keyfork/crates/keyfork-shard/src/openpgp.rs

//! OpenPGP Shard functionality.

use std::{
    collections::{HashMap, VecDeque},
    io::{stdin, stdout, Read, Write},
    path::Path,
    str::FromStr,
};

use aes_gcm::{
    aead::{consts::U12, Aead},
    aes::cipher::InvalidLength,
    Aes256Gcm, Error as AesError, KeyInit, Nonce,
};
use hkdf::{Hkdf, InvalidLength as HkdfInvalidLength};
use keyfork_derive_openpgp::{
    derive_util::{DerivationPath, PathError, VariableLengthSeed},
    XPrv,
};
use keyfork_mnemonic_util::{Mnemonic, MnemonicFromStrError, MnemonicGenerationError, Wordlist};
use keyfork_prompt::{
    validators::{mnemonic::MnemonicSetValidator, Validator},
    Error as PromptError, Message as PromptMessage, PromptHandler, Terminal,
};
use openpgp::{
    armor::{Kind, Writer},
    cert::{Cert, CertParser, ValidCert},
    packet::{Packet, Tag, UserID, PKESK, SEIP},
    parse::{
        stream::{DecryptionHelper, DecryptorBuilder, VerificationHelper},
        Parse,
    },
    policy::{NullPolicy, Policy, StandardPolicy},
    serialize::{
        stream::{ArbitraryWriter, Encryptor2, LiteralWriter, Message, Recipient, Signer},
        Marshal,
    },
    types::KeyFlags,
    Fingerprint, KeyID, PacketPile,
};
pub use sequoia_openpgp as openpgp;
use sha2::Sha256;
use sharks::{Share, Sharks};
use x25519_dalek::{EphemeralSecret, PublicKey};

mod keyring;
use keyring::Keyring;

mod smartcard;
use smartcard::SmartcardManager;

/// Shard metadata verson 1:
/// 1 byte: Version
/// 1 byte: Threshold
/// Packet Pile of Certs
const SHARD_METADATA_VERSION: u8 = 1;
const SHARD_METADATA_OFFSET: usize = 2;

use super::{
    InvalidData, SharksError, HUNK_VERSION, QRCODE_COULDNT_READ, QRCODE_ERROR, QRCODE_PROMPT,
    QRCODE_TIMEOUT,
};

// 256 bit share is 49 bytes + some amount of hunk bytes, gives us reasonable padding
const ENC_LEN: u8 = 4 * 16;

/// Errors encountered while performing operations using OpenPGP.
#[derive(Debug, thiserror::Error)]
pub enum Error {
    /// Errors encountered while creating or combining shares.
    #[error("{0}")]
    Sharks(#[from] SharksError),

    /// Unable to decrypt a share.
    #[error("Error decrypting share: {0}")]
    SymDecryptShare(#[from] AesError),

    /// The generated AES key is of an invalid length.
    #[error("Invalid length of AES key: {0}")]
    AesLength(#[from] InvalidLength),

    /// The HKDF function was given an input of an invalid length.
    #[error("Invalid KDF length: {0}")]
    HkdfLength(#[from] HkdfInvalidLength),

    /// The secret did not match the previously-known secret fingerprint.
    #[error("Derived secret hash {0} != expected {1}")]
    InvalidSecret(Fingerprint, Fingerprint),

    /// An error occurred while performing an OpenPGP operation.
    #[error("OpenPGP error: {0}")]
    Sequoia(#[source] anyhow::Error),

    /// An IO error occurred while performing an OpenPGP operation.
    #[error("OpenPGP IO error: {0}")]
    SequoiaIo(#[source] std::io::Error),

    /// An error occurred while using a keyring.
    #[error("Keyring error: {0}")]
    Keyring(#[from] keyring::Error),

    /// An error occurred while using a smartcard.
    #[error("Smartcard error: {0}")]
    Smartcard(#[from] smartcard::Error),

    /// An error occurred while displaying a prompt.
    #[error("Prompt error: {0}")]
    Prompt(#[from] PromptError),

    /// An error occurred while generating a mnemonic.
    #[error("Mnemonic generation error: {0}")]
    MnemonicGeneration(#[from] MnemonicGenerationError),

