Secrets kept by mathematics rather than by trust.
Blockchain is the public, distributed ledger Nakamoto built for bitcoin in 2008–2009. It uses cryptographic hashing, proof-of-work, and difficulty adjustment to prevent double-spending without central control. It works—but at high energy cost, low throughput, and contested governance.
AES is a narrow, standardised, symmetric cipher mandated by NIST in 2001. It works reliably only when implemented correctly. It solves no problems beyond confidentiality of bulk data under fixed parameters. It demands careful key handling and side-channel mitigation — which it does not provide.
RSA is the first publicly described, usable public-key cryptosystem. It works by exploiting the asymmetry between modular exponentiation and root extraction modulo a composite number whose factorisation is secret. Its security depends on factoring difficulty—but that dependence is not proven equivalent. No published attack breaks it with large keys. It changed cryptography by enabling open protocols, trustless key exchange, and standardised role names like Alice and Bob.
SFTP is a file protocol built for SSH-2. It assumes security is already solved. It delivers precise, stateful filesystem control. It never became an RFC. It remains widely deployed—not because it succeeded, but because alternatives failed.
ECC is a public-key method based on elliptic curves over finite fields. Proposed independently by Koblitz and Miller in 1985. Delivers equivalent security to RSA and ElGamal with smaller keys. Security rests on the hardness of the elliptic curve discrete logarithm problem — specifically, recovering scalar k from points P and Q = kP — and on efficient yet irreversible point multiplication for appropriate parameters.