pixelpass/src/common/identity.rs

//! Persistent node identity at `~/.config/pixelpass/identity.key`.
//!
//! Without this, [`super::endpoint::bind`] would let iroh mint a fresh random
//! keypair on every launch, so a peer's `EndpointId` would change each run.
//! The friends system identifies people by that id (it's the public key already
//! embedded in every share code), so it must stay stable across launches — and
//! across roles: the same machine gets the same id whether it's hosting,
//! viewing, or just sitting in the GUI.
//!
//! The key is the ed25519 secret (32 bytes) stored as hex on its own line, in a
//! `0600` file separate from `config.toml` — it's a secret, not a preference,
//! and keeping it out of the TOML means a hand-edit or a config reset can't
//! clobber your identity.

use anyhow::{Context, Result, bail};
use iroh::SecretKey;
use std::fs;
use std::io::Write;
use std::path::PathBuf;

/// Returns `~/.config/pixelpass/identity.key` (or the XDG equivalent). Shares
/// the config directory with [`super::config`]; the parent is created on save.
pub fn identity_path() -> Result<PathBuf> {
    Ok(super::config::config_path()?
        .parent()
        .context("config path has no parent directory")?
        .join("identity.key"))
}

/// Load the persisted secret key, or generate-and-save one on first run.
///
/// A malformed file is a hard error rather than a silent regenerate: silently
/// minting a new identity would orphan every friend who has the old id, so we'd
/// rather fail loud and let the user notice (and decide) than lose it quietly.
pub fn load_or_create() -> Result<SecretKey> {
    let path = identity_path()?;
    match fs::read_to_string(&path) {
        Ok(s) => parse_key(s.trim())
            .with_context(|| format!("failed to parse the identity key at {}", path.display())),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
            let key = SecretKey::generate();
            save(&key)?;
            tracing::info!(id = %key.public(), "generated a new persistent identity");
            Ok(key)
        }
        Err(e) => Err(e).with_context(|| format!("failed to read {}", path.display())),
    }
}

fn parse_key(hex: &str) -> Result<SecretKey> {
    let bytes = decode_hex(hex)?;
    let arr: [u8; 32] = bytes
        .try_into()
        .map_err(|_| anyhow::anyhow!("identity key must be 32 bytes (64 hex chars)"))?;
    Ok(SecretKey::from_bytes(&arr))
}

/// Atomic, `0600` write: tempfile-in-same-dir, chmod, then rename. Same
/// approach as [`super::config::save`], but with restrictive perms applied
/// before the rename so the secret is never briefly world-readable.
pub fn save(key: &SecretKey) -> Result<()> {
    let path = identity_path()?;
    let parent = path
        .parent()
        .context("identity path has no parent directory")?;
    fs::create_dir_all(parent).with_context(|| format!("failed to create {}", parent.display()))?;

    let tmp = parent.join(format!(".identity.key.tmp.{}", std::process::id()));
    {
        let mut f = fs::File::create(&tmp)
            .with_context(|| format!("failed to create {}", tmp.display()))?;
        #[cfg(unix)]
        {
            use std::os::unix::fs::PermissionsExt;
            f.set_permissions(fs::Permissions::from_mode(0o600))
                .with_context(|| format!("failed to chmod {}", tmp.display()))?;
        }
        f.write_all(encode_hex(&key.to_bytes()).as_bytes())
            .with_context(|| format!("failed to write {}", tmp.display()))?;
        f.write_all(b"\n").ok();
        f.sync_all().ok();
    }
    fs::rename(&tmp, &path)
        .with_context(|| format!("failed to rename {} -> {}", tmp.display(), path.display()))?;
    Ok(())
}

fn encode_hex(bytes: &[u8]) -> String {
    let mut s = String::with_capacity(bytes.len() * 2);
    for b in bytes {
        s.push_str(&format!("{b:02x}"));
    }
    s
}

fn decode_hex(s: &str) -> Result<Vec<u8>> {
    if !s.len().is_multiple_of(2) {
        bail!("hex string has an odd length");
    }
    (0..s.len())
        .step_by(2)
        .map(|i| {
            u8::from_str_radix(&s[i..i + 2], 16)
                .with_context(|| format!("invalid hex byte at offset {i}"))
        })
        .collect()
}

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

    #[test]
    fn hex_round_trips() {
        let bytes: Vec<u8> = (0u8..=255).collect();
        let encoded = encode_hex(&bytes);
        assert_eq!(encoded.len(), bytes.len() * 2);
        assert_eq!(decode_hex(&encoded).unwrap(), bytes);
    }

    #[test]
    fn key_round_trips_through_hex() {
        let key = SecretKey::generate();
        let hex = encode_hex(&key.to_bytes());
        let parsed = parse_key(&hex).unwrap();
        assert_eq!(parsed.to_bytes(), key.to_bytes());
        assert_eq!(parsed.public(), key.public());
    }

    #[test]
    fn rejects_wrong_length() {
        assert!(parse_key("dead").is_err());
        assert!(parse_key("").is_err());
    }

    #[test]
    fn rejects_odd_and_nonhex() {
        assert!(decode_hex("abc").is_err());
        assert!(decode_hex("zz").is_err());
    }
}