Crypto glossary
BLS signature
A BLS signature is a scheme whose signatures can be combined, so thousands from different signers merge into one short, verifiable signature.
Named after Dan Boneh, Ben Lynn and Hovav Shacham, who described it in 2001, the scheme uses pairing-friendly elliptic curves, most commonly BLS12-381. Its key property is aggregation: many signatures on the same or different messages can be added together into a single signature, and the matching public keys can be combined to verify it in one step.
That makes BLS ideal for proof-of-stake consensus with large validator sets. Ethereum's consensus layer uses it so that attestations from hundreds of thousands of validators can be compressed into a few aggregates per block, which keeps bandwidth and verification manageable. Chia, Filecoin and several other networks also rely on it, and it is a building block for threshold signatures and distributed validator technology.
BLS verification is slower than ECDSA or Schnorr for a single signature, because pairings are expensive, so it pays off mainly when many signatures are aggregated. Naive aggregation is vulnerable to rogue-key attacks unless signers prove they own their keys. BLS signatures are not used for ordinary Ethereum user transactions, which still rely on ECDSA.
Example
In each Ethereum slot, thousands of validators in a committee sign the same attestation; an aggregator combines their BLS signatures into one that the block includes along with a bitfield of who signed.