pqfile 4.2.2

/// Credential-store hardware backend.
///
/// Stores the private key seed in the OS credential store:
/// - **Windows**: Windows Credential Manager (user-bound; optionally TPM-backed
///   on machines where the user profile is protected by Windows Hello / TPM)
/// - **macOS**: macOS Keychain (user-session-bound)
/// - **Linux**: Linux kernel keyutils (session keyring; no daemon required)
///
/// The seed is hex-encoded before storage (credential stores expect string values).
/// The credential is keyed by `"pqfile:{label}"` under the service name `"pqfile"`.
use std::collections::HashMap;
use std::sync::OnceLock;

use keyring_core::{set_default_store, Entry, Error as KeyringError};
use zeroize::Zeroizing;

use crate::error::PqfileError;

use super::stub::HardwareKeyRef;

pub(super) struct CredentialStoreBackend;

static STORE_INIT: OnceLock<Result<(), String>> = OnceLock::new();

fn ensure_store() -> Result<(), PqfileError> {
    STORE_INIT
        .get_or_init(init_platform_store)
        .as_ref()
        .map(|_| ())
        .map_err(|e| {
            PqfileError::Io(std::io::Error::other(format!(
                "hardware key store init failed: {e}"
            )))
        })
}

#[cfg(target_os = "windows")]
fn init_platform_store() -> Result<(), String> {
    use windows_native_keyring_store::Store;
    set_default_store(Store::new_with_configuration(&HashMap::new()).map_err(|e| e.to_string())?);
    Ok(())
}

#[cfg(target_os = "macos")]
fn init_platform_store() -> Result<(), String> {
    use apple_native_keyring_store::keychain::Store;
    set_default_store(Store::new_with_configuration(&HashMap::new()).map_err(|e| e.to_string())?);
    Ok(())
}

#[cfg(target_os = "linux")]
fn init_platform_store() -> Result<(), String> {
    use linux_keyutils_keyring_store::Store;
    set_default_store(Store::new_with_configuration(&HashMap::new()).map_err(|e| e.to_string())?);
    Ok(())
}

#[cfg(not(any(target_os = "windows", target_os = "macos", target_os = "linux")))]
fn init_platform_store() -> Result<(), String> {
    Err("hardware keys are not supported on this platform".to_string())
}

impl CredentialStoreBackend {
    /// Stores `seed` in the credential store under the key derived from `key_ref`.
    pub fn store_seed(&self, key_ref: &HardwareKeyRef, seed: &[u8]) -> Result<(), PqfileError> {
        ensure_store()?;
        let account = account_name(&key_ref.label);
        let entry = Entry::new("pqfile", &account).map_err(hw_err)?;
        let encoded = Zeroizing::new(hex_encode(seed));
        entry.set_password(encoded.as_str()).map_err(hw_err)
    }

    /// Loads the seed from the credential store.
    ///
    /// Returns `Io(NotFound)` if no credential exists for this label.
    pub fn load_seed(&self, key_ref: &HardwareKeyRef) -> Result<Zeroizing<Vec<u8>>, PqfileError> {
        ensure_store()?;
        let account = account_name(&key_ref.label);
        let entry = Entry::new("pqfile", &account).map_err(hw_err)?;
        let encoded = Zeroizing::new(entry.get_password().map_err(|e| match e {
            KeyringError::NoEntry => PqfileError::Io(std::io::Error::new(
                std::io::ErrorKind::NotFound,
                format!(
                    "hardware key '{}' not found in the OS credential store; \
                         was it generated on a different machine or user account?",
                    key_ref.label
                ),
            )),
            other => hw_err(other),
        })?);
        hex_decode(encoded.as_str()).map_err(|_| {
            // Truncate the label to 32 chars in the error message so that
            // sensitive identifiers embedded in label names do not appear in logs.
            let label_preview = key_ref.label.chars().take(32).collect::<String>();
            PqfileError::InvalidPem(format!(
                "hardware key '{label_preview}' credential store entry is corrupted"
            ))
        })
    }

    /// Removes the credential from the store.
    #[allow(dead_code)]
    pub fn delete_seed(&self, key_ref: &HardwareKeyRef) -> Result<(), PqfileError> {
        ensure_store()?;
        let account = account_name(&key_ref.label);
        let entry = Entry::new("pqfile", &account).map_err(hw_err)?;
        entry.delete_credential().map_err(hw_err)
    }
}

fn account_name(label: &str) -> String {
    format!("pqfile:{label}")
}

fn hw_err(e: impl std::fmt::Display) -> PqfileError {
    PqfileError::Io(std::io::Error::other(format!(
        "hardware key credential store error: {e}"
    )))
}

fn hex_encode(bytes: &[u8]) -> String {
    // Write nibbles directly into a pre-sized buffer to avoid per-byte heap
    // allocations that would not be zeroized on drop.
    const HEX: &[u8; 16] = b"0123456789abcdef";
    let mut out = String::with_capacity(bytes.len() * 2);
    for &b in bytes {
        out.push(HEX[(b >> 4) as usize] as char);
        out.push(HEX[(b & 0xF) as usize] as char);
    }
    out
}

fn hex_decode(s: &str) -> Result<Zeroizing<Vec<u8>>, ()> {
    if !s.len().is_multiple_of(2) {
        return Err(());
    }
    let bytes: Result<Vec<u8>, _> = (0..s.len())
        .step_by(2)
        .map(|i| u8::from_str_radix(&s[i..i + 2], 16).map_err(|_| ()))
        .collect();
    bytes.map(Zeroizing::new)
}

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

    #[test]
    fn hex_encode_decode_roundtrip() {
        let data = vec![0xDE, 0xAD, 0xBE, 0xEF, 0x00, 0xFF];
        let encoded = hex_encode(&data);
        assert_eq!(encoded, "deadbeef00ff");
        let decoded = hex_decode(&encoded).unwrap();
        assert_eq!(*decoded, data);
    }

    #[test]
    fn hex_decode_rejects_odd_length() {
        assert!(hex_decode("abc").is_err());
    }

    #[test]
    fn hex_decode_rejects_non_hex() {
        assert!(hex_decode("zz").is_err());
    }
}