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

@@ -7,33 +7,52 @@ use std::{
     process::ExitCode,
 };
 
-use keyfork_shard::{openpgp::OpenPGP, Format};
+use keyfork_shard::openpgp::{combine, discover_certs, openpgp::Cert, parse_messages};
 
 type Result<T, E = Box<dyn std::error::Error>> = std::result::Result<T, E>;
 
 fn validate(
     shard: impl AsRef<Path>,
     key_discovery: Option<&str>,
-) -> Result<(File, Option<PathBuf>)> {
+) -> Result<(File, Vec<Cert>)> {
     let key_discovery = key_discovery.map(PathBuf::from);
     key_discovery.as_ref().map(std::fs::metadata).transpose()?;
 
-    Ok((File::open(shard)?, key_discovery))
+    // Load certs from path
+    let certs = key_discovery
+        .map(discover_certs)
+        .transpose()?
+        .unwrap_or(vec![]);
+
+    Ok((File::open(shard)?, certs))
 }
 
 fn run() -> Result<()> {
     let mut args = env::args();
     let program_name = args.next().expect("program name");
     let args = args.collect::<Vec<_>>();
-    let (messages_file, key_discovery) = match args.as_slice() {
+    let (messages_file, cert_list) = match args.as_slice() {
         [shard, key_discovery] => validate(shard, Some(key_discovery))?,
         [shard] => validate(shard, None)?,
         _ => panic!("Usage: {program_name} <shard> [key_discovery]"),
     };
 
-    let openpgp = OpenPGP;
+    let mut encrypted_messages = parse_messages(messages_file)?;
-    let bytes = openpgp.decrypt_all_shards_to_secret(key_discovery.as_deref(), messages_file)?;
+
-    print!("{}", smex::encode(bytes));
+    let encrypted_metadata = encrypted_messages
+        .pop_front()
+        .expect("any pgp encrypted message");
+
+    let mut bytes = vec![];
+
+    combine(
+        cert_list,
+        &encrypted_metadata,
+        encrypted_messages.into(),
+        &mut bytes,
+    )?;
+
+    print!("{}", smex::encode(&bytes));
 
     Ok(())
 }

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

@@ -7,33 +7,47 @@ use std::{
     process::ExitCode,
 };
 
-use keyfork_shard::{Format, openpgp::OpenPGP};
+use keyfork_shard::openpgp::{decrypt, discover_certs, openpgp::Cert, parse_messages};
 
 type Result<T, E = Box<dyn std::error::Error>> = std::result::Result<T, E>;
 
-fn validate(
+fn validate<'a>(
-    shard: impl AsRef<Path>,
+    messages_file: impl AsRef<Path>,
-    key_discovery: Option<&str>,
+    key_discovery: impl Into<Option<&'a str>>,
-) -> Result<(File, Option<PathBuf>)> {
+) -> Result<(File, Vec<Cert>)> {
-    let key_discovery = key_discovery.map(PathBuf::from);
+    let key_discovery = key_discovery.into().map(PathBuf::from);
     key_discovery.as_ref().map(std::fs::metadata).transpose()?;
 
-    Ok((File::open(shard)?, key_discovery))
+    // Load certs from path
+    let certs = key_discovery
+        .map(discover_certs)
+        .transpose()?
+        .unwrap_or(vec![]);
+
+    Ok((File::open(messages_file)?, certs))
 }
 
 fn run() -> Result<()> {
     let mut args = env::args();
     let program_name = args.next().expect("program name");
     let args = args.collect::<Vec<_>>();
-    let (messages_file, key_discovery) = match args.as_slice() {
+    let (messages_file, cert_list) = match args.as_slice() {
-        [shard, key_discovery] => validate(shard, Some(key_discovery))?,
+        [messages_file, key_discovery] => validate(messages_file, key_discovery.as_str())?,
-        [shard] => validate(shard, None)?,
+        [messages_file] => validate(messages_file, None)?,
-        _ => panic!("Usage: {program_name} <shard> [key_discovery]"),
+        _ => panic!("Usage: {program_name} messages_file [key_discovery]"),
     };
 
-    let openpgp = OpenPGP;
+    let mut encrypted_messages = parse_messages(messages_file)?;
 
-    openpgp.decrypt_one_shard_for_transport(key_discovery.as_deref(), messages_file)?;
+    let encrypted_metadata = encrypted_messages
+        .pop_front()
+        .expect("any pgp encrypted message");
+
+    decrypt(
+        &cert_list,
+        &encrypted_metadata,
+        encrypted_messages.make_contiguous(),
+    )?;
 
     Ok(())
 }

crates/keyfork-shard/src/bin/keyfork-shard-remote.rs

@@ -20,7 +20,7 @@ fn run() -> Result<()> {
 
     let mut bytes = vec![];
     remote_decrypt(&mut bytes)?;
-    print!("{}", smex::encode(bytes));
+    print!("{}", smex::encode(&bytes));
 
     Ok(())
 }

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

@@ -2,12 +2,14 @@
 
 use std::{env, path::PathBuf, process::ExitCode, str::FromStr};
 
-use keyfork_shard::{Format, openpgp::OpenPGP};
+use keyfork_shard::openpgp::{discover_certs, openpgp::Cert, split};
 
