Applied Cryptography Vocabulary

AES-GCM combines the AES block cipher in Galois/Counter Mode to provide authenticated encryption with associated data (AEAD). It encrypts the plaintext for confidentiality and simultaneously produces an authentication tag that guarantees integrity and authenticity, so tampering is detected on decryption. GCM is fast and parallelizable. The critical rule is that a nonce (IV) must never be reused with the same key — nonce reuse catastrophically breaks both confidentiality and authentication. AES-GCM is the default AEAD in TLS 1.3 and most modern protocols.

RSA is a public-key (asymmetric) cryptosystem whose security rests on the practical difficulty of factoring the product of two large prime numbers. It uses a mathematically linked key pair: a public key for encryption or signature verification and a private key for decryption or signing. RSA is commonly used for key transport and digital signatures rather than bulk data, since it is slow compared with symmetric ciphers. Secure use requires proper padding (OAEP for encryption, PSS for signatures) and adequately large key sizes (2048 bits or more).

ECDSA (Elliptic Curve Digital Signature Algorithm) produces and verifies digital signatures using elliptic-curve mathematics. Compared with RSA it achieves equivalent security with much smaller keys — a 256-bit ECDSA key roughly matches a 3072-bit RSA key — yielding faster operations and shorter signatures. A signer uses their private key to sign a message hash; anyone can verify with the public key. A crucial implementation detail is that each signature needs a unique, unpredictable random nonce; reusing or leaking it exposes the private key, as several high-profile breaches have demonstrated.

An HMAC (Hash-based Message Authentication Code) combines a cryptographic hash function (like SHA-256) with a secret key to produce a fixed-size tag. Anyone holding the shared key can recompute the tag to verify that a message was not altered (integrity) and came from someone with the key (authenticity). Unlike a plain hash, HMAC's nested-key construction resists length-extension attacks. It is widely used for API request signing, JWT signatures (HS256), and verifying webhooks. Note that HMAC provides authentication, not confidentiality — the message itself is not encrypted.

An X.509 certificate is a standardized data structure that binds a public key to an identity (such as a domain name), signed by a trusted Certificate Authority. It includes the subject, issuer, validity period, and the CA's signature, forming the basis of TLS trust chains. Key rotation is the practice of periodically generating new keys and retiring old ones. It limits the blast radius if a key is compromised, reduces the amount of data exposed under any single key, and is often mandated by compliance. Automated rotation avoids outages from expired or leaked credentials.