Is a MIT licensed cryptographical library develop by Satoshi Labs. The algorithms are heavily optimized for use in embedded devices which may lack necessary memory, CPU power or both for conventional/naive implementations of algorithms.
List of features is here:
- AES/Rijndael encryption/decryption
 - Big Number (256 bit) Arithmetics
 - BIP32 Hierarchical Deterministic Wallets
 - BIP39 Mnemonic code
 - ECDSA signing/verifying (supports secp256k1 and nist256p1 curves, uses RFC6979 for deterministic signatures)
 - ECDSA public key derivation
 - Base32 (RFC4648 and custom alphabets)
 - Base58 address representation
 - Ed25519 signing/verifying (also SHA3 and Keccak variants)
 - ECDH using secp256k1, nist256p1 and Curve25519
 - HMAC-SHA256 and HMAC-SHA512
 - PBKDF2
 - RIPEMD-160
 - SHA1
 - SHA2-256/SHA2-512
 - SHA3/Keccak
 - BLAKE2s/BLAKE2b
 - Chacha20-Poly1305
 - unit tests (using Check - check.sf.net; in test_check.c)
 - tests against OpenSSL (in test_openssl.c)
 - integrated Wycheproof testsi
 
Relevant links: https://github.com/trezor/trezor-crypto https://github.com/trezor/trezor-firmware/tree/master/crypto