 #[derive(Clone, Debug)]
 enum Error {
     Usage(String),
     Input,
+    Threshold(u8, u8),
+    InvalidCertCount(usize, u8),
 }
 
 impl std::fmt::Display for Error {
@@ -17,6 +19,15 @@ impl std::fmt::Display for Error {
                 write!(f, "Usage: {program_name} threshold max key_discovery")
             }
             Error::Input => f.write_str("Expected hex encoded input"),
+            Error::Threshold(threshold, max) => {
+                write!(
+                    f,
+                    "Invalid threshold: 0 < threshold {threshold} <= max {max} < 256"
+                )
+            }
+            Error::InvalidCertCount(count, max) => {
+                write!(f, "Invalid cert count: count {count} != max {max}")
+            }
         }
     }
 }
@@ -25,20 +36,31 @@ impl std::error::Error for Error {}
 
 type Result<T, E = Box<dyn std::error::Error>> = std::result::Result<T, E>;
 
-fn validate(threshold: &str, max: &str, key_discovery: &str) -> Result<(u8, u8, PathBuf)> {
+fn validate(threshold: &str, max: &str, key_discovery: &str) -> Result<(u8, Vec<Cert>)> {
     let threshold = u8::from_str(threshold)?;
     let max = u8::from_str(max)?;
     let key_discovery = PathBuf::from(key_discovery);
+    if threshold > max {
+        return Err(Error::Threshold(threshold, max).into());
+    }
+
+    // Verify path exists
     std::fs::metadata(&key_discovery)?;
 
-    Ok((threshold, max, key_discovery))
+    // Load certs from path
+    let certs = discover_certs(key_discovery)?;
+    if certs.len() != max.into() {
+        return Err(Error::InvalidCertCount(certs.len(), max).into());
+    }
+
+    Ok((threshold, certs))
 }
 
 fn run() -> Result<()> {
     let mut args = env::args();
     let program_name = args.next().expect("program name");
     let args = args.collect::<Vec<_>>();
-    let (threshold, max, key_discovery) = match args.as_slice() {
+    let (threshold, cert_list) = match args.as_slice() {
         [threshold, max, key_discovery] => validate(threshold, max, key_discovery)?,
         _ => return Err(Error::Usage(program_name).into()),
     };
@@ -50,9 +72,8 @@ fn run() -> Result<()> {
         smex::decode(line?)?
     };
 
-    let openpgp = OpenPGP;
+    split(threshold, cert_list, &input, std::io::stdout())?;
 
-    openpgp.shard_and_encrypt(threshold, max, &input, key_discovery.as_path(), std::io::stdout())?;
     Ok(())
 }
 

crates/keyfork-shard/src/lib.rs

@@ -1,10 +1,10 @@
 #![doc = include_str!("../README.md")]
 
-use std::io::{stdin, stdout, Read, Write};
+use std::io::{stdin, stdout, Write};
 
 use aes_gcm::{
-    aead::{consts::U12, Aead, AeadCore, OsRng},
+    aead::{Aead, AeadCore, OsRng},
-    Aes256Gcm, KeyInit, Nonce,
+    Aes256Gcm, KeyInit,
 };
 use hkdf::Hkdf;
 use keyfork_mnemonic_util::{English, Mnemonic};
@@ -16,338 +16,9 @@ use sha2::Sha256;
 use sharks::{Share, Sharks};
 use x25519_dalek::{EphemeralSecret, PublicKey};
 
-// 256 bit share encrypted is 49 bytes, couple more bytes before we reach max size
-const ENC_LEN: u8 = 4 * 16;
-
 #[cfg(feature = "openpgp")]
 pub mod openpgp;
 
