v2fly/external/github.com/refraction-networking/utls/README.md

uTLS

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.
Golang 1.11+ is required.
If you have any questions, bug reports or contributions, you are welcome to publish those on GitHub. If you want to do so in private, you can contact one of developers personally via sergey.frolov@colorado.edu

Features

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 handshake

This package can generate randomized ClientHello using only extensions and cipherSuites "crypto/tls" already supports. This provides a solid moving target without any compatibility or parrot-is-dead attack risks.
Feedback about opinionated implementation details of randomized handshake is appreciated.

Parroting

This package can be used to parrot ClientHello of popular browsers. There are some caveats to this parroting:

• We are forced to offer ciphersuites and tls extensions that are not supported by crypto/tls. This is not a problem, if you fully control the server and turn unsupported things off on server side.
• Parroting could be imperfect, and there is no parroting beyond ClientHello.

Compatibility risks of available parrots

Parrot	Ciphers*	Signature*	Unsupported extensions	TLS Fingerprint ID
Chrome 62	no	no	ChannelID	0a4a74aeebd1bb66
Chrome 70	no	no	ChannelID, Encrypted Certs	bc4c7e42f4961cd7
Firefox 56	very low	no	None	c884bad7f40bee56
Firefox 63	very low	no	MaxRecordSize	6bfedc5d5c740d58
iOS 11.1	low**	no	None	71a81bafd58e1301

* Denotes very rough guesstimate of likelihood that unsupported things will get echoed back by the server in the wild, visibly breaking the connection.
** No risk, if utls.EnableWeakCiphers() is called prior to using it.

Parrots FAQ

Does it really look like, say, Google Chrome with all the GREASE and stuff?

It LGTM, but please open up Wireshark and check. If you see something — say something.

Aren't there side channels? Everybody knows that the bird is a wordparrot is dead

There sure are. If you found one that approaches practicality at line speed — please tell us.

Things to implement in Golang to make parrots better

uTLS is fundamentially limited in parroting, because Golang's "crypto/tls" doesn't support many things. Would be nice to have:

• ChannelID extension
• In general, any modern crypto is likely to be useful going forward.

Custom Handshake

It is possible to create custom handshake by

1. Use HelloCustom as an argument for UClient() to get empty config
2. Fill tls header fields: UConn.Hello.{Random, CipherSuites, CompressionMethods}, if needed, or stick to defaults.
3. Configure and add various TLS Extensions to UConn.Extensions: they will be marshaled in order.
4. Set Session and SessionCache, as needed.

If you need to manually control all the bytes on the wire(certainly not recommended!), you can set UConn.HandshakeStateBuilt = true, and marshal clientHello into UConn.HandshakeState.Hello.raw yourself. In this case you will be responsible for modifying other parts of Config and ClientHelloMsg to reflect your setup and not confuse "crypto/tls", which will be processing response from server.

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. Currently, there is a simple function to set session ticket to any desired state:

// If you want you session tickets to be reused - use same cache on following connections
func (uconn *UConn) SetSessionState(session *ClientSessionState)

Note that session tickets (fake ones or otherwise) are not reused.
To reuse tickets, create a shared cache and set it on current and further configs:

// If you want you session tickets to be reused - use same cache on following connections
func (uconn *UConn) SetSessionCache(cache ClientSessionCache)

Client Hello IDs

See full list of clientHelloID values here.
There are different behaviors you can get, depending on your clientHelloID:

1. utls.HelloRandomized adds/reorders extensions, ciphersuites, etc. randomly.
   HelloRandomized adds ALPN in 50% of cases, you may want to use HelloRandomizedALPN or HelloRandomizedNoALPN to choose specific behavior explicitly, as ALPN might affect application layer.
2. utls.HelloGolang HelloGolang will use default "crypto/tls" handshake marshaling codepath, which WILL overwrite your changes to Hello(Config, Session are fine). You might want to call BuildHandshakeState() before applying any changes. UConn.Extensions will be completely ignored.
3. utls.HelloCustom will prepare ClientHello with empty uconn.Extensions so you can fill it with TLSExtension's manually.
4. The rest will will parrot given browser. Such parrots include, for example:
   • utls.HelloChrome_Auto- parrots recommended(usually latest) Google Chrome version
   • utls.HelloChrome_58 - parrots Google Chrome 58
   • utls.HelloFirefox_Auto - parrots recommended(usually latest) Firefox version
   • utls.HelloFirefox_55 - parrots Firefox 55

Usage

Examples

Find basic examples here.
Here's a more advanced example showing how to generate randomized ClientHello, modify generated ciphersuites a bit, and proceed with the handshake.

Migrating from "crypto/tls"

Here's how default "crypto/tls" is typically used:

    dialConn, err := net.Dial("tcp", "172.217.11.46:443")
    if err != nil {
        fmt.Printf("net.Dial() failed: %+v\n", err)
        return
    }

    config := tls.Config{ServerName: "www.google.com"}
    tlsConn := tls.Client(dialConn, &config)
    n, err = tlsConn.Write("Hello, World!")
    //...

To start using using uTLS:

1. Import this library (e.g. import tls "github.com/refraction-networking/utls")
2. Pick the Client Hello ID
3. Simply substitute tlsConn := tls.Client(dialConn, &config) with tlsConn := tls.UClient(dialConn, &config, tls.clientHelloID)

Customizing handshake

Some customizations(such as setting session ticket/clientHello) have easy-to-use functions for them. The idea is to make common manipulations easy:

    cRandom := []byte{100, 101, 102, 103, 104, 105, 106, 107, 108, 109,
        110, 111, 112, 113, 114, 115, 116, 117, 118, 119,
        120, 121, 122, 123, 124, 125, 126, 127, 128, 129,
        130, 131}
    tlsConn.SetClientRandom(cRandom)
    masterSecret := make([]byte, 48)
    copy(masterSecret, []byte("masterSecret is NOT sent over the wire")) // you may use it for real security

    // Create a session ticket that wasn't actually issued by the server.
    sessionState := utls.MakeClientSessionState(sessionTicket, uint16(tls.VersionTLS12),
        tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
        masterSecret,
        nil, nil)
    tlsConn.SetSessionState(sessionState)

For other customizations there are following functions

// you can use this to build the state manually and change it
// for example use Randomized ClientHello, and add more extensions
func (uconn *UConn) BuildHandshakeState() error

// Then apply the changes and marshal final bytes, which will be sent
func (uconn *UConn) MarshalClientHello() error

Contributors' guide

Please refer to this document if you're interested in internals

Credits

The initial development of uTLS was completed during an internship at Google Jigsaw. This is not an official Google product.