cryptographie

Cryptographie, or cryptography, is the practice and study of techniques for secure communication in the presence of adversaries. It encompasses the design of cryptographic algorithms, protocols, and key management to achieve confidentiality, integrity, authenticity, and non-repudiation.

Core concepts include encryption, which transforms plaintext into ciphertext; symmetric cryptography uses the same key for encryption and decryption, while asymmetric (public-key) cryptography uses a public and a private key. Hash functions produce fixed-length digests and are used for data integrity and authentication. Digital signatures verify authorship and non-repudiation. Secure key exchange and randomness generation are essential components of practical systems.

History spans from ancient ciphers to modern mathematics-based cryptography. The 1970s brought public-key cryptography, enabling secure key exchange and digital signatures (notably Diffie-Hellman and RSA). Elliptic-curve cryptography provides comparable security with smaller key sizes. The field has evolved with advances in algorithms, cryptanalysis, and secure implementations.

Common algorithms and protocols include symmetric ciphers such as AES, public-key systems such as RSA and ECC, and hash functions such as SHA-256. Protocols like TLS, PGP, and SSH use these primitives to secure communications, emails, and remote access. Cryptographic standards and evaluations are maintained by organizations such as NIST and ISO.

Security considerations involve selecting appropriate algorithms and key lengths, ensuring correct implementation, defending against side-channel attacks, and adhering to standards. The security of modern systems relies on computational hardness assumptions and trustworthy randomness, with ongoing research into quantum-resistant approaches. Cryptography underpins secure communications, financial services, and data protection worldwide.