SChannel

Schannel, short for Secure Channel, is the Windows security support provider (SSP) that implements TLS and SSL for network communications. It is part of the Security Support Provider Interface (SSPI) and provides the cryptographic operations, certificate handling, and session management required to establish secure connections between clients and servers running Windows.

Schannel supports a range of TLS/SSL protocols, depending on the operating system version, and the cipher suites that are permitted by policy. It relies on the Windows certificate stores to locate trusted root and personal certificates, performs certificate validation and chain building, and can support mutual TLS authentication. It handles the TLS handshake, session resumption, and record-layer encryption, and it can be used by both client and server applications.

Applications on Windows typically interact with Schannel through the Security Support Provider Interface (SSPI) by calling functions such as AcquireCredentialsHandle and InitializeSecurityContext. Common Windows components, including IIS and HTTP.sys, .NET network libraries, and various VPN and email servers, use Schannel to provide HTTPS, FTPS, SMTPS, and other TLS-based protocols. It is loaded as a system-wide security package named Schannel and is integrated with the Windows certificate store and policy framework.

Configuration and security considerations: Schannel behavior can be configured via registry keys under HKLM\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL, including protocol enablement, cipher suites, and certificate validation settings. Best practices include keeping the operating system and Schannel up to date, disabling outdated protocols such as TLS 1.0/1.1 where possible, and enforcing strong ciphers and certificate validation.