This part of ISO/IEC 9798 specifies entity authentication mechanisms using zero-knowledge techniques. &63719; Clause 5 specifies mechanisms (already present in the first edition, ISO/IEC 9798-4:1999) based on identities and providing unilateral authentication. They have been repaired after the withdrawal of ISO/IEC 9796:1991. &63719; Clause 6 specifies mechanisms (inserted in this second edition) based on integer factorization and providing unilateral authentication, etc.