uTLS is a fork of “crypto/tls”, which provides ClientHello fingerprinting resistance, low-level access to handshake, fake session tickets and some other features. Handshake is still performed by “crypto/tls”, this library merely changes ClientHello part of it and provides low-level access.
Low-level access to handshake
- Read/write access to all bits of client hello message.
- Read access to fields of ClientHandshakeState, which, among other things, includes ServerHello and MasterSecret.
- Read keystream. Can be used, for example, to “write” something in ciphertext.
ClientHello fingerprinting resistance
Golang’s ClientHello has a very unique fingerprint, which especially sticks out on mobile clients, where Golang is not too popular yet. Some members of anti-censorship community are concerned that their tools could be trivially blocked based on ClientHello with relatively small collateral damage. There are multiple solutions to this issue.
Randomized Fingerprints are supposedly good at defeating blacklists, since those fingerprints have random ciphersuites and extensions in random order. Note that all used ciphersuites and extensions are fully supported by uTLS, which provides a solid moving target without any compatibility or parrot-is-dead attack risks.
Fake Session Tickets
Fake session tickets is a very nifty trick that allows power users to hide parts of handshake, which may have some very fingerprintable features of handshake, and saves 1 RTT.