client Bob to server Alice in the initial ClientHello...
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