    /// An error occurred while parsing a mnemonic.
    #[error("Mnemonic parse error: {0}")]
    MnemonicFromStr(#[from] MnemonicFromStrError),

    /// An error occurred while converting mnemonic data.
    #[error("{0}")]
    InvalidMnemonicData(#[from] InvalidData),

    /// An IO error occurred.
    #[error("IO error: {0}")]
    Io(#[source] std::io::Error),

    /// An error occurred while deriving data.
    #[error("Derivation: {0}")]
    Derivation(#[from] keyfork_derive_openpgp::derive_util::extended_key::private_key::Error),

    /// An error occurred while parsing a derivation path.
    #[error("Derivation path: {0}")]
    DerivationPath(#[from] PathError),

    /// An error occurred while requesting derivation.
    #[error("Derivation request: {0}")]
    DerivationRequest(#[from] keyfork_derive_openpgp::derive_util::request::DerivationError),

    /// An error occurred while decoding hex.
    #[error("Unable to decode hex: {0}")]
    HexDecode(#[from] smex::DecodeError),

    /// An error occurred while creating an OpenPGP cert.
    #[error("Keyfork OpenPGP: {0}")]
    KeyforkOpenPGP(#[from] keyfork_derive_openpgp::Error),
}

#[allow(missing_docs)]
pub type Result<T, E = Error> = std::result::Result<T, E>;

/// An OpenPGP encrypted message and public-key-encrypted-secret-key packets.
#[derive(Debug, Clone)]
pub struct EncryptedMessage {
    pkesks: Vec<PKESK>,
    message: SEIP,
}

impl EncryptedMessage {
    /// Create a new EncryptedMessage from known parts.
    pub fn new(pkesks: &mut Vec<PKESK>, seip: SEIP) -> Self {
        Self {
            pkesks: std::mem::take(pkesks),
            message: seip,
        }
    }

    /// Decrypt the message with a Sequoia policy and decryptor.
    ///
    /// This method creates a container containing the packets and passes the serialized container
    /// to a DecryptorBuilder, which is used to decrypt the message.
    ///
    /// # Errors
    /// The method may return an error if it is unable to rebuild the message to decrypt or if it
    /// is unable to decrypt the message.
    pub fn decrypt_with<H>(&self, policy: &'_ dyn Policy, decryptor: H) -> Result<Vec<u8>>
    where
        H: VerificationHelper + DecryptionHelper,
    {
        let mut packets = vec![];

        for pkesk in &self.pkesks {
            let mut packet = vec![];
            pkesk.serialize(&mut packet).map_err(Error::Sequoia)?;
            let message = Message::new(&mut packets);
            let mut message = ArbitraryWriter::new(message, Tag::PKESK).map_err(Error::Sequoia)?;
            message.write_all(&packet).map_err(Error::SequoiaIo)?;
            message.finalize().map_err(Error::Sequoia)?;
        }
        let mut packet = vec![];
        self.message
            .serialize(&mut packet)
            .map_err(Error::Sequoia)?;
        let message = Message::new(&mut packets);
        let mut message = ArbitraryWriter::new(message, Tag::SEIP).map_err(Error::Sequoia)?;
        message.write_all(&packet).map_err(Error::SequoiaIo)?;
        message.finalize().map_err(Error::Sequoia)?;

        let mut decryptor = DecryptorBuilder::from_bytes(&packets)
            .map_err(Error::Sequoia)?
            .with_policy(policy, None, decryptor)
            .map_err(Error::Sequoia)?;

        let mut content = vec![];
        decryptor
            .read_to_end(&mut content)
            .map_err(Error::SequoiaIo)?;
        Ok(content)
    }
}

/// Read all OpenPGP certificates in a path and return a [`Vec`] of them. Certificates are read
/// from a file, or from files one level deep in a directory.
///
/// # Errors
/// The function may return an error if it is unable to read the directory or if Sequoia is unable
/// to load certificates from the file.
pub fn discover_certs(path: impl AsRef<Path>) -> Result<Vec<Cert>> {
    let path = path.as_ref();

    if path.is_file() {
        let mut vec = vec![];
        for cert in CertParser::from_file(path).map_err(Error::Sequoia)? {
            vec.push(cert.map_err(Error::Sequoia)?);
        }
        Ok(vec)
    } else {
        let mut vec = vec![];
        for entry in path
            .read_dir()
            .map_err(Error::Io)?
            .filter_map(Result::ok)
            .filter(|p| p.path().is_file())
        {
            vec.push(Cert::from_file(entry.path()).map_err(Error::Sequoia)?);
        }
        Ok(vec)
    }
}

