cryptosystem

A cryptosystem is a formal framework that uses cryptographic algorithms, protocols, keys, and procedures to secure information and communications. It is designed to provide security services such as confidentiality, integrity, authenticity, and non-repudiation, depending on the intended use. A cryptosystem is implemented through a combination of mathematical primitives, software, and hardware, and its security relies on both algorithmic hardness and correct operation of its components.

Cryptosystems are commonly classified as symmetric or asymmetric. In symmetric systems, the same key encrypts and decrypts data, with examples such as AES and ChaCha20-Poly1305. In asymmetric systems, a key pair enables public-key encryption, digital signatures, and key exchange; examples include RSA and elliptic-curve cryptography. Hybrid schemes combine both approaches, using public-key methods to exchange a symmetric session key, then fast symmetric encryption for data.

Components of a cryptosystem typically include the encryption algorithm, a key management scheme, and a protocol that governs how keys are generated, distributed, stored, and retired. Secure operation also requires appropriate modes of operation, hash functions, and, for signatures, verification procedures. The security of a cryptosystem depends on the strength of the underlying algorithms, the size of the keys, and disciplined key management.

Security is defined with formal models that specify adversary capabilities, such as IND-CPA and IND-CCA for encryption, or EUF-CMA for signatures. Real-world deployments are subject to implementation flaws, side-channel attacks, network protocol weaknesses, and poor operational practices. Cryptosystems should be designed with cryptographic agility to adapt to new standards and threat models.

Examples of widely deployed cryptosystems include TLS for securing web traffic, PGP/GPG and S/MIME for email, SSH for remote login, and various digital signature standards. Standards bodies such as NIST, ISO, and the IETF publish algorithms, key sizes, and protocols to guide interoperability and security assurance.