XLS-??d Quantum-Resistant Signatures

Title: Quantum-Resistant Signatures
Revision: 1 (2025-07-08)
Type: Draft
Author:
    Atharva Lele, Trinity College Dublin
    Denis Angell, XRPL Labs

Abstract

This proposal introduces quantum-resistant digital signatures to the XRP Ledger (XRPL) using the Dilithium post-quantum cryptographic algorithm. The amendment provides accounts with the ability to use quantum-resistant signatures for enhanced security against future quantum computing threats while maintaining backward compatibility with existing signature schemes.

Motivation and Rationale

As quantum computing advances, current cryptographic signatures (secp256k1, ed25519) may become vulnerable to quantum attacks. This proposal adds support for Dilithium, a NIST-standardized post-quantum signature algorithm, ensuring long-term security for XRPL accounts.

Amendment

This feature enables accounts to use quantum-resistant signatures with an optional enforcement mechanism.

The amendment adds:

• Support for Dilithium signature algorithm (KeyType::dilithium = 2)

• New account flag lsfForceQuantum to enforce quantum-resistant signatures

• Updated key generation, encoding, and verification systems

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Development Branch

Implementation Repository

The quantum-resistant signatures implementation is currently under active development in the following branch:

Repository: Transia-RnD/rippled Branch: dilithium-full

Development Status

This branch contains the working implementation of the quantum-resistant signature system, including:

• Core Dilithium Integration: Implementation of the Dilithium post-quantum signature algorithm

• Key Management Updates: Modified key generation, storage, and retrieval systems

• Signature Verification: Updated transaction signing and verification processes

• Account Flag Implementation: lsfForceQuantum flag enforcement mechanisms

• Backward Compatibility: Maintained support for existing signature schemes

Testing and Validation

The quantum branch includes:

• Unit tests for Dilithium key operations

• Integration tests for quantum-resistant transaction processing

• Performance benchmarks comparing signature verification times

• Compatibility tests ensuring existing functionality remains intact

Contributing

Developers interested in contributing to the quantum-resistant signatures implementation should:

1. Fork the repository and checkout the quantum branch

2. Review the existing implementation and test coverage

3. Submit pull requests against the quantum branch

4. Ensure all tests pass and maintain backward compatibility

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Implementation Details

Key Specifications

Aspect	secp256k1	ed25519	Dilithium
Public Key Size	33 bytes	33 bytes	1312 bytes
Secret Key Size	32 bytes	32 bytes	2528 bytes
Signature Size	~70 bytes	64 bytes	~2420 bytes
Security Level	128-bit	128-bit	128-bit (quantum-resistant)

Key Generation

// Generate quantum-resistant keys
auto keyPair = generateKeyPair(KeyType::dilithium, seed);
auto secretKey = randomSecretKey(KeyType::dilithium);

Public Key Detection

std::optional<KeyType> publicKeyType(Slice const& slice) {
    if (slice.size() == 33) {
        if (slice[0] == 0xED) return KeyType::ed25519;
        if (slice[0] == 0x02 || slice[0] == 0x03) return KeyType::secp256k1;
    }
    else if (slice.size() == CRYPTO_PUBLICKEYBYTES) {
        return KeyType::dilithium;  // 1312 bytes
    }
    return std::nullopt;
}

---

Account Flag: Force Quantum Signatures

lsfForceQuantum Flag

Field	Value	Description
lsfForceQuantum	0x02000000	When set, account requires quantum-resistant signatures
asfForceQuantum	11	AccountSet flag to enable/disable quantum requirement

Usage

{
  "TransactionType": "AccountSet",
  "Account": "rAccount...",
  "SetFlag": 11  // Enable quantum-only signatures
}

Enforcement

if (account.isFlag(lsfForceQuantum) && publicKey.size() != DILITHIUM_PK_SIZE)
    return telBAD_PUBLIC_KEY;

---

Signature Operations

Signature Generation

case KeyType::dilithium: {
    uint8_t sig[CRYPTO_BYTES];
    size_t len;
    crypto_sign_signature(sig, &len, message.data(), message.size(), secretKey.data());
    return Buffer{sig, len};
}

Signature Verification

if (keyType == KeyType::dilithium) {
    return crypto_sign_verify(
        sig.data(), sig.size(), 
        message.data(), message.size(), 
        publicKey.data()) == 0;
}

---

Migration Strategy

Gradual Adoption

1. Optional Phase: Quantum signatures available but not required

2. Account Choice: Individual accounts can enable lsfForceQuantum

3. Network Transition: Networks can mandate quantum signatures over time

Backward Compatibility

• Existing accounts continue using current signature types

• No breaking changes to existing functionality

• Smooth upgrade path for enhanced security

---

Error Codes

Error Code	Description
telBAD_PUBLIC_KEY	Non-quantum signature used with lsfForceQuantum account

---

Future Requirements

Validator Infrastructure Updates

As quantum-resistant signatures become standard, several validator-related components will require updates:

Validator Code Updates

• rippled: Core validator software must support quantum-resistant key generation and signature verification

• Consensus Algorithm: Ensure quantum-resistant signatures are properly validated during consensus

• Peer Communication: Update peer-to-peer communication to handle larger quantum signatures

UNL (Unique Node List) Generation

• UNL Tools: Update UNL generation tools to support quantum-resistant validator keys

• Key Format: Modify UNL file format to accommodate larger Dilithium public keys (1312 bytes)

• Validation: Ensure UNL validation processes can verify quantum-resistant signatures

validator-keys Repository

• Key Generation: Update validator-keys tool to generate Dilithium key pairs

• Key Management: Modify key storage and management for larger quantum keys

• Migration Tools: Provide utilities for existing validators to transition to quantum-resistant keys

• Documentation: Update validator setup guides for quantum key generation

Network Transition Considerations

• Phased Rollout: Gradual migration of validators to quantum-resistant keys

• Backward Compatibility: Maintain support for existing validator keys during transition

• Performance Impact: Account for increased signature verification time and bandwidth usage

---

Dependencies

• Dilithium Library: pq-crystals/dilithium reference implementation

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Example Usage

Generate Quantum-Resistant Keys

// From seed
auto seed = generateSeed("masterpassphrase");
auto keyPair = generateKeyPair(KeyType::dilithium, seed);

// Random generation
auto secretKey = randomSecretKey(KeyType::dilithium);
auto publicKey = derivePublicKey(KeyType::dilithium, secretKey);

Enable Quantum-Only Account

{
  "TransactionType": "AccountSet",
  "Account": "rQuantumAccount...",
  "SetFlag": 11
}

Sign Transaction with Quantum Key

auto signature = sign(publicKey, secretKey, transactionData);
bool isValid = verify(publicKey, transactionData, signature);