/// Parse messages from a type implementing [`Read`] and store them as [`EncryptedMessage`].
///
/// # Errors
/// The function may return an error if the reader has run out of data or if the data is not
/// properly formatted OpenPGP messages.
///
/// # Panics
/// When given packets that are not a list of PKESK packets and SEIP packets, the function panics.
/// The `split` utility should never give packets that are not in this format.
pub fn parse_messages(reader: impl Read + Send + Sync) -> Result<VecDeque<EncryptedMessage>> {
    let mut pkesks = Vec::new();
    let mut encrypted_messages = VecDeque::new();

    for packet in PacketPile::from_reader(reader)
        .map_err(Error::Sequoia)?
        .into_children()
    {
        match packet {
            Packet::PKESK(p) => pkesks.push(p),
            Packet::SEIP(s) => {
                encrypted_messages.push_back(EncryptedMessage::new(&mut pkesks, s));
            }
            s => {
                panic!("Invalid variant found: {}", s.tag());
            }
        }
    }

    Ok(encrypted_messages)
}

fn get_encryption_keys<'a>(
    cert: &'a ValidCert,
) -> openpgp::cert::prelude::ValidKeyAmalgamationIter<
    'a,
    openpgp::packet::key::PublicParts,
    openpgp::packet::key::UnspecifiedRole,
> {
    cert.keys()
        .alive()
        .revoked(false)
        .supported()
        .for_storage_encryption()
}

fn get_decryption_keys<'a>(
    cert: &'a ValidCert,
) -> openpgp::cert::prelude::ValidKeyAmalgamationIter<
    'a,
    openpgp::packet::key::SecretParts,
    openpgp::packet::key::UnspecifiedRole,
> {
    cert.keys()
        /*
        .alive()
        .revoked(false)
        .supported()
        */
        .for_storage_encryption()
        .secret()
}

fn decode_metadata_v1(buf: &[u8]) -> Result<(u8, Cert, Vec<Cert>)> {
    assert_eq!(
        SHARD_METADATA_VERSION, buf[0],
        "Incompatible metadata version"
    );
    let threshold = buf[1];

    let mut cert_parser =
        CertParser::from_bytes(&buf[SHARD_METADATA_OFFSET..]).map_err(Error::Sequoia)?;

    let root_cert = match cert_parser.next() {
        Some(Ok(c)) => c,
        Some(Err(e)) => return Err(Error::Sequoia(e)),
        None => panic!("No data found"),
    };

    let certs = cert_parser
        .collect::<openpgp::Result<Vec<_>>>()
        .map_err(Error::Sequoia)?;

    Ok((threshold, root_cert, certs))
}

// NOTE: When using single-decryptor mechanism, use this method with `threshold = 1` to return a
// single message.
fn decrypt_with_manager(
    threshold: u8,
    messages: &mut HashMap<KeyID, EncryptedMessage>,
    certs: &[Cert],
    policy: &dyn Policy,
    manager: &mut SmartcardManager,
) -> Result<HashMap<KeyID, Vec<u8>>> {
    let mut decrypted_messages = HashMap::new();

    while threshold as usize - decrypted_messages.len() > 0 {
        // Build list of fingerprints that haven't yet been used for decrypting
        let mut cert_by_fingerprint = HashMap::new();
        let mut unused_fingerprints = vec![];
        for valid_cert in certs
            .iter()
            .filter(|cert| !decrypted_messages.contains_key(&cert.keyid()))
            .map(|cert| cert.with_policy(policy, None))
        {
            let valid_cert = valid_cert.map_err(Error::Sequoia)?;
            let fp = valid_cert
                .keys()
                .for_storage_encryption()
                .map(|k| k.fingerprint())
                .collect::<Vec<_>>();
            for fp in &fp {
                cert_by_fingerprint.insert(fp.clone(), valid_cert.keyid());
            }
            unused_fingerprints.extend(fp.into_iter());
        }

