MPT-Crypto: Cryptographic Primitives for Confidential Assets

Overview

MPT-Crypto is a specialized C library implementing the cryptographic building blocks for Confidential Multi-Purpose Tokens (MPT) on the XRP Ledger. It provides implementations of homomorphic encryption, aggregated range proofs, and specialized zero-knowledge proofs. The library is built on top of libsecp256k1 for elliptic curve arithmetic and OpenSSL for hashing and randomness.

Features

1. Confidential Balances (EC-ElGamal)

• Additive Homomorphic Encryption: Enables the ledger to aggregate encrypted balances (e.g., Enc(A) + Enc(B) = Enc(A+B)) without decryption.
• Canonical Zero: Deterministic encryption of zero balances to prevent ledger state bloat and ensure consistency.

2. Range Proofs (Bulletproofs)

• Aggregated Proofs: Supports proving that $m$ values are within the range $[0, 2^{64})$ in a single proof with logarithmic size $\mathcal{O}(\log n)$.
• Inner Product Argument (IPA): Implements the standard Bulletproofs IPA for succinct verification.
• Fiat-Shamir: Secure non-interactive challenge generation with strict domain separation.

3. Zero-Knowledge Proofs (Sigma Protocols)

• Plaintext Equality: Proves two or more ciphertexts encrypt the same amount under different keys.
• Linkage Proof: Proves consistency between an ElGamal ciphertext (used for transfer) and a Pedersen Commitment (used for the range proof).
• Proof of Knowledge (PoK): Proves ownership of the secret key during account registration to prevent rogue key attacks.

Building and Testing

Prerequisites

Before building, ensure you have the following installed:

• CMake (version 3.10 or higher)
• C Compiler (GCC, Clang, or AppleClang)
• OpenSSL 3.x (development headers and libraries)

On macOS with Homebrew:

brew install cmake openssl@3

On Ubuntu/Debian:

sudo apt-get install cmake libssl-dev build-essential

Dependency Setup

This library requires libsecp256k1 as a sibling directory. Clone it from the bitcoin-core repository:

# From the parent directory of mpt-crypto
cd ..
git clone https://github.com/bitcoin-core/secp256k1.git

Your directory structure should look like:

Projects/
├── mpt-crypto/
└── secp256k1/

Build Instructions

1. Create the build directory and configure:

   cd mpt-crypto
   mkdir -p build && cd build
   cmake ..

2. Build the library and tests:

   make -j4

Platform-Specific Notes

macOS (Apple Silicon): If you encounter architecture mismatch errors with OpenSSL, explicitly set the architecture:

cmake -DCMAKE_OSX_ARCHITECTURES=arm64 ..

macOS (Intel): Use x86_64 instead:

cmake -DCMAKE_OSX_ARCHITECTURES=x86_64 ..

Running Tests

After building, run the test suite using CTest:

cd build
ctest --output-on-failure

Or run individual tests directly:

./tests/test_elgamal
./tests/test_bulletproof_agg
./tests/test_commitments

Expected Results

The following tests should pass:

• test_bulletproof_agg - Aggregated Bulletproof range proofs
• test_commitments - Pedersen commitments
• test_elgamal - ElGamal encryption/decryption
• test_elgamal_verify - ElGamal verification
• test_equality_proof - Equality proofs
• test_ipa - Inner Product Argument (IPA) Core Logic
• test_link_proof - Linkage proofs
• test_pok_sk - Proof of knowledge of secret key
• test_same_plaintext - Same plaintext proofs
• test_same_plaintext_multi - Multi-recipient same plaintext proofs
• test_same_plaintext_multi_shared_r - Shared randomness variant

Note: test_bulletproof.c is excluded from the build because the aggregated implementation (bulletproof_aggregated.c) is fully general; verifying the m=1 case is now covered by test_bulletproof_agg.c.