RSA
acronymcryptography
Stands for: Rivest-Shamir-Adleman
The classic public-key algorithm for signatures and, historically, key transport.
Rivest-Shamir-Adleman (RFC 8017) is a public-key cryptosystem based on the difficulty of factoring large numbers. In modern TLS it authenticates the server via its certificate; its use for key transport is deprecated because it lacks forward secrecy.
RSA was the first practical public key system and for decades it was the default answer to both key exchange and signatures. Its security rests on factoring being hard: multiplying two large primes is easy, recovering them from the product is not, and everything else follows from that asymmetry.
The reason it is receding is arithmetic rather than any break. Equivalent security requires much larger keys than elliptic curve alternatives, roughly 3072 bits of RSA to match a 256-bit curve, and the gap widens as you climb. Larger keys mean slower operations and bigger handshakes, which is why modern deployments have drifted to ECDSA and Ed25519 for signatures while RSA persists mostly in certificates and in systems too entrenched to change.
Two practical cautions outlive the algorithm choice. Textbook RSA, used without proper padding, is broken in ways that are easy to demonstrate, which is why OAEP and PSS exist and why nobody should implement the primitive directly. And factoring is exactly the problem a sufficiently large quantum computer would solve efficiently, so RSA sits squarely in the category of cryptography with a known expiry condition, which is what post-quantum migration planning is about.
Also known as: rsa