2016-fifield-fingerprintability
findings extracted from this paper
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Snowflake exclusively uses WebRTC data channels (on-wire protocol: DTLS), whereas the majority of WebRTC applications use media channels (DTLS-SRTP or SRTP/SDES); a censor can therefore block Snowflake by filtering data-channel flows alone without blocking WebRTC media applications, incurring minimal collateral damage and reducing the overblocking deterrent.
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The authors extend Houmansadr et al.'s 'parrot is dead' argument to WebRTC: because WebRTC is a large multi-protocol framework, superficial mimicry that fails to replicate exact DTLS version, cipher suite ordering, certificate common name ('WebRTC'), 30-day validity period, STUN server selection, and ICE packet sequence leaves detectable residual distinguishers, making deep fingerprint conformance especially hard for standalone non-browser implementations such as Snowflake's client.
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Among the five WebRTC applications analyzed (Google Hangouts, Facebook Messenger, OpenTokRTC, Sharefest, Snowflake), Snowflake is uniquely identifiable by its use of DTLSv1.2 (all others use DTLSv1.0), its 17 offered cipher suites, and its exclusive selection of TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256—a cipher suite not chosen by any other application in the study.
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STUN and TURN packets carry a SOFTWARE attribute that explicitly names the server implementation (e.g., 'Citrix-3.2.5.1 Marshal West' for OpenTokRTC), and the choice of STUN servers, forced-TURN usage, and STUN message-type sequence (Binding-only vs. Allocate+CreatePermission vs. send-indication) differ across applications, providing a passive censor with reliable application-level fingerprints orthogonal to the DTLS layer.
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A DTLS fingerprinting script run on one full day of network traffic at Lawrence Berkeley National Laboratory found only 7 DTLS handshakes with 3 unique client fingerprints and 3 unique server fingerprints, suggesting there may not be enough naturally occurring WebRTC traffic to provide meaningful cover for a WebRTC-based circumvention system.