How do you lock in a choice now but reveal it only later — without being able to change it, and without giving it away early? Hash your value together with a random blinder and publish the hash: that’s the sealed envelope. It HIDES the value (the hash tells nobody), yet BINDS you (you can’t find a different value that hashes the same). Later you open it and everyone checks. It’s the backbone of fair coin-flips, sealed bids, and zero-knowledge. Slide to commit, then reveal.
A commitment scheme is a cryptographic sealed envelope with two properties: HIDING (the commitment reveals nothing about the value) and BINDING (you cannot open it to a different value). The simplest is hash-based: commit c=H(m ‖ r) with a random blinder r; later reveal (m,r) and anyone verifies H(m ‖ r)=c. Hiding comes from r making the input unguessable; binding from the hash’s collision resistance (finding m′≠m with the same c is infeasible — see [[the-birthday-attack]]). It underpins fair coin-flipping, sealed-bid auctions, verifiable secret sharing, and [[the-zero-knowledge|zero-knowledge]] proofs. A fail-loud self-check throws unless a correct reveal verifies and a tampered value fails. ◆ real cryptography, node-verified.
Illustrated with a TOY hash; real schemes use SHA-256 (computationally binding) or Pedersen commitments (information-theoretically hiding, homomorphic). Hiding requires a truly random blinder r. The commit/reveal/verify logic is exact.