soft3/mudra/src/seed.rs

rust
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// ---
// tags: mudra, rust
// crystal-type: source
// crystal-domain: comp
// ---
//! Seeds and hierarchical-deterministic keys.
//!
//! A holder's wallet is a BIP-39 mnemonic. From it we derive the secp256k1 key
//! at the Cosmos derivation path so we can reproduce the *exact* on-chain
//! account a spacepussy neuron controls. This is how the bridge "computes real
//! neurons" from a seed.

use bip32::XPrv;

use crate::{Error, SigningKey};

/// The Cosmos-SDK derivation path โ€” coin type **118**, account 0, external
/// chain, first address. spacepussy and bostrom both use this.
pub const COSMOS_PATH: &str = "m/44'/118'/0'/0/0";

/// Derive the 64-byte BIP-39 seed from a mnemonic and optional passphrase.
///
/// The passphrase (BIP-39 "25th word") is `""` for a standard wallet.
pub fn seed(mnemonic: &str, passphrase: &str) -> Result<[u8; 64], Error> {
    let mn = bip39::Mnemonic::parse(mnemonic).map_err(|e| Error::Mnemonic(e.to_string()))?;
    Ok(mn.to_seed(passphrase))
}

/// Derive the secp256k1 signing key at `path` from a seed.
pub fn signing_key(seed: &[u8; 64], path: &str) -> Result<SigningKey, Error> {
    let path = path.parse().map_err(|e: bip32::Error| Error::Derive(e.to_string()))?;
    let xprv = XPrv::derive_from_path(seed, &path).map_err(|e| Error::Derive(e.to_string()))?;
    Ok(xprv.into())
}

/// Convenience: mnemonic โ†’ signing key at the Cosmos path (coin type 118).
///
/// This is the one-call path from a holder's wallet phrase to the key that
/// controls their spacepussy account.
pub fn cosmos_key(mnemonic: &str, passphrase: &str) -> Result<SigningKey, Error> {
    let seed = seed(mnemonic, passphrase)?;
    signing_key(&seed, COSMOS_PATH)
}

/// Generate a fresh 24-word BIP-39 mnemonic (256 bits of entropy). The phrase is
/// the sole key to the derived account and neuron โ€” store it safely.
pub fn generate_mnemonic() -> Result<String, Error> {
    bip39::Mnemonic::generate(24)
        .map(|m| m.to_string())
        .map_err(|e| Error::Mnemonic(e.to_string()))
}

#[cfg(test)]
mod tests {
    use super::*;

    // The canonical all-`abandon` BIP-39 test vector.
    const MNEMONIC: &str =
        "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about";

    #[test]
    fn derivation_is_deterministic() {
        let a = cosmos_key(MNEMONIC, "").unwrap();
        let b = cosmos_key(MNEMONIC, "").unwrap();
        assert_eq!(a.to_bytes(), b.to_bytes(), "same mnemonic โ†’ same key");
    }

    #[test]
    fn passphrase_changes_the_key() {
        let a = cosmos_key(MNEMONIC, "").unwrap();
        let b = cosmos_key(MNEMONIC, "trezor").unwrap();
        assert_ne!(a.to_bytes(), b.to_bytes(), "the 25th word forks the wallet");
    }

    #[test]
    fn bad_mnemonic_is_rejected() {
        assert!(cosmos_key("not a real mnemonic phrase at all", "").is_err());
    }
}

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