Initial commit - 611 cybersecurity skills across all subdomains

skills/implementing-zero-knowledge-proof-for-authentication/SKILL.md

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+---
+name: implementing-zero-knowledge-proof-for-authentication
+description: Zero-Knowledge Proofs (ZKPs) allow a prover to demonstrate knowledge of a secret (such as a password or private key) without revealing the secret itself. This skill implements the Schnorr identificati
+domain: cybersecurity
+subdomain: cryptography
+tags: [cryptography, zero-knowledge-proof, authentication, privacy, zkp]
+version: "1.0"
+author: mahipal
+license: MIT
+---
+# Implementing Zero-Knowledge Proof for Authentication
+
+## Overview
+
+Zero-Knowledge Proofs (ZKPs) allow a prover to demonstrate knowledge of a secret (such as a password or private key) without revealing the secret itself. This skill implements the Schnorr identification protocol and a simplified ZKPP (Zero-Knowledge Password Proof) using the discrete logarithm problem, enabling authentication where the server never learns the user's password.
+
+## Objectives
+
+- Implement Schnorr's identification protocol for ZKP authentication
+- Build a non-interactive ZKP using Fiat-Shamir heuristic
+- Implement zero-knowledge password proof (ZKPP)
+- Demonstrate completeness, soundness, and zero-knowledge properties
+- Compare ZKP authentication with traditional password verification
+
+## Key Concepts
+
+### ZKP Properties
+
+| Property | Description |
+|----------|------------|
+| Completeness | Honest prover always convinces honest verifier |
+| Soundness | Dishonest prover cannot convince verifier (except negligible probability) |
+| Zero-Knowledge | Verifier learns nothing beyond the statement's truth |
+
+### Schnorr Protocol
+
+1. **Setup**: Public generator g, prime p, q (order of g)
+2. **Registration**: Prover computes y = g^x mod p (public key from secret x)
+3. **Commitment**: Prover sends t = g^r mod p (random r)
+4. **Challenge**: Verifier sends random c
+5. **Response**: Prover sends s = r + c*x mod q
+6. **Verify**: Check g^s == t * y^c mod p
+
+## Security Considerations
+
+- Use cryptographically secure random number generators
+- Challenge must be unpredictable (from verifier's perspective)
+- For non-interactive proofs, use Fiat-Shamir with collision-resistant hash
+- ZKP alone does not provide forward secrecy; combine with TLS
+
+## Validation Criteria
+
+- [ ] Honest prover always verifies successfully (completeness)
+- [ ] Random response without secret does not verify (soundness)
+- [ ] Server never receives the secret value
+- [ ] Non-interactive proof is verifiable offline
+- [ ] Multiple authentications produce different transcripts
+- [ ] Protocol resists replay attacks