        // Iterate over all fingerprints and use key_by_fingerprints to assoc with Enc. Message
        if let Some(fp) = manager.load_any_fingerprint(unused_fingerprints)? {
            let cert_keyid = cert_by_fingerprint.get(&fp).unwrap().clone();
            if let Some(message) = messages.remove(&cert_keyid) {
                let message = message.decrypt_with(policy, &mut *manager)?;
                decrypted_messages.insert(cert_keyid, message);
            }
        }
    }

    Ok(decrypted_messages)
}

// NOTE: When using single-decryptor mechanism, only a single key should be provided in Keyring to
// decrypt messages with.
fn decrypt_with_keyring(
    messages: &mut HashMap<KeyID, EncryptedMessage>,
    certs: &[Cert],
    policy: &NullPolicy,
    keyring: &mut Keyring,
) -> Result<HashMap<KeyID, Vec<u8>>, Error> {
    let mut decrypted_messages = HashMap::new();

    for valid_cert in certs.iter().map(|cert| cert.with_policy(policy, None)) {
        let valid_cert = valid_cert.map_err(Error::Sequoia)?;
        let Some(secret_cert) = keyring.get_cert_for_primary_keyid(&valid_cert.keyid()) else {
            continue;
        };
        let secret_cert = secret_cert
            .with_policy(policy, None)
            .map_err(Error::Sequoia)?;
        let keys = get_decryption_keys(&secret_cert).collect::<Vec<_>>();
        if !keys.is_empty() {
            if let Some(message) = messages.get_mut(&valid_cert.keyid()) {
                for (pkesk, key) in message.pkesks.iter_mut().zip(keys) {
                    pkesk.set_recipient(key.keyid());
                }
                // we have a pkesk, decrypt via keyring
                decrypted_messages.insert(
                    valid_cert.keyid(),
                    message.decrypt_with(policy, &mut *keyring)?,
                );
            }
        }
    }

    Ok(decrypted_messages)
}

fn decrypt_metadata(
    message: &EncryptedMessage,
    policy: &NullPolicy,
    keyring: &mut Keyring,
    manager: &mut SmartcardManager,
) -> Result<Vec<u8>> {
    Ok(if keyring.is_empty() {
        manager.load_any_card()?;
        message.decrypt_with(policy, manager)?
    } else {
        message.decrypt_with(policy, keyring)?
    })
}

fn decrypt_one(
    messages: Vec<EncryptedMessage>,
    certs: &[Cert],
    metadata: &EncryptedMessage,
) -> Result<(Vec<u8>, u8, Cert)> {
    let policy = NullPolicy::new();

    let mut keyring = Keyring::new(certs)?;
    let mut manager = SmartcardManager::new()?;

    let content = decrypt_metadata(metadata, &policy, &mut keyring, &mut manager)?;

    let (threshold, root_cert, certs) = decode_metadata_v1(&content)?;

    keyring.set_root_cert(root_cert.clone());
    manager.set_root_cert(root_cert.clone());

    let mut messages: HashMap<KeyID, EncryptedMessage> =
        certs.iter().map(Cert::keyid).zip(messages).collect();

    let decrypted_messages = decrypt_with_keyring(&mut messages, &certs, &policy, &mut keyring)?;

    if let Some(message) = decrypted_messages.into_values().next() {
        return Ok((message, threshold, root_cert));
    }

    let decrypted_messages = decrypt_with_manager(1, &mut messages, &certs, &policy, &mut manager)?;

    if let Some(message) = decrypted_messages.into_values().next() {
        return Ok((message, threshold, root_cert));
    }

    unreachable!("smartcard manager should always decrypt")
}

/// Decrypt a single shard, encrypt to a remote operator, and present the transport shard as a QR
/// code and mnemonic to be sent to the remote operator.
///
/// # Errors
///
/// The function may error if an error occurs while displaying a prompt or while decrypting the
/// shard. An error will not be returned if the camera has a hardware error while scanning a QR
/// code; instead, a mnemonic prompt will be used.
///
/// # Panics
///
/// The function may panic if a share is decrypted but has a length larger than 256 bits. This is
/// atypical usage and should not be encountered in normal usage, unless something that is not a
/// Keyfork seed has been fed into [`split`].
pub fn decrypt(
    certs: &[Cert],
    metadata: &EncryptedMessage,
    encrypted_messages: &[EncryptedMessage],
) -> Result<()> {
    let mut pm = Terminal::new(stdin(), stdout())?;
    let wordlist = Wordlist::default();

    let mut nonce_data: Option<[u8; 12]> = None;
    let mut pubkey_data: Option<[u8; 32]> = None;

