ClientHelloviestillä

ClientHelloviestillä, or the ClientHello message, is the initial handshake message sent by a TLS client to a server to begin a secure connection. It advertises the client’s protocol versions, cryptographic capabilities, and other parameters that will shape the ensuing handshake and session keys. The server uses this information to decide on the protocol version and cipher suite for the session and to determine what extensions, if any, it will support.

In TLS 1.2 and earlier, the ClientHello typically contains a legacy_version field, a 32‑byte random value, a session_id, a list of supported cipher_suites, a list of compression_methods, and an extensions block. The extensions may carry data such as the server name (SNI), application layer protocol negotiation (ALPN), supported_groups (elliptic curves), and signature_algorithms. In TLS 1.3, the structure shifts toward more flexible negotiation, with fields like legacy_version and legacy_session_id retained for compatibility, and an emphasis on extensions such as supported_versions, supported_groups, and key_share to establish post‑handshake keys efficiently.

Security and privacy implications accompany the ClientHello. Because most of its payload is sent in the clear before encryption begins, observers can fingerprint clients by their offered cipher suites and extensions (a technique known as JA3 fingerprinting). The Server Name Indication (SNI) reveals the target host in many deployments, though efforts to encrypt or conceal client hello data (e.g., encrypted client hello) are under development or deployment in limited scenarios.

Overall, the ClientHelloviestillä is a core element of the TLS handshake, enabling the secure negotiation that follows.