Re Implementing Zero Knowledge Proof For Authentication

---
name: re-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.
domain: re
category: cryptography
hide: true
license: Apache-2.0
metadata:
  manifest_entry: src/corpus/re-implementing-zero-knowledge-proof-for-authentication/SKILL.md
---
# 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.


## When to Use

- When deploying or configuring implementing zero knowledge proof for authentication capabilities in your environment
- When establishing security controls aligned to compliance requirements
- When building or improving security architecture for this domain
- When conducting security assessments that require this implementation

## Common Misconfigurations & Verification

- **Predictable challenge / nonce reuse:** in Schnorr, reusing the commitment randomness `r` across two challenges leaks the secret `x` via `x = (s1 - s2)/(c1 - c2)`. Generate `r` from a CSPRNG per proof and never reuse it. Verify two runs produce different `t` values.
- **Weak Fiat-Shamir transcript:** the non-interactive challenge must hash the *full* transcript including the public key and commitment (`c = H(g, y, t, msg)`), not just `t` — otherwise proofs are forgeable or transferable. Use a collision-resistant hash.
- **Verifier-chosen challenge skipped / replay:** without binding to a fresh server nonce or message, a captured transcript can be replayed. Include a server-supplied challenge or message and reject duplicates.
- **Insecure group parameters:** use a safe prime `p` and prime-order subgroup `q`; validate that `y` lies in the correct subgroup to avoid small-subgroup attacks.
- **Secret transmitted or logged:** confirm the server never receives or stores `x`.
- **Mandatory tests:** (1) honest prover always verifies (**completeness**); (2) a random/forged response without `x` **fails** verification (**soundness**); (3) the server transcript never contains the secret (**zero-knowledge**); (4) repeated authentications yield distinct transcripts; (5) a replayed transcript is rejected.

## Prerequisites

- Familiarity with cryptography concepts and tools
- Access to a test or lab environment for safe execution
- Python 3.8+ with required dependencies installed
- Appropriate authorization for any testing activities

## 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