    #[cfg(feature = "qrcode")]
    {
        pm.prompt_message(PromptMessage::Text(QRCODE_PROMPT.to_string()))?;
        if let Ok(Some(hex)) =
            keyfork_qrcode::scan_camera(std::time::Duration::from_secs(QRCODE_TIMEOUT), 0)
        {
            let decoded_data = smex::decode(&hex)?;
            let _ = nonce_data.insert(decoded_data[..12].try_into().map_err(|_| InvalidData)?);
            let _ = pubkey_data.insert(decoded_data[12..].try_into().map_err(|_| InvalidData)?);
        } else {
            pm.prompt_message(PromptMessage::Text(QRCODE_ERROR.to_string()))?;
        };
    }

    let (nonce, pubkey) = match (nonce_data, pubkey_data) {
        (Some(nonce), Some(pubkey)) => (nonce, pubkey),
        _ => {
            let validator = MnemonicSetValidator {
                word_lengths: [9, 24],
            };
            let [nonce_mnemonic, pubkey_mnemonic] =
                pm.prompt_validated_wordlist(QRCODE_COULDNT_READ, &wordlist, 3, validator.to_fn())?;

            let nonce = nonce_mnemonic
                .as_bytes()
                .try_into()
                .map_err(|_| InvalidData)?;
            let pubkey = pubkey_mnemonic
                .as_bytes()
                .try_into()
                .map_err(|_| InvalidData)?;
            (nonce, pubkey)
        }
    };

    let nonce = Nonce::<U12>::from_slice(&nonce);

    let our_key = EphemeralSecret::random();
    let our_pubkey_mnemonic =
        Mnemonic::from_bytes(PublicKey::from(&our_key).as_bytes(), Default::default())?;

    let shared_secret = our_key.diffie_hellman(&PublicKey::from(pubkey)).to_bytes();

    let (mut share, threshold, ..) = decrypt_one(encrypted_messages.to_vec(), certs, metadata)?;
    share.insert(0, HUNK_VERSION);
    share.insert(1, threshold);
    assert!(
        share.len() <= ENC_LEN as usize,
        "invalid share length (too long, max {ENC_LEN} bytes)"
    );

    let hkdf = Hkdf::<Sha256>::new(None, &shared_secret);
    let mut hkdf_output = [0u8; 256 / 8];
    hkdf.expand(&[], &mut hkdf_output)?;
    let shared_key = Aes256Gcm::new_from_slice(&hkdf_output)?;

    let bytes = shared_key.encrypt(nonce, share.as_slice())?;
    shared_key.decrypt(nonce, &bytes[..])?;

    // NOTE: Padding length is less than u8::MAX because ENC_LEN < u8::MAX
    // NOTE: This previously used a single value as the padding byte, but resulted in
    // difficulty when entering in prompts manually, as one's place could be lost due to repeated
    // keywords. This is done below by having sequentially increasing numbers up to but not
    // including the last byte.
    #[allow(clippy::assertions_on_constants)]
    {
        assert!(ENC_LEN < u8::MAX, "padding byte can be u8");
    }
    #[allow(clippy::cast_possible_truncation)]
    let mut out_bytes = [bytes.len() as u8; ENC_LEN as usize];
    assert!(
        bytes.len() < out_bytes.len(),
        "encrypted payload larger than acceptable limit"
    );
    out_bytes[..bytes.len()].clone_from_slice(&bytes);
    #[allow(clippy::cast_possible_truncation)]
    for (i, byte) in (out_bytes[bytes.len()..(ENC_LEN as usize - 1)])
        .iter_mut()
        .enumerate()
    {
        *byte = (i % u8::MAX as usize) as u8;
    }

