libssh2 SSH Protocol Library

libssh2 is a client-side C library implementing the SSH2 protocol, providing a lightweight and portable API for secure shell operations. Key features: full SSH2 protocol implementation including authentication via password, public key, and keyboard-interactive methods. Supports SSH sessions with command execution, interactive shell via PTY allocation, and subsystem channels (SFTP). SFTP file transfer subsystem for secure file upload, download, directory listing, and file attribute management. Port forwarding including local, remote, and dynamic (SOCKS) forwarding. Public key authentication with RSA, DSA, ECDSA, and Ed25519 keys. Key exchange algorithms including Curve25519-SHA256, ECDH, and DH Group Exchange. Strong encryption with AES-GCM, ChaCha20-Poly1305, AES-CBC, and 3DES. Message Authentication Codes with HMAC-SHA256, HMAC-SHA512, and ETM modes. Compact C library with minimal dependencies, suitable for embedded systems and resource-constrained environments. Thread-safety considerations for multi-threaded applications. Cross-platform on Windows, macOS, Linux, and embedded RTOS platforms. Used by libcurl, PHP, Python paramiko internals, and many other SSH tools. Open source under BSD license.