open-standards/wai/wai-quantum-cli/src/keys.rs

//! The key a receipt is signed with.
//!
//! - `--demo-key`: the published demo key, for receipts that repeat byte for
//!   byte everywhere. Anyone can sign with it, so it proves nothing about who
//!   signed.
//! - `--key FILE`, or `$WAI_QUANTUM_KEY` naming a file: that key.
//! - Otherwise this user's key, `$XDG_CONFIG_HOME/wai/quantum-key` (or
//!   `~/.config/wai/quantum-key`), made from the operating system's random
//!   source on first use and readable by its owner only.
//!
//! A key file holds the 32-byte Ed25519 seed as 64 hex digits.

use std::path::{Path, PathBuf};
use wai_quantum::ed25519_dalek::SigningKey;

/// The published demo key's seed.
pub const DEMO_SEED: [u8; 32] = [42; 32];

/// The key to sign with, and where it came from.
pub struct Key {
    pub key: SigningKey,
    /// The file it was read from or made in; `None` for the demo key.
    pub path: Option<PathBuf>,
    /// Whether this call made the file.
    pub made: bool,
}

/// This user's key file: `$XDG_CONFIG_HOME/wai/quantum-key`, or
/// `~/.config/wai/quantum-key`.
pub fn user_path() -> Option<PathBuf> {
    let base = std::env::var_os("XDG_CONFIG_HOME")
        .map(PathBuf::from)
        .or_else(|| std::env::var_os("HOME").map(|h| Path::new(&h).join(".config")))?;
    Some(base.join("wai").join("quantum-key"))
}

fn parse(text: &str) -> Option<[u8; 32]> {
    let t = text.trim();
    if t.len() != 64 {
        return None;
    }
    let mut out = [0u8; 32];
    for (i, b) in out.iter_mut().enumerate() {
        *b = u8::from_str_radix(t.get(2 * i..2 * i + 2)?, 16).ok()?;
    }
    Some(out)
}

/// The key `demo` and `given` (`--demo-key`, `--key FILE`) name: a named file
/// must exist; this user's own is made when there is none.
pub fn load(given: Option<&Path>, demo: bool) -> Result<Key, String> {
    if demo {
        return Ok(Key { key: SigningKey::from_bytes(&DEMO_SEED), path: None, made: false });
    }
    let named = given.map(Path::to_path_buf).or_else(|| std::env::var_os("WAI_QUANTUM_KEY").map(PathBuf::from));
    let path = match &named {
        Some(p) => p.clone(),
        None => user_path().ok_or("no HOME to keep a key in: pass --key FILE")?,
    };
    match std::fs::read_to_string(&path) {
        Ok(text) => {
            let seed = parse(&text).ok_or_else(|| format!("{}: a key file holds 64 hex digits", path.display()))?;
            Ok(Key { key: SigningKey::from_bytes(&seed), path: Some(path), made: false })
        }
        Err(e) if e.kind() == std::io::ErrorKind::NotFound && named.is_none() => {
            let mut seed = [0u8; 32];
            getrandom::fill(&mut seed).map_err(|e| format!("no random source for a new key: {e}"))?;
            if let Some(dir) = path.parent() {
                std::fs::create_dir_all(dir).map_err(|e| format!("{}: {e}", dir.display()))?;
            }
            let text: String = seed.iter().map(|b| format!("{b:02x}")).collect();
            write_private(&path, &format!("{text}\n")).map_err(|e| format!("{}: {e}", path.display()))?;
            Ok(Key { key: SigningKey::from_bytes(&seed), path: Some(path), made: true })
        }
        Err(e) => Err(format!("{}: {e}", path.display())),
    }
}

fn write_private(path: &Path, text: &str) -> std::io::Result<()> {
    use std::io::Write as _;
    let mut f = std::fs::OpenOptions::new();
    f.write(true).create_new(true);
    #[cfg(unix)]
    {
        use std::os::unix::fs::OpenOptionsExt;
        f.mode(0o600);
    }
    f.open(path)?.write_all(text.as_bytes())
}

/// What `--demo-key` and a new key say on standard error.
pub fn notes(k: &Key) {
    match &k.path {
        None => eprintln!("note: signed with the published demo key; the receipt shows the result is unaltered, not who signed it"),
        Some(p) if k.made => eprintln!("note: made a signing key at {}", p.display()),
        Some(_) => {}
    }
}

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

    #[test]
    fn a_key_is_made_once_private_then_read_and_a_named_file_must_exist() {
        let dir = std::env::temp_dir().join(format!("wai-quantum-cli-key-{}", std::process::id()));
        let _ = std::fs::remove_dir_all(&dir);
        let path = dir.join("k");
        // A named file that is not there is refused, never made.
        assert!(load(Some(&path), false).is_err());
        assert!(!path.exists());
        // The demo key is the published one.
        let demo = load(None, true).unwrap();
        assert_eq!(demo.key.to_bytes(), DEMO_SEED);
        assert!(demo.path.is_none());
        // A written key reads back; a malformed one is refused.
        std::fs::create_dir_all(&dir).unwrap();
        let seed: String = (0..32).map(|i| format!("{:02x}", i * 7)).collect();
        std::fs::write(&path, format!("{seed}\n")).unwrap();
        let k = load(Some(&path), false).unwrap();
        assert_eq!(k.key.to_bytes()[1], 7);
        assert!(!k.made);
        std::fs::write(&path, "not a key").unwrap();
        assert!(load(Some(&path), false).is_err());
        // A made key is the owner's alone.
        let fresh = dir.join("fresh");
        write_private(&fresh, "x").unwrap();
        #[cfg(unix)]
        {
            use std::os::unix::fs::PermissionsExt;
            assert_eq!(std::fs::metadata(&fresh).unwrap().permissions().mode() & 0o777, 0o600);
        }
        let _ = std::fs::remove_dir_all(&dir);
    }
}