    // safety: size of out_bytes is constant and always % 4 == 0
    let payload_mnemonic = unsafe { Mnemonic::from_raw_bytes(&out_bytes, Default::default()) };

    #[cfg(feature = "qrcode")]
    {
        use keyfork_qrcode::{qrencode, ErrorCorrection};
        let mut qrcode_data = our_pubkey_mnemonic.to_bytes();
        qrcode_data.extend(payload_mnemonic.as_bytes());
        if let Ok(qrcode) = qrencode(&smex::encode(&qrcode_data), ErrorCorrection::Highest) {
            pm.prompt_message(PromptMessage::Text(
                concat!(
                    "A QR code will be displayed after this prompt. ",
                    "Send the QR code back to the operator combining the shards. ",
                    "Nobody else should scan this QR code."
                )
                .to_string(),
            ))?;
            pm.prompt_message(PromptMessage::Data(qrcode))?;
        }
    }

    pm.prompt_message(PromptMessage::Text(format!(
        "Upon request, these words should be sent: {our_pubkey_mnemonic} {payload_mnemonic}"
    )))?;

    Ok(())
}

/// Combine mulitple shards into a secret.
///
/// # Errors
/// The function may return an error if an error occurs while decrypting shards, parsing shards, or
/// combining the shards into a secret.
pub fn combine(
    certs: Vec<Cert>,
    metadata: &EncryptedMessage,
    messages: Vec<EncryptedMessage>,
    mut output: impl Write,
) -> Result<()> {
    // Be as liberal as possible when decrypting.
    // We don't want to invalidate someone's keys just because the old sig expired.
    let policy = NullPolicy::new();

    let mut keyring = Keyring::new(certs)?;
    let mut manager = SmartcardManager::new()?;
    let content = decrypt_metadata(metadata, &policy, &mut keyring, &mut manager)?;

    let (threshold, root_cert, certs) = decode_metadata_v1(&content)?;

    keyring.set_root_cert(root_cert.clone());
    manager.set_root_cert(root_cert.clone());

    // Generate a controlled binding from certificates to encrypted messages. This is stable
    // because we control the order packets are encrypted and certificates are stored.

    let mut messages: HashMap<KeyID, EncryptedMessage> =
        certs.iter().map(Cert::keyid).zip(messages).collect();

    let mut decrypted_messages =
        decrypt_with_keyring(&mut messages, &certs, &policy, &mut keyring)?;

    // clean decrypted messages from encrypted messages
    messages.retain(|k, _v| !decrypted_messages.contains_key(k));

    let left_from_threshold = threshold as usize - decrypted_messages.len();
    if left_from_threshold > 0 {
        #[allow(clippy::cast_possible_truncation)]
        let new_messages = decrypt_with_manager(
            left_from_threshold as u8,
            &mut messages,
            &certs,
            &policy,
            &mut manager,
        )?;
        decrypted_messages.extend(new_messages);
    }

    let shares = decrypted_messages
        .values()
        .map(|message| Share::try_from(message.as_slice()))
        .collect::<Result<Vec<_>, &str>>()
        .map_err(|e| SharksError::Share(e.to_string()))?;
    let secret = Sharks(threshold)
        .recover(&shares)
        .map_err(|e| SharksError::CombineShare(e.to_string()))?;

    // TODO: extract as function
    let userid = UserID::from("keyfork-sss");
    let path = DerivationPath::from_str("m/7366512'/0'")?;
    let seed = VariableLengthSeed::new(&secret);
    let xprv = XPrv::new(seed).derive_path(&path)?;
    let derived_cert = keyfork_derive_openpgp::derive(
        xprv,
        &[KeyFlags::empty().set_certification().set_signing()],
        &userid,
    )?;

    // NOTE: Signatures on certs will be different. Compare fingerprints instead.
    let derived_fp = derived_cert.fingerprint();
    let expected_fp = root_cert.fingerprint();
    if derived_fp != expected_fp {
        return Err(Error::InvalidSecret(derived_fp, expected_fp));
    }

    output.write_all(&secret).map_err(Error::Io)?;

