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.

Minimum Go Version: Go 1.21

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~~.

You can contact one of developers personally via gaukas.wang@colorado.edu.

Documentation below may not keep up with all the changes and new features at all times, so you are encouraged to use godoc.

Note: Information provided below in this README.md could be obsolete. We welcome any contributions to refresh the documentations in addition to code contributions.

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.

It is highly recommended to use multiple fingeprints, including randomized ones to avoid relying on a single fingerprint. utls.Roller does this automatically.

Randomized Fingerprint

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.

But note that there's a small chance that generated fingerprint won't work, so you may want to keep generating until a working one is found, and then keep reusing the working fingerprint to avoid suspicious behavior of constantly changing fingerprints. utls.Roller reuses working fingerprint automatically.

Generating randomized fingerprints

To generate a randomized fingerprint, simply do:

uTlsConn := tls.UClient(tcpConn, &config, tls.HelloRandomized)

you can use helloRandomizedALPN or helloRandomizedNoALPN to ensure presence or absence of ALPN(Application-Layer Protocol Negotiation) extension. It is recommended, but certainly not required to include ALPN (or use helloRandomized which may or may not include ALPN). If you do use ALPN, you will want to correctly handle potential application layer protocols (likely h2 or http/1.1).

Reusing randomized fingerprint

// oldConn is an old connection that worked before, so we want to reuse it
// newConn is a new connection we'd like to establish
newConn := tls.UClient(tcpConn, &config, oldConn.ClientHelloID)

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 | | Chrome 72 | no | no | ChannelID, Encrypted Certs | bbf04e5f1881f506 | | Chrome 83 | no | no | ChannelID, Encrypted Certs | 9c673fd64a32c8dc | | Firefox 56 | very low | no | None | c884bad7f40bee56 | | Firefox 65 | very low | no | MaxRecordSize | 6bfedc5d5c740d58 | | iOS 11.1 | low** | no | None | 71a81bafd58e1301 | | iOS 12.1 | low** | no | None | ec55e5b4136c7949 |

* 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 word~~parrot is dead

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

However, there is a difference between this sort of parroting and techniques like SkypeMorth. Namely, TLS is highly standardized protocol, therefore simply not that many subtle things in TLS protocol could be different and/or suddenly change in one of mimicked implementation(potentially undermining the mimicry). It is possible that we have a distinguisher right now, but amount of those potential distinguishers is limited.

Custom Handshake

It is possible to create custom handshake by

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

Fingerprinting Captured Client Hello

You can use a captured client hello to generate new ones that mimic/have the same properties as the original. The generated client hellos should look like they were generated from the same client software as the original fingerprinted bytes. In order to do this:

Create a ClientHelloSpec from the raw bytes of the original client hello
Use HelloCustom as an argument for UClient() to get empty config
Use ApplyPreset with the generated ClientHelloSpec to set the appropriate connection properties

uConn := UClient(&net.TCPConn{}, nil, HelloCustom)
fingerprinter := &Fingerprinter{}
generatedSpec, err := fingerprinter.FingerprintClientHello(rawCapturedClientHelloBytes)
if err != nil {
  panic("fingerprinting failed: %v", err)
}
if err := uConn.ApplyPreset(generatedSpec); err != nil {
  panic("applying generated spec failed: %v", err)
}

The rawCapturedClientHelloBytes should be the full tls record, including the record type/version/length header.

Roller

A simple wrapper, that allows to easily use multiple latest(auto-updated) fingerprints.

// NewRoller creates Roller object with default range of HelloIDs to cycle
// through until a working/unblocked one is found.
func NewRoller() (*Roller, error)

// Dial attempts to connect to given address using different HelloIDs.
// If a working HelloID is found, it is used again for subsequent Dials.
// If tcp connection fails or all HelloIDs are tried, returns with last error.
//
// Usage examples:
//
// Dial("tcp4", "google.com:443", "google.com")
// Dial("tcp", "10.23.144.22:443", "mywebserver.org")
func (c *Roller) Dial(network, addr, serverName string) (*UConn, error)

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)

Utls

Install / Use

README