Add --hex-secret option to exportwallet and importwallet commands

[horologger]

Summary

This PR adds a new --hex-secret (short option -x) to both the exportwallet and importwallet subcommands in space-cli, enabling users to work with hex-encoded private keys directly.

Features Added

Export Wallet with Hex Secret (exportwallet -x)

• New option: --hex-secret / -x flag for space-cli exportwallet
• Output enhancement: When -x is provided, the exported JSON includes a hex-secret field containing the hex-encoded private key
• Placement: The hex-secret field is positioned after the label element in the JSON output
• Backward compatibility: Existing export functionality remains unchanged when -x is not used

Import Wallet with Hex Secret (importwallet -x)

• New option: --hex-secret / -x flag for space-cli importwallet
• Hex-to-xprv conversion: Automatically converts hex secrets to proper xprv (extended private key) format
• Descriptor generation: Creates valid Bitcoin taproot descriptors using the /86'/0'/0'/0/* derivation path
• Flexible input: When -x is used, the descriptor field becomes optional in the import JSON
• Validation: Ensures hex secrets are exactly 32 bytes (64 hex characters)

Technical Implementation

Core Changes

1. Wallet Export Enhancement (wallet/src/export.rs, wallet/src/lib.rs)
   Added hex_secret field to WalletExport struct
   Implemented extract_hex_secret() method in SpacesWallet
   Modified export_wallet() to conditionally include hex secret

2. RPC Interface Updates (client/src/rpc.rs)
   Updated wallet_export RPC method to accept hex_secret parameter
   Added xpriv_from_hex_secret() function for proper hex-to-xprv conversion
   Implemented hex-based descriptor generation in wallet creation process
   Added remove_checksum() helper function for descriptor formatting

3. CLI Command Enhancements (client/src/bin/space-cli.rs)
   Added --hex-secret / -x options to both export and import commands
   Implemented logic to skip descriptor field validation when using hex import
   Added proper error handling for hex secret validation

Key Technical Details

• Hex Secret Validation: Ensures 32-byte (64 hex character) input
• Xprv Conversion: Uses Bitcoin's secp256k1 to create proper extended private keys
• Descriptor Generation: Creates BIP86 taproot descriptors with proper derivation paths
• Wallet Creation: Uses temporary wallet creation to extract properly formatted descriptor strings
• Error Handling: Comprehensive validation and user-friendly error messages

Usage Examples

Export with Hex Secret

space-cli exportwallet -x ./wallet.json
Output includes:

{
  "descriptor": "tr([xprv...]/86'/0'/0'/0/*)",
  "blockheight": 910000,
  "label": "my_wallet",
  "hex_secret": "8e91ef66b88fd8ce4daefeb357e300a6906ead06dc25882915a9c5d4624ebd5e"
}

Import with Hex Secret

space-cli importwallet -x ./wallet.json
Input JSON (descriptor field optional):

{
  "blockheight": 910000,
  "label": "imported_wallet",
  "hex_secret": "8e91ef66b88fd8ce4daefeb357e300a6906ead06dc25882915a9c5d4624ebd5e"
}

Testing

• ✅ Import Success: Successfully imports wallets using hex secrets
• ✅ Descriptor Generation: Creates valid Bitcoin taproot descriptors
• ✅ Validation: Properly validates hex secret format and length
• ✅ Error Handling: Clear error messages for invalid inputs
• ✅ Backward Compatibility: Existing functionality remains unchanged

Files Modified

• client/src/bin/space-cli.rs - CLI command definitions and argument handling
• client/src/rpc.rs - RPC interface and wallet creation logic
• wallet/src/export.rs - Wallet export structure and serialization
• wallet/src/lib.rs - Core wallet functionality for hex secret extraction

Breaking Changes

None. This is a purely additive feature that maintains full backward compatibility.

Related Issues

Resolves the need for direct hex private key support in wallet import/export operations, enabling easier integration with external key management systems and tools that work with raw hex-encoded private keys.

[horologger]

Testing showed hex of secret was being used as seed not directly. round trip created different hex. Fixed.