XMSS Security API

Side-channel claim limit. Nothing on this page claims constant-time behaviour or side-channel resistance for any target: the side-channel harness covers nothing (finding A48) and no measurement exists. The Rust and C cores are designed for constant time and unmeasured; managed runtimes (JIT, GC, bounds checks) make no timing claim. Where this page names constant-time or side-channel techniques, in prose or in code, they are design intent or recommendations, not measured properties.

Version: 1.0.0
Last Updated: 2025-01-02
Security Classification: Stateful Hash-Based Signature
Author: MetaMUI Security Team

Overview

XMSS (eXtended Merkle Signature Scheme) is a stateful hash-based signature scheme providing post-quantum security with careful state management.

• Algorithm Family: Hash-based signature (stateful)
• Security Levels: 128/192/256-bit
• State Management: Critical for security
• Tree Height: Configurable (10, 16, 20)
• Standard: RFC 8391

Security Warnings ⚠️

1. STATE MANAGEMENT CRITICAL: Reusing state breaks security completely
2. One-Time Signatures: Each leaf can only be used once
3. Backup Considerations: State must be backed up atomically
4. No Parallelization: Signatures must be sequential

API Functions

Key Generation

def xmss_keygen(height: int, security_level: int) -> Tuple[PublicKey, SecretKey, State]:
    """Generate XMSS keypair with initial state"""

Signing (Stateful)

def xmss_sign(message: bytes, secret_key: SecretKey, state: State) -> Tuple[Signature, State]:
    """Sign message and update state (MUST save new state)"""

Verification (Stateless)

def xmss_verify(message: bytes, signature: Signature, public_key: PublicKey) -> bool:
    """Verify XMSS signature (stateless operation)"""

Security Best Practices

State Management Requirements

1. Atomic Updates: State must be updated atomically
2. No Rollback: Never revert to previous state
3. Secure Storage: Store state in secure, persistent storage
4. Backup Strategy: Implement secure state backup

Implementation Checklist

• Atomic state updates
• Persistent state storage
• State backup mechanism
• Recovery procedures
• Exhaustion detection

Common Vulnerabilities

1. State Reuse: Fatal security breach
2. Power Failure: Can cause state loss
3. Concurrent Access: Must serialize signing

Security Analysis

Threat Model: XMSS Threat Model

The comprehensive threat analysis covers:

• Algorithm-specific attack vectors
• Implementation vulnerabilities
• Side-channel considerations
• Quantum resistance analysis (where applicable)
• Deployment recommendations

For complete security analysis and risk assessment, see the dedicated threat model documentation.

References

1. RFC 8391: XMSS
2. Stateful Hash-Based Signatures

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