-/// A trait to specify where keys can be discovered from, such as a Rust-native type or a path on
-/// the filesystem that keys may be read from.
-pub trait KeyDiscovery<F: Format + ?Sized> {
-    /// Discover public keys for the associated format.
-    ///
-    /// # Errors
-    /// The method may return an error if public keys could not be loaded from the given discovery
-    /// mechanism. A discovery mechanism _must_ be able to detect public keys.
-    fn discover_public_keys(&self) -> Result<Vec<F::PublicKey>, F::Error>;
-
-    /// Discover private keys for the associated format.
-    ///
-    /// # Errors
-    /// The method may return an error if private keys could not be loaded from the given
-    /// discovery mechanism. Keys may exist off-system (such as with smartcards), in which case the
-    /// PrivateKeyData type of the asssociated format should be either `()` (if the keys may never
-    /// exist on-system) or an empty container (such as an empty Vec); in either case, this method
-    /// _must not_ return an error if keys are accessible but can't be transferred into memory.
-    fn discover_private_keys(&self) -> Result<F::PrivateKeyData, F::Error>;
-}
-
-/// A format to use for splitting and combining secrets.
-pub trait Format {
-    /// The error type returned from any failed operations.
-    type Error: std::error::Error + 'static;
-
-    /// A type encapsulating a single public key recipient.
-    type PublicKey;
-
-    /// A type encapsulating the private key recipients of shards.
-    type PrivateKeyData;
-
-    /// A type representing a Signer derived from the secret.
-    type SigningKey;
-
-    /// A type representing the parsed, but encrypted, Shard data.
-    type EncryptedData;
-
-    /// Derive a signer
-    fn derive_signing_key(&self, seed: &[u8]) -> Self::SigningKey;
-
-    /// Format a header containing necessary metadata. Such metadata contains a version byte, a
-    /// threshold byte, a public version of the [`Format::SigningKey`], and the public keys used to
-    /// encrypt shards. The public keys must be kept _in order_ to the encrypted shards. Keyfork
-    /// will use the same key_data for both, ensuring an iteration of this method will match with
-    /// iterations in methods called later.
-    ///
-    /// # Errors
-    /// The method may return an error if encryption to any of the public keys fails.
-    fn format_encrypted_header(
-        &self,
-        signing_key: &Self::SigningKey,
-        key_data: &[Self::PublicKey],
-        threshold: u8,
-    ) -> Result<Self::EncryptedData, Self::Error>;
-
-    /// Format a shard encrypted to the given public key, signing with the private key.
-    ///
-    /// # Errors
-    /// The method may return an error if the public key used to encrypt the shard is unsuitable
-    /// for encryption, or if an error occurs while encrypting.
-    fn encrypt_shard(
-        &self,
-        shard: &[u8],
-        public_key: &Self::PublicKey,
-        signing_key: &mut Self::SigningKey,
-    ) -> Result<Self::EncryptedData, Self::Error>;
-
-    /// Parse the Shard file into a processable type.
-    ///
-    /// # Errors
-    /// The method may return an error if the Shard file could not be read from or if the Shard
-    /// file could not be properly parsed.
-    fn parse_shard_file(
-        &self,
-        shard_file: impl Read + Send + Sync,
-    ) -> Result<Vec<Self::EncryptedData>, Self::Error>;
-
-    /// Write the Shard data to a Shard file.
-    ///
-    /// # Errors
-    /// The method may return an error if the Shard data could not be properly serialized or if the
-    /// Shard file could not be written to.
-    fn format_shard_file(
-        &self,
-        encrypted_data: &[Self::EncryptedData],
-        shard_file: impl Write + Send + Sync,
-    ) -> Result<(), Self::Error>;
-
-    /// Decrypt shares and associated metadata from a readable input. For the current version of
-    /// Keyfork, the only associated metadata is a u8 representing the threshold to combine
-    /// secrets.
-    ///
-    /// # Errors
-    /// The method may return an error if the shardfile couldn't be read from, if all shards
-    /// could not be decrypted, or if a shard could not be parsed from the decrypted data.
-    fn decrypt_all_shards(
-        &self,
-        private_keys: Option<Self::PrivateKeyData>,
-        encrypted_messages: &[Self::EncryptedData],
-    ) -> Result<(Vec<Share>, u8), Self::Error>;
-
-    /// Decrypt a single share and associated metadata from a reaable input. For the current
-    /// version of Keyfork, the only associated metadata is a u8 representing the threshold to
-    /// combine secrets.
-    ///
-    /// # Errors
-    /// The method may return an error if the shardfile couldn't be read from, if a shard could not
-    /// be decrypted, or if a shard could not be parsed from the decrypted data.
-    fn decrypt_one_shard(
-        &self,
-        private_keys: Option<Self::PrivateKeyData>,
-        encrypted_data: &[Self::EncryptedData],
-    ) -> Result<(Share, u8), Self::Error>;
-
-    /// Decrypt multiple shares and combine them to recreate a secret.
-    ///
-    /// # Errors
-    /// The method may return an error if the shares can't be decrypted or if the shares can't
-    /// be combined into a secret.
-    fn decrypt_all_shards_to_secret(
-        &self,
-        private_key_discovery: Option<impl KeyDiscovery<Self>>,
-        reader: impl Read + Send + Sync,
-    ) -> Result<Vec<u8>, Box<dyn std::error::Error>> {
-        let private_keys = private_key_discovery
-            .map(|p| p.discover_private_keys())
-            .transpose()?;
-        let encrypted_messages = self.parse_shard_file(reader)?;
-        let (shares, threshold) = self.decrypt_all_shards(private_keys, &encrypted_messages)?;
-
-        let secret = Sharks(threshold)
-            .recover(&shares)
-            .map_err(|e| SharksError::CombineShare(e.to_string()))?;
-
-        Ok(secret)
-    }
-
-    /// Establish an AES-256-GCM transport key using ECDH, decrypt a single shard, and encrypt the
-    /// shard to the AES key.
-    ///
-    /// # Errors
-    /// The method may return an error if a share can't be decrypted. The method will not return an
-    /// error if the camera is inaccessible or if a hardware error is encountered while scanning a
-    /// QR code; instead, a mnemonic prompt will be used.
-    fn decrypt_one_shard_for_transport(
-        &self,
-        private_key_discovery: Option<impl KeyDiscovery<Self>>,
-        reader: impl Read + Send + Sync,
-    ) -> Result<(), Box<dyn std::error::Error>> {
-        let mut pm = Terminal::new(stdin(), stdout())?;
-
-        // parse input
-        let private_keys = private_key_discovery
-            .map(|p| p.discover_private_keys())
-            .transpose()?;
-        let encrypted_messages = self.parse_shard_file(reader)?;
-
-        // establish AES-256-GCM key via ECDH
-        let mut nonce_data: Option<[u8; 12]> = None;
-        let mut pubkey_data: Option<[u8; 32]> = None;
-
-        // receive remote data via scanning QR code from camera
-        #[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(30), 0)
-            {
-                let decoded_data = smex::decode(&hex)?;
-                nonce_data = Some(decoded_data[..12].try_into().map_err(|_| InvalidData)?);
-                pubkey_data = Some(decoded_data[12..].try_into().map_err(|_| InvalidData)?)
-            } else {
-                pm.prompt_message(PromptMessage::Text(QRCODE_ERROR.to_string()))?;
-            };
-        }
-
-        // if QR code scanning failed or was unavailable, read from a set of mnemonics
-        let (nonce, their_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::<English, _>(
-                        QRCODE_COULDNT_READ,
-                        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)
-            }
-        };
-
-        // create our shared key
-        let our_key = EphemeralSecret::random();
-        let our_pubkey_mnemonic = Mnemonic::from_bytes(PublicKey::from(&our_key).as_bytes())?;
-        let shared_secret = our_key
-            .diffie_hellman(&PublicKey::from(their_pubkey))
-            .to_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)?;
-
-        // decrypt a single shard and create the payload
-        let (share, threshold) = self.decrypt_one_shard(private_keys, &encrypted_messages)?;
-        let mut payload = Vec::from(&share);
-        payload.insert(0, HUNK_VERSION);
-        payload.insert(1, threshold);
-        assert!(
-            payload.len() <= ENC_LEN as usize,
-            "invalid share length (too long, max {ENC_LEN} bytes)"
-        );
-
-        // encrypt data
-        let nonce = Nonce::<U12>::from_slice(&nonce);
-        let payload_bytes = shared_key.encrypt(nonce, payload.as_slice())?;
-
-        // convert data to a static-size payload
-        // NOTE: Padding length is less than u8::MAX because ENC_LEN < u8::MAX
-        #[allow(clippy::assertions_on_constants)]
-        {
-            assert!(ENC_LEN < u8::MAX, "padding byte can be u8");
-        }
-        #[allow(clippy::cast_possible_truncation)]
-        let mut out_bytes = [payload_bytes.len() as u8; ENC_LEN as usize];
-        assert!(
-            payload_bytes.len() < out_bytes.len(),
-            "encrypted payload larger than acceptable limit"
-        );
-        out_bytes[..payload_bytes.len()].clone_from_slice(&payload_bytes);
-
-        // NOTE: This previously used a single repeated 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 resolved below by having sequentially increasing numbers up to
-        // but not including the last byte.
-        #[allow(clippy::cast_possible_truncation)]
-        for (i, byte) in (out_bytes[payload_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) };
-
-        #[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(())
-    }
-
-    /// Split a secret into a shard for every shard in keys, with the given Shamir's Secret Sharing
-    /// threshold.
-    ///
-    /// # Errors
-    /// The method may return an error if the shares can't be encrypted.
-    fn shard_and_encrypt(
-        &self,
-        threshold: u8,
-        max: u8,
-        secret: &[u8],
-        public_key_discovery: impl KeyDiscovery<Self>,
-        writer: impl Write + Send + Sync,
-    ) -> Result<(), Box<dyn std::error::Error>> {
-        let mut signing_key = self.derive_signing_key(secret);
-
-        let sharks = Sharks(threshold);
-        let dealer = sharks.dealer(secret);
-
-        let public_keys = public_key_discovery.discover_public_keys()?;
-        assert!(
-            public_keys.len() < u8::MAX as usize,
-            "must have less than u8::MAX public keys"
-        );
-        assert_eq!(
-            max,
-            public_keys.len() as u8,
-            "max must be equal to amount of public keys"
-        );
-        let max = public_keys.len() as u8;
-        assert!(max >= threshold, "threshold must not exceed max keys");
-
-        let header = self.format_encrypted_header(&signing_key, &public_keys, threshold)?;
-        let mut messages = vec![header];
-        for (pk, share) in public_keys.iter().zip(dealer) {
-            let shard = Vec::from(&share);
-            messages.push(self.encrypt_shard(&shard, pk, &mut signing_key)?);
-        }
-
-        self.format_shard_file(&messages, writer)?;
-
-        Ok(())
-    }
-}
-
 /// Errors encountered while creating or combining shares using Shamir's Secret Sharing.
 #[derive(thiserror::Error, Debug)]
 pub enum SharksError {

