Dear Ibrahim,

zero-knowledge protocol is a method by which one party (the prover) can prove to another party (the verifier) that a given statement is true, without conveying any information apart from the fact that the statement is indeed true.  https://en.wikipedia.org/wiki/Zero-knowledge_proof

Some of the protocols are discussed at  https://en.wikipedia.org/wiki/Proof_of_knowledge

The attached file by Eli Biham describes more details. Also, you can refer OR protocol lecture by Yevgeniy Dodis in another attachment.

AR Reddy

Cramer Damgard Schoonmakers Crypto 94 proceedings, or DeSantis, Di Crescenzo, Persiano and Yung FOCS 94 deal with the or proofs etc. that may be useful (can be viewed as extending the so called sigma-protocols techniques).