Monocypher
Single-file C crypto library, ~2000 LoC. Loup Vaillant. Designed for embedded systems where libsodium is too large. ChaCha20, Poly1305, Ed25519, Argon2, X25519.
Headers-only deployment: copy monocypher.c + .h into your project. No build deps, no dynamic-link issues. Constant-time, audited. Use case: firmware on microcontrollers (ESP32, STM32), embedded Linux where binary size counts. Trade-off vs libsodium: fewer features (no crypto_box-style helper), must compose primitives yourself. Modern audit by Cure53 (2017) found no issues. Classic choice in Rust ecosystem: rust-crypto via RustCrypto org.