covertDTLS

covertDTLS is a library inspired by uTLS for offering fingerprint-resistance features to pion/dtls.

Why does this library exists?

The censorship circumvention system Snowflake has previously been blocked by fingerprinting the DTLS handshake. This library is a module that extends the pion/dtls library by hooking and manipulating handshake messages to make them indistinguishable from other DTLS implementations used for WebRTC traffic.

Fingerprint generation

This repo contains a workflow (.github/workflows/fingerprint.yaml) for automatically generating fresh DTLS handshakes (fingerprints) of new browser versions (Firefox and Chrome) by using a minimal WebRTC example application and Selenium. Fresh handshakes are captured each day and stored as pcap artifacts and the fingerprints-captures directory. The pcaps are further parsed and a fingerprint is added to pkg/mimicry/fingerprints.go

main.go contains a script for parsing pcaps, extracting the fingerprints and adding them to pkg/mimicry/fingerprints.go

Validation

This library was developed as part of a Master thesis: "Reducing distinguishability of DTLS for usage in Snowflake". Additionally, dfind was created for analyzing and finding passive field-based fingerprints of DTLS. dfind was used to validate this library, finding that mimicked ClientHello messages was indistinguishable from the fresh browser handshakes . Analysis also found that randomization of extensions was especially effective against fingerprinting, while randomization of ciphers has potential, but must be configured properly. To provide more effective randomization, it is recommended to use this library with configuring as many supported ciphers as possible (using Config.CipherSuites).

Features

• Mimicking/replaying ClientHello
• Randomization of ClientHello
  • cipher suites: shuffle and random size
  • extensions: shuffle
  • use_srtp: shuffle and random size
  • supported_groups: shuffle and random size
  • signature_algorithm: shuffle and random size
  • ALPN: add random ALPN of common protocols

Note: using these features might make handshakes unstable as unsupported features might be announced in the ClientHello message.

Planned

• Mimicking ServerHello
• Mimicking CertificateRequest

Examples

Mimicry

import  (
  "github.com/pion/dtls/v2"
  "github.com/theodorsm/covert-dtls/pkg/fingerprints"
  "github.com/theodorsm/covert-dtls/pkg/mimicry"
)

// Get a specific fingerprint
fingerprint := fingerprints.Mozilla_Firefox_125_0_1

clientHello := mimicry.MimickedClientHello{}

// If no specific fingerprint is loaded, the most recent one will be used
clientHello.LoadFingerprint(fingerprint)

cfg := &dtls.Config{
    // SRTP needs to be enabled as the fingerprints are from webrtc traffic, thus containing the use_srtp extension.
    SRTPProtectionProfiles: []dtls.SRTPProtectionProfile{dtls.SRTP_AES128_CM_HMAC_SHA1_80, dtls.SRTP_AES128_CM_HMAC_SHA1_32, dtls.SRTP_AEAD_AES_128_GCM, dtls.SRTP_AEAD_AES_256_GCM},
    ClientHelloMessageHook: clientHello.Hook,
}

// Use config with connection...

Randomization

import  (
  "github.com/pion/dtls/v2"
  "github.com/theodorsm/covert-dtls/pkg/randomize"
)

clientHello := randomize.RandomizedMessageClientHello{RandomALPN: true}

cfg := &dtls.Config{
    // Enable all ciphers for making randomization more effective. Optional step.
    CipherSuites: randomize.DefaultCipherSuites() ,
    ClientHelloMessageHook: clientHello.Hook,
}

// Use config with connection...