crates/keyfork-shard/src/openpgp.rs

@@ -1,16 +1,27 @@
 //! OpenPGP Shard functionality.
 
 use std::{
-    collections::HashMap,
+    collections::{HashMap, VecDeque},
-    io::{Read, Write},
+    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, VariableLengthSeed},
+    derive_util::{DerivationPath, PathError, VariableLengthSeed},
     XPrv,
 };
+use keyfork_mnemonic_util::{English, Mnemonic, MnemonicFromStrError, MnemonicGenerationError};
+use keyfork_prompt::{
+    validators::{mnemonic::MnemonicSetValidator, Validator},
+    Error as PromptError, Message as PromptMessage, PromptHandler, Terminal,
+};
 use openpgp::{
     armor::{Kind, Writer},
     cert::{Cert, CertParser, ValidCert},
@@ -25,10 +36,12 @@ use openpgp::{
         Marshal,
     },
     types::KeyFlags,
-    KeyID, PacketPile,
+    Fingerprint, KeyID, PacketPile,
 };
 pub use sequoia_openpgp as openpgp;
-use sharks::Share;
+use sha2::Sha256;
+use sharks::{Share, Sharks};
+use x25519_dalek::{EphemeralSecret, PublicKey};
 
 mod keyring;
 use keyring::Keyring;
@@ -43,7 +56,13 @@ use smartcard::SmartcardManager;
 const SHARD_METADATA_VERSION: u8 = 1;
 const SHARD_METADATA_OFFSET: usize = 2;
 
