Zilliqa employs Elliptic Curve Based Schnorr Signature Algorithm (EC-Schnorr) as the base signing algorithm. Schnorr allows for multisignatures, is faster than ECDSA, and has a smaller signature size (64 bytes).
The Schnorr algorithm was initially based on section 4.2.3 page 24 of version 1.0 of BSI TR-03111 Elliptic Curve Cryptography (ECC). A more complete discussion of the algorithm is also contained in the Zilliqa whitepaper.
The Schnorr algorithm is used during the consensus protocol, message signing, and generally anywhere where a signature is needed within the protocol. Zilliqa nodes are also identified by their Schnorr public keys, alongside their IP information.