The Subliminal Handshake: Hiding Data in TLS Signatures | John Mathot | BSides312 2026

Target Audience: anyone interested in cryptography, privacy, covert data exchange, and how digital signatures are used in transport layer security. Takeaways: 1. A 'subliminal channel' is a way to hide data inside digital signatures while preserving the validity of the signature. They exist in many digital signing algorithms. 2. These subliminal channels can be used to send data wherever signed messages are exchanged, and signatures that contain subliminal data are indistinguishable from those that do not. 3. Using a subliminal channel, a TLS 1.3 handshakes can contain command and control messages inside the signatures sent between the server and client, and this traffic will be indistinguishable from a normal TLS 1.3 handshake. --- About John Mathot: Security Researcher I'm a security professional with a background in physical security and penetration testing. I currently do bug bounties and independent security research. Recorded live at BSides312 2026. --- Chapters: 0:00 Introduction and Outline 0:48 About John 1:17 What "Subliminal" Means Here 1:53 A Live Example: Signing "Hello World" 2:49 Proving the Signature Is Actually Valid 3:07 "Follow the White Rabbit" 3:47 Defining a Cryptographic Subliminal Message 4:01 Where They're Found, and What They Could Hide In 4:37 A Botnet Taking Orders Over a Blockchain 5:05 How Edwards-Curve DSA Signatures Work 5:56 The Small r Value and the Nonce 6:55 The PlayStation 3 Root Key 7:38 Deliberately Ignoring RFC 8032 8:21 Recovering the Message 8:44 The Second Channel: When You Don't Have the Key 9:32 Recap: Broadband vs. Narrowband 10:38 Part Two: The TLS Handshake 11:13 A Fast TLS 1.3 Review 11:53 Prior Art, and Why It's Easy to Detect 13:11 Why CertificateVerify Is Ideal 14:17 The Implementation Problem 15:28 Patching wolfSSL 16:12 What It Looks Like Running 16:33 77 Handshakes to Exchange Keys 17:16 The Packet Capture 18:02 The whoami Command, Decoded 19:02 Detection, Proxies, and Limits 20:06 Implications and Closing Thoughts 21:44 Q&A --- Mentioned in this talk: - Ed25519 / EdDSA - RFC 8032 - wolfSSL - OpenSSL - TLS 1.3 CertificateVerify Full BSides312 2026 playlist: https://www.youtube.com/playlist?list=PLZnyzZE8j8jw