-use super::{Format, KeyDiscovery, SharksError};
+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)]
@@ -52,6 +71,22 @@ pub enum Error {
     #[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),
@@ -68,9 +103,45 @@ pub enum Error {
     #[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)]
@@ -92,61 +163,6 @@ impl EncryptedMessage {
         }
     }
 
-    /// Parse OpenPGP packets for encrypted messages.
-    ///
-    /// # Errors
-    /// The function may return an error if Sequoia is unable to parse packets.
-    ///
-    /// # Panics
-    /// The function may panic if an unexpected packet is encountered.
-    pub fn from_reader(input: impl Read + Send + Sync) -> openpgp::Result<Vec<Self>> {
-        let mut pkesks = Vec::new();
-        let mut encrypted_messages = vec![];
-
-        for packet in PacketPile::from_reader(input)
-            .map_err(Error::Sequoia)?
-            .into_children()
-        {
-            match packet {
-                Packet::PKESK(p) => pkesks.push(p),
-                Packet::SEIP(s) => {
-                    encrypted_messages.push(EncryptedMessage::new(&mut pkesks, s));
-                }
-                s => {
-                    panic!("Invalid variant found: {}", s.tag());
-                }
-            }
-        }
-
-        Ok(encrypted_messages)
-    }
-
-    /// Serialize all contents of the message to a writer.
-    ///
-    /// # Errors
-    /// The function may error for any condition in Sequoia's Serialize trait.
-    fn serialize(&self, mut o: impl std::io::Write + Send + Sync) -> openpgp::Result<()> {
-        for pkesk in &self.pkesks {
-            let mut packet = vec![];
-            pkesk.serialize(&mut packet).map_err(Error::Sequoia)?;
-            let message = Message::new(&mut o);
-            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 o);
-        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)?;
-
-        Ok(())
-    }
-
     /// Decrypt the message with a Sequoia policy and decryptor.
     ///
     /// This method creates a container containing the packets and passes the serialized container
@@ -160,7 +176,23 @@ impl EncryptedMessage {
         H: VerificationHelper + DecryptionHelper,
     {
         let mut packets = vec![];
-        self.serialize(&mut packets).map_err(Error::Sequoia)?;
+
+        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)?
@@ -175,17 +207,13 @@ impl EncryptedMessage {
     }
 }
 
+/// 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.
 ///
-pub struct OpenPGP;
+/// # Errors
-
+/// The function may return an error if it is unable to read the directory or if Sequoia is unable
-impl OpenPGP {
+/// to load certificates from the file.
-    /// Read all OpenPGP certificates in a path and return a [`Vec`] of them. Certificates are read
+pub fn discover_certs(path: impl AsRef<Path>) -> Result<Vec<Cert>> {
-    /// 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() {
@@ -206,317 +234,37 @@ impl OpenPGP {
         }
         Ok(vec)
     }
-    }
 }
 
-impl Format for OpenPGP {
+/// Parse messages from a type implementing [`Read`] and store them as [`EncryptedMessage`].
-    type Error = Error;
+///
-    type PublicKey = Cert;
+/// # Errors
-    type PrivateKeyData = Vec<Cert>;
+/// The function may return an error if the reader has run out of data or if the data is not
-    type SigningKey = Cert;
+/// properly formatted OpenPGP messages.
-    type EncryptedData = EncryptedMessage;
+///
+/// # 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();
 
