keyfork/keyfork-derive-openpgp/src/lib.rs

use std::time::{Duration, SystemTime, SystemTimeError};

use ed25519_dalek::SigningKey;
use keyfork_derive_util::{
    request::{DerivationResponse, TryFromDerivationResponseError},
    DerivationIndex, ExtendedPrivateKey, PrivateKey,
};
use sequoia_openpgp::{
    packet::{
        key::{Key4, PrimaryRole, SubordinateRole},
        signature::SignatureBuilder,
        Key, UserID,
    },
    types::{KeyFlags, SignatureType},
    Cert, Packet,
};

#[derive(Debug, thiserror::Error)]
pub enum Error {
    #[error("{0}")]
    Anyhow(#[from] anyhow::Error),

    #[error("Incorrect derived data: {0}")]
    IncorrectDerivedData(#[from] TryFromDerivationResponseError),

    #[error("Could not create derivation index: {0}")]
    Index(#[from] keyfork_derive_util::index::Error),

    #[error("Could not perform operation against private key: {0}")]
    PrivateKey(#[from] keyfork_derive_util::extended_key::private_key::Error),

    #[error("Invalid system time: {0}")]
    SystemTime(#[from] SystemTimeError),

    #[error("First key in certificate must have certify capability")]
    NotCert,

    #[error("Index out of bounds: {0}")]
    IndexOutOfBounds(#[from] std::num::TryFromIntError),
}

pub type Result<T, E = Error> = std::result::Result<T, E>;

pub fn derive(data: DerivationResponse, keys: &[KeyFlags], userid: UserID) -> Result<Cert> {
    let primary_key_flags = match keys.get(0) {
        Some(kf) if kf.for_certification() => kf,
        _ => return Err(Error::NotCert),
    };

    let epoch = SystemTime::UNIX_EPOCH + Duration::from_secs(1);
    let one_day = SystemTime::now() + Duration::from_secs(60 * 60 * 24);

    // Create certificate with initial key and signature
    let xprv = ExtendedPrivateKey::<SigningKey>::try_from(data)?;
    let derived_primary_key = xprv.derive_child(&DerivationIndex::new(0, true)?)?;
    let primary_key = Key::from(Key4::<_, PrimaryRole>::import_secret_ed25519(
        &PrivateKey::to_bytes(derived_primary_key.private_key()),
        epoch,
    )?);
    let cert = Cert::from_packets(vec![Packet::SecretKey(primary_key.clone())].into_iter())?;

    // Sign and attach primary key and primary userid
    let builder = SignatureBuilder::new(SignatureType::PositiveCertification)
        .set_key_validity_period(one_day.duration_since(epoch)?)?
        .set_signature_creation_time(epoch)?
        .set_key_flags(primary_key_flags.clone())?;
    let binding = userid.bind(&mut primary_key.clone().into_keypair()?, &cert, builder)?;
    let cert = cert.insert_packets(vec![Packet::from(userid.clone()), binding.into()])?;
    let policy = sequoia_openpgp::policy::StandardPolicy::new();

    // Set certificate expiration to one day
    let mut keypair = primary_key.clone().into_keypair()?;
    let signatures = cert.set_expiration_time(&policy, None, &mut keypair, Some(one_day))?;
    let cert = cert.insert_packets(signatures)?;

    let mut cert = cert;
    for (index, subkey_flags) in keys.iter().enumerate().skip(1) {
        // Generate subkey
        let index = u32::try_from(index)?;
        let derived_key = xprv.derive_child(&DerivationIndex::new(index, true)?)?;
        let subkey = Key::from(
            Key4::<_, SubordinateRole>::import_secret_ed25519(
                &PrivateKey::to_bytes(derived_key.private_key()),
                epoch,
            )
            .unwrap(),
        );

        // As per OpenPGP spec, signing keys must backsig the primary key
        let builder = if subkey_flags.for_signing() {
            SignatureBuilder::new(SignatureType::SubkeyBinding)
                .set_key_flags(subkey_flags.clone())?
                .set_signature_creation_time(epoch)?
                .set_key_validity_period(one_day.duration_since(epoch)?)?
                .set_embedded_signature(
                    SignatureBuilder::new(SignatureType::PrimaryKeyBinding)
                        .set_signature_creation_time(epoch)?
                        .sign_primary_key_binding(
                            &mut subkey.clone().into_keypair()?,
                            &primary_key,
                            &subkey,
                        )?,
                )?
        } else {
            SignatureBuilder::new(SignatureType::SubkeyBinding)
                .set_key_flags(subkey_flags.clone())?
                .set_signature_creation_time(epoch)?
                .set_key_validity_period(one_day.duration_since(epoch)?)?
        };

        // Sign subkey with primary key and attach to cert
        let binding =
            builder.sign_subkey_binding(&mut primary_key.clone().into_keypair()?, None, &subkey)?;
        cert = cert.insert_packets(vec![
            Packet::SecretSubkey(subkey.clone()),
            Packet::Signature(binding),
        ])?;
    }

    Ok(cert)
}