    Ok(())
}

/// Split a secret into an OpenPGP formatted Shard file.
///
/// # Errors
///
/// The function may return an error if the shards can't be encrypted to the provided OpenPGP
/// certs or if an error happens while writing the Shard file.
///
/// # Panics
///
/// The function may panic if the metadata can't properly store the certificates used to generate
/// the encrypted shares.
pub fn split(threshold: u8, certs: Vec<Cert>, secret: &[u8], output: impl Write) -> Result<()> {
    let seed = VariableLengthSeed::new(secret);
    // build cert to sign encrypted shares
    let userid = UserID::from("keyfork-sss");
    let path = DerivationPath::from_str("m/7366512'/0'")?;
    let xprv = XPrv::new(seed).derive_path(&path)?;
    let derived_cert = keyfork_derive_openpgp::derive(
        xprv,
        &[KeyFlags::empty().set_certification().set_signing()],
        &userid,
    )?;
    let signing_key = derived_cert
        .primary_key()
        .parts_into_secret()
        .map_err(Error::Sequoia)?
        .key()
        .clone()
        .into_keypair()
        .map_err(Error::Sequoia)?;

    let sharks = Sharks(threshold);
    let dealer = sharks.dealer(secret);
    let generated_shares = dealer.map(|s| Vec::from(&s)).collect::<Vec<_>>();
    let policy = StandardPolicy::new();
    let mut writer = Writer::new(output, Kind::Message).map_err(Error::SequoiaIo)?;

    let mut total_recipients = vec![];
    let mut messages = vec![];

    for (share, cert) in generated_shares.iter().zip(certs) {
        total_recipients.push(cert.clone());
        let valid_cert = cert.with_policy(&policy, None).map_err(Error::Sequoia)?;
        let encryption_keys = get_encryption_keys(&valid_cert).collect::<Vec<_>>();

        let mut message_output = vec![];
        let message = Message::new(&mut message_output);
        let message = Encryptor2::for_recipients(
            message,
            encryption_keys
                .iter()
                .map(|k| Recipient::new(KeyID::wildcard(), k.key())),
        )
        .build()
        .map_err(Error::Sequoia)?;
        let message = Signer::new(message, signing_key.clone())
            .build()
            .map_err(Error::Sequoia)?;
        let mut message = LiteralWriter::new(message)
            .build()
            .map_err(Error::Sequoia)?;
        message.write_all(share).map_err(Error::SequoiaIo)?;
        message.finalize().map_err(Error::Sequoia)?;

        messages.push(message_output);
    }

    let mut pp = vec![SHARD_METADATA_VERSION, threshold];
    // store derived cert to verify provided shares
    derived_cert.serialize(&mut pp).map_err(Error::Sequoia)?;
    for recipient in &total_recipients {
        recipient.serialize(&mut pp).map_err(Error::Sequoia)?;
    }

    // verify packet pile
    for (packet_cert, cert) in openpgp::cert::CertParser::from_bytes(&pp[2..])
        .map_err(Error::Sequoia)?
        .skip(1)
        .zip(total_recipients.iter())
    {
        assert_eq!(
            &packet_cert.map_err(Error::Sequoia)?,
            cert,
            "packet pile could not recreate cert: {}",
            cert.fingerprint()
        );
    }

    let valid_certs = total_recipients
        .iter()
        .map(|c| c.with_policy(&policy, None))
        .collect::<openpgp::Result<Vec<_>>>()
        .map_err(Error::Sequoia)?;

    let total_recipients = valid_certs.iter().flat_map(|vc| {
        get_encryption_keys(vc).map(|key| Recipient::new(KeyID::wildcard(), key.key()))
    });

    // metadata
    let mut message_output = vec![];
    let message = Message::new(&mut message_output);
    let message = Encryptor2::for_recipients(message, total_recipients)
        .build()
        .map_err(Error::Sequoia)?;
    let mut message = LiteralWriter::new(message)
        .build()
        .map_err(Error::Sequoia)?;
    message.write_all(&pp).map_err(Error::SequoiaIo)?;
    message.finalize().map_err(Error::Sequoia)?;
    writer
        .write_all(&message_output)
        .map_err(Error::SequoiaIo)?;

    for message in messages {
        writer.write_all(&message).map_err(Error::SequoiaIo)?;
    }

    writer.finalize().map_err(Error::SequoiaIo)?;

    Ok(())
}