-    /// Derive an OpenPGP Shard certificate from the given seed.
+    for packet in PacketPile::from_reader(reader)
-    fn derive_signing_key(&self, seed: &[u8]) -> Self::SigningKey {
-        let seed = VariableLengthSeed::new(seed);
-        // build cert to sign encrypted shares
-        let userid = UserID::from("keyfork-sss");
-        let path = DerivationPath::from_str("m/7366512'/0'").expect("valid derivation path");
-        let xprv = XPrv::new(seed)
-            .derive_path(&path)
-            .expect("valid derivation");
-        keyfork_derive_openpgp::derive(
-            xprv,
-            &[KeyFlags::empty().set_certification().set_signing()],
-            &userid,
-        )
-        .expect("valid cert creation")
-    }
-
-    fn format_encrypted_header(
-        &self,
-        signing_key: &Self::SigningKey,
-        key_data: &[Self::PublicKey],
-        threshold: u8,
-    ) -> Result<Self::EncryptedData, Self::Error> {
-        let policy = StandardPolicy::new();
-        let mut pp = vec![SHARD_METADATA_VERSION, threshold];
-        // Note: Sequoia does not export private keys on a Cert, only on a TSK
-        signing_key
-            .serialize(&mut pp)
-            .expect("serialize cert into bytes");
-        for cert in key_data {
-            cert.serialize(&mut pp)
-                .expect("serialize pubkey into bytes");
-        }
-
-        // verify packet pile
-        let mut iter = openpgp::cert::CertParser::from_bytes(&pp[SHARD_METADATA_OFFSET..])
-            .expect("should have certs");
-        let first_cert = iter.next().transpose().ok().flatten().expect("first cert");
-        assert_eq!(signing_key, &first_cert);
-
-        for (packet_cert, cert) in iter.zip(key_data) {
-            assert_eq!(
-                &packet_cert.expect("parsed packet cert"),
-                cert,
-                "packet pile could not recreate cert: {}",
-                cert.fingerprint(),
-            );
-        }
-
-        let valid_certs = key_data
-            .iter()
-            .map(|c| c.with_policy(&policy, None))
-            .collect::<openpgp::Result<Vec<_>>>()
-            .map_err(Error::Sequoia)?;
-        let recipients = valid_certs.iter().flat_map(|vc| {
-            get_encryption_keys(vc).map(|key| Recipient::new(KeyID::wildcard(), key.key()))
-        });
-
-        // Process is as follows:
-        // * Any OpenPGP message
-        // * An encrypted message
-        // * A literal message
-        // * The packet pile
-        //
-        // When decrypting, OpenPGP will see:
-        // * A message, and parse it
-        // * An encrypted message, and decrypt it
-        // * A literal message, and extract it
-        // * The packet pile
-        let mut output = vec![];
-        let message = Message::new(&mut output);
-        let encrypted_message = Encryptor2::for_recipients(message, recipients)
-            .build()
-            .map_err(Error::Sequoia)?;
-        let mut literal_message = LiteralWriter::new(encrypted_message)
-            .build()
-            .map_err(Error::Sequoia)?;
-        literal_message.write_all(&pp).map_err(Error::SequoiaIo)?;
-        literal_message.finalize().map_err(Error::Sequoia)?;
-
-        // Parse it into an EncryptedMessage. Yes, this takes a serialized message
-        // and deserializes it. Don't think about it too hard. It's easier this way.
-
-        let mut pkesks = vec![];
-        for packet in PacketPile::from_reader(output.as_slice())
         .map_err(Error::Sequoia)?
         .into_children()
     {
         match packet {
             Packet::PKESK(p) => pkesks.push(p),
-                Packet::SEIP(s) => return Ok(EncryptedMessage::new(&mut pkesks, s)),
+            Packet::SEIP(s) => {
-                s => panic!("Invalid variant found: {}", s.tag()),
+                encrypted_messages.push_back(EncryptedMessage::new(&mut pkesks, s));
+            }
+            s => {
+                panic!("Invalid variant found: {}", s.tag());
+            }
         }
     }
 
-        panic!("Unable to build EncryptedMessage from PacketPile");
+    Ok(encrypted_messages)
-    }
-
-    fn encrypt_shard(
-        &self,
-        shard: &[u8],
-        public_key: &Cert,
-        signing_key: &mut Self::SigningKey,
-    ) -> Result<EncryptedMessage> {
-        let policy = StandardPolicy::new();
-        let valid_cert = public_key
-            .with_policy(&policy, None)
-            .map_err(Error::Sequoia)?;
-        let encryption_keys = get_encryption_keys(&valid_cert).collect::<Vec<_>>();
-
-        let signing_key = signing_key
-            .primary_key()
-            .parts_into_secret()
-            .map_err(Error::Sequoia)?
-            .key()
-            .clone()
-            .into_keypair()
-            .map_err(Error::Sequoia)?;
-
-        // Process is as follows:
-        // * Any OpenPGP message
-        // * An encrypted message
-        // * A signed message
-        // * A literal message
-        // * The shard itself
-        //
-        // When decrypting, OpenPGP will see:
-        // * A message, and parse it
-        // * An encrypted message, and decrypt it
-        // * A signed message, and verify it
-        // * A literal message, and extract it
-        // * The shard itself
-        let mut message_output = vec![];
-        let message = Message::new(&mut message_output);
-        let encrypted_message = Encryptor2::for_recipients(
-            message,
-            encryption_keys
-                .iter()
-                .map(|k| Recipient::new(KeyID::wildcard(), k.key())),
-        )
-        .build()
-        .map_err(Error::Sequoia)?;
-        let signed_message = Signer::new(encrypted_message, signing_key)
-            .build()
-            .map_err(Error::Sequoia)?;
-        let mut message = LiteralWriter::new(signed_message)
-            .build()
-            .map_err(Error::Sequoia)?;
-        message.write_all(shard).map_err(Error::SequoiaIo)?;
-        message.finalize().map_err(Error::Sequoia)?;
-
-        let message = EncryptedMessage::from_reader(message_output.as_slice())
-            .map_err(Error::Sequoia)?
-            .into_iter()
-            .next()
-            .expect("serialized message should be parseable");
-
-        Ok(message)
-    }
-
-    fn parse_shard_file(
-        &self,
-        shard_file: impl Read + Send + Sync,
-    ) -> Result<Vec<Self::EncryptedData>, Self::Error> {
-        EncryptedMessage::from_reader(shard_file).map_err(Error::Sequoia)
-    }
-
-    fn format_shard_file(
-        &self,
-        encrypted_data: &[Self::EncryptedData],
-        shard_file: impl Write + Send + Sync,
-    ) -> Result<(), Self::Error> {
-        let mut writer = Writer::new(shard_file, Kind::Message).map_err(Error::SequoiaIo)?;
-        for message in encrypted_data {
-            message.serialize(&mut writer).map_err(Error::Sequoia)?;
-        }
-        writer.finalize().map_err(Error::SequoiaIo)?;
-        Ok(())
-    }
-
-    fn decrypt_all_shards(
-        &self,
-        private_keys: Option<Self::PrivateKeyData>,
-        encrypted_data: &[Self::EncryptedData],
-    ) -> std::result::Result<(Vec<Share>, u8), Self::Error> {
-        // 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(private_keys.unwrap_or_default())?;
-        let mut manager = SmartcardManager::new()?;
-
-        let mut encrypted_messages = encrypted_data.iter();
-
-        let metadata = encrypted_messages.next().expect("metdata");
-        let metadata_content = decrypt_metadata(metadata, &policy, &mut keyring, &mut manager)?;
-
-        let (threshold, root_cert, certs) = decode_metadata_v1(&metadata_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.
-
-        // TODO: remove alloc, convert EncryptedMessage to &EncryptedMessage
-        let mut messages: HashMap<KeyID, EncryptedMessage> = certs
-            .iter()
-            .map(Cert::keyid)
-            .zip(encrypted_messages.cloned())
-            .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()))?;
-        Ok((shares, threshold))
-    }
-
-    fn decrypt_one_shard(
-        &self,
-        private_keys: Option<Self::PrivateKeyData>,
-        encrypted_data: &[Self::EncryptedData],
-    ) -> std::result::Result<(Share, u8), Self::Error> {
-        let policy = NullPolicy::new();
-        let mut keyring = Keyring::new(private_keys.unwrap_or_default())?;
-        let mut manager = SmartcardManager::new()?;
-
-        let mut encrypted_messages = encrypted_data.iter();
-
-        let metadata = encrypted_messages.next().expect("metadata");
-        let metadata_content = decrypt_metadata(metadata, &policy, &mut keyring, &mut manager)?;
-
-        let (threshold, root_cert, certs) = decode_metadata_v1(&metadata_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(encrypted_messages.cloned())
-            .collect();
-
-        let decrypted_messages =
-            decrypt_with_keyring(&mut messages, &certs, &policy, &mut keyring)?;
-
-        if let Some(message) = decrypted_messages.into_values().next() {
-            let share = Share::try_from(message.as_slice())
-                .map_err(|e| SharksError::Share(e.to_string()))?;
-            return Ok((share, threshold));
-        }
-
-        let decrypted_messages =
-            decrypt_with_manager(1, &mut messages, &certs, &policy, &mut manager)?;
-
-        if let Some(message) = decrypted_messages.into_values().next() {
-            let share = Share::try_from(message.as_slice())
-                .map_err(|e| SharksError::Share(e.to_string()))?;
-            return Ok((share, threshold));
-        }
-
-        panic!("unable to decrypt shard");
-    }
-}
-
-impl KeyDiscovery<OpenPGP> for &Path {
-    fn discover_public_keys(&self) -> Result<Vec<<OpenPGP as Format>::PublicKey>> {
-        OpenPGP::discover_certs(self)
-    }
-
-    fn discover_private_keys(&self) -> Result<<OpenPGP as Format>::PrivateKeyData> {
-        todo!()
-    }
-}
-
-impl KeyDiscovery<OpenPGP> for &[Cert] {
-    fn discover_public_keys(&self) -> Result<Vec<<OpenPGP as Format>::PublicKey>> {
-        Ok(self.to_vec())
-    }
-
-    fn discover_private_keys(&self) -> Result<<OpenPGP as Format>::PrivateKeyData> {
-        Ok(self.to_vec())
-    }
 }
 
