cryptosystems

Cryptosystems are structured sets of cryptographic algorithms and procedures designed to protect information. They specify how plaintext is converted to ciphertext and how it is recovered, how keys are generated and managed, and how additional properties such as authenticity and integrity are provided. A typical cryptosystem includes a key generation method, an encryption algorithm, a decryption algorithm, and often digital signing and verification algorithms, along with protocols for key distribution and revocation. Security in a cryptosystem is defined with respect to an adversarial model and is typically described in terms of confidentiality, integrity, authenticity, and non-repudiation.

Cryptosystems are broadly categorized as symmetric-key systems, where the same key is used for encryption and decryption, and public-key (asymmetric) systems, which use related but separate keys for encryption and decryption. Symmetric systems rely on algorithms such as block and stream ciphers (for example AES) and require secure key exchange through a separate channel or protocol. Public-key systems (for example RSA, ElGamal, and elliptic-curve variants) enable key exchange, encryption of small amounts of data, and digital signatures. In practice, many systems combine both approaches in hybrid schemes, using public-key operations to exchange a symmetric key and then a fast symmetric cipher for data.

Security is assessed under concrete hardness assumptions, such as factoring, discrete logarithms, or lattice problems, and may include proofs of security or reductions. Cryptosystems are standardized and audited and are used in a wide range of applications, including securing communications, ensuring data integrity, and enabling digital signatures.