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Hardware Wallet Integration with sigma-rust#

Introduction#

Integrating Ergo support into hardware wallets (like Ledger, Trezor, Keystone, etc.) presents unique challenges due to the resource-constrained nature of these devices (limited memory, processing power, and no standard library/OS). The standard sigma-rust library, while comprehensive, relies on Rust's standard library (std) and certain dependencies that might be too heavy or unsuitable for embedded environments.

This guide provides developers with pointers and strategies for adapting sigma-rust functionality for hardware wallet integration, based on community discussions and efforts.

Key Challenges#

  • No std: Hardware wallets typically run bare-metal or on a minimal RTOS, lacking Rust's standard library (std). Code must be compatible with no_std.
  • Limited Resources: Memory (RAM and flash) and CPU cycles are scarce. Libraries and cryptographic operations must be efficient.
  • Dependency Bloat: Large dependencies can quickly exceed storage limits.
  • Cryptographic Primitives: Hardware wallets often have optimized, built-in implementations of core cryptographic primitives (like secp256k1) that should ideally be leveraged instead of pulling in separate library implementations.

Adapting sigma-rust: Strategies and Tools#

1. Using sigma-rust-mini and no_std#

A community fork/effort, often referred to as sigma-rust-mini, aims to provide a no_std-compatible subset of sigma-rust.

  • Repository (Example Fork): github.com/Alesfatalis/sigma-rust-mini/tree/no_std (Note: Check for the latest official or community-maintained versions).
  • no_std Feature Flag: When using such forks or potentially future versions of sigma-rust, look for a no_std feature flag in Cargo.toml to enable compatibility. This typically excludes parts of the library relying on std.

2. Replacing Cryptographic Backends (k256 vs. secp256k1)#

sigma-rust often uses the k256 crate for secp256k1 operations. Hardware wallets usually have their own optimized secp256k1 implementations (often using the secp256k1 crate or a C library). To avoid duplicate code and leverage hardware optimizations, you'll likely need to:

  • Fork sigma-rust (or sigma-rust-mini): Modify the necessary parts of the library to use the hardware wallet's preferred secp256k1 backend instead of k256.
  • Focus Areas: Pay close attention to areas involving key generation, signing, verification, and Diffie-Hellman operations.
  • Key Types: When working with the secp256k1 crate, you'll typically use types like:
    • secp256k1::SecretKey
    • secp256k1::PublicKey
    • secp256k1::ecdsa::Signature
  • Relevant Methods (Hints from Dev Chat): Community members have pointed towards needing methods like:
    • PublicKey::mul_tweak: For operations related to key derivation or tweaking.
    • PublicKey::combine: For combining public keys (e.g., in multi-sig or aggregated signatures).
    • Consult the secp256k1 crate documentation for details on using its API.

3. Minimizing Dependencies#

Carefully review the dependency tree of the sigma-rust components you intend to use. Remove or replace dependencies that are too large or rely on std. This might involve:

  • Using feature flags to disable unused functionality.
  • Replacing crates with lighter-weight or no_std alternatives where possible.
  • Potentially re-implementing certain non-cryptographic helper functions if their dependencies are problematic.

Core Functionality to Port#

The essential functions needed for basic hardware wallet support typically include:

  • Key Derivation: Deriving child keys from a master seed according to EIP-3.
  • Address Generation: Generating Ergo addresses from public keys.
  • Transaction Parsing: Securely parsing transaction details for display and user confirmation.
  • Transaction Signing: Signing the transaction digest using the derived private key. This is the most critical part and must use the hardware wallet's secure key storage and signing mechanism.
  • ErgoTree Serialization/Hashing: Potentially needed for constructing parts of the transaction message to be signed.

Conclusion#

Integrating Ergo with hardware wallets requires careful adaptation of existing libraries like sigma-rust. Leveraging no_std compatible forks/versions, replacing cryptographic backends to use the device's optimized implementations (like secp256k1), and minimizing dependencies are key strategies. This is a complex task often requiring deep knowledge of both the Ergo protocol and embedded Rust development. Collaboration within the developer community is crucial for advancing hardware wallet support.