 fn get_encryption_keys<'a>(
@@ -667,3 +415,378 @@ fn decrypt_metadata(
         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 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::<English, _>(
+                QRCODE_COULDNT_READ,
+                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())?;
+
+    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) };
+
+    #[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(())
+}

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

@@ -111,10 +111,12 @@ impl DecryptionHelper for &mut Keyring {
                 pkesk.recipient().is_wildcard()
                     || cert.keys().any(|k| &k.keyid() == pkesk.recipient())
             }) {
+                #[allow(deprecated, clippy::map_flatten)]
                 let name = cert
                     .userids()
                     .next()
-                    .and_then(|userid| userid.userid().name2().transpose())
+                    .map(|userid| userid.userid().name().transpose())
+                    .flatten()
                     .transpose()
                     .ok()
                     .flatten();

crates/keyfork/src/cli/recover.rs

@@ -3,7 +3,10 @@ use clap::{Parser, Subcommand};
 use std::path::PathBuf;
 
 use keyfork_mnemonic_util::{English, Mnemonic};
-use keyfork_shard::{remote_decrypt, Format};
+use keyfork_shard::{
+    openpgp::{combine, discover_certs, parse_messages},
+    remote_decrypt,
+};
 
 type Result<T, E = Box<dyn std::error::Error>> = std::result::Result<T, E>;
 
@@ -34,10 +37,15 @@ impl RecoverSubcommands {
             } => {
                 let content = std::fs::read_to_string(shard_file)?;
                 if content.contains("BEGIN PGP MESSAGE") {
-                    let openpgp = keyfork_shard::openpgp::OpenPGP;
+                    let certs = key_discovery
-                    // TODO: remove .clone() by making handle() consume self
+                        .as_ref()
-                    let seed = openpgp
+                        .map(discover_certs)
-                        .decrypt_all_shards_to_secret(key_discovery.as_deref(), content.as_bytes())?;
+                        .transpose()?
+                        .unwrap_or(vec![]);
+                    let mut messages = parse_messages(content.as_bytes())?;
+                    let metadata = messages.pop_front().expect("any pgp encrypted message");
+                    let mut seed = vec![];
+                    combine(certs, &metadata, messages.into(), &mut seed)?;
                     Ok(seed)
                 } else {
                     panic!("unknown format of shard file");

crates/keyfork/src/cli/shard.rs

@@ -1,6 +1,5 @@
 use super::Keyfork;
 use clap::{builder::PossibleValue, Parser, Subcommand, ValueEnum};
-use keyfork_shard::Format as _;
 use std::{
     io::{stdin, stdout, Read, Write},
     path::{Path, PathBuf},
@@ -32,23 +31,27 @@ trait ShardExec {
         &self,
         threshold: u8,
         max: u8,
-        key_discovery: &Path,
+        key_discovery: impl AsRef<Path>,
         secret: &[u8],
-        output: &mut (impl Write + Send + Sync),
-    ) -> Result<(), Box<dyn std::error::Error>>;
-
-    fn combine(
-        &self,
-        key_discovery: Option<&Path>,
-        input: impl Read + Send + Sync,
         output: &mut impl Write,
     ) -> Result<(), Box<dyn std::error::Error>>;
 
-    fn decrypt(
+    fn combine<T>(
         &self,
-        key_discovery: Option<&Path>,
+        key_discovery: Option<T>,
         input: impl Read + Send + Sync,
-    ) -> Result<(), Box<dyn std::error::Error>>;
+        output: &mut impl Write,
+    ) -> Result<(), Box<dyn std::error::Error>>
+    where
+        T: AsRef<Path>;
+
+    fn decrypt<T>(
+        &self,
+        key_discovery: Option<T>,
+        input: impl Read + Send + Sync,
+    ) -> Result<(), Box<dyn std::error::Error>>
+    where
+        T: AsRef<Path>;
 }
 
 #[derive(Clone, Debug)]
@@ -59,36 +62,77 @@ impl ShardExec for OpenPGP {
         &self,
         threshold: u8,
         max: u8,
-        key_discovery: &Path,
+        key_discovery: impl AsRef<Path>,
         secret: &[u8],
-        output: &mut (impl Write + Send + Sync),
+        output: &mut impl Write,
     ) -> Result<(), Box<dyn std::error::Error>> {
-        let opgp = keyfork_shard::openpgp::OpenPGP;
+        // Get certs and input
-        opgp.shard_and_encrypt(threshold, max, secret, key_discovery, output)
+        let certs = keyfork_shard::openpgp::discover_certs(key_discovery.as_ref())?;
+        assert_eq!(
+            certs.len(),
+            max.into(),
+            "cert count {} != max {max}",
+            certs.len()
+        );
+        keyfork_shard::openpgp::split(threshold, certs, secret, output).map_err(Into::into)
     }
 
-    fn combine(
+    fn combine<T>(
         &self,
-        key_discovery: Option<&Path>,
+        key_discovery: Option<T>,
         input: impl Read + Send + Sync,
         output: &mut impl Write,
     ) -> Result<(), Box<dyn std::error::Error>>
+    where
+        T: AsRef<Path>,
     {
-        let openpgp = keyfork_shard::openpgp::OpenPGP;
+        let certs = key_discovery
-        let bytes = openpgp.decrypt_all_shards_to_secret(key_discovery, input)?;
+            .map(|kd| keyfork_shard::openpgp::discover_certs(kd.as_ref()))
-        write!(output, "{}", smex::encode(bytes))?;
+            .transpose()?
+            .unwrap_or(vec![]);
+
+        let mut encrypted_messages = keyfork_shard::openpgp::parse_messages(input)?;
+        let encrypted_metadata = encrypted_messages
+            .pop_front()
+            .expect("any pgp encrypted message");
+
+        let mut bytes = vec![];
+
+        keyfork_shard::openpgp::combine(
+            certs,
+            &encrypted_metadata,
+            encrypted_messages.into(),
+            &mut bytes,
+        )?;
+
+        write!(output, "{}", smex::encode(&bytes))?;
 
         Ok(())
     }
 
-    fn decrypt(
+    fn decrypt<T>(
         &self,
-        key_discovery: Option<&Path>,
+        key_discovery: Option<T>,
         input: impl Read + Send + Sync,
     ) -> Result<(), Box<dyn std::error::Error>>
+    where
+        T: AsRef<Path>,
     {
-        let openpgp = keyfork_shard::openpgp::OpenPGP;
+        let certs = key_discovery
-        openpgp.decrypt_one_shard_for_transport(key_discovery, input)?;
+            .map(|kd| keyfork_shard::openpgp::discover_certs(kd.as_ref()))
+            .transpose()?
+            .unwrap_or(vec![]);
+
+        let mut encrypted_messages = keyfork_shard::openpgp::parse_messages(input)?;
+        let encrypted_metadata = encrypted_messages
+            .pop_front()
+            .expect("any pgp encrypted message");
+
+        keyfork_shard::openpgp::decrypt(
+            &certs,
+            &encrypted_metadata,
+            encrypted_messages.make_contiguous(),
+        )?;
         Ok(())
     }
 }
@@ -181,7 +225,7 @@ impl ShardSubcommands {
 
                 match format {
                     Some(Format::OpenPGP(o)) => {
-                        o.decrypt(key_discovery.as_deref(), shard_content.as_bytes())
+                        o.decrypt(key_discovery.as_ref(), shard_content.as_bytes())
                     }
                     Some(Format::P256(_p)) => todo!(),
                     None => panic!("{COULD_NOT_DETERMINE_FORMAT}"),
@@ -198,7 +242,7 @@ impl ShardSubcommands {
 
                 match format {
                     Some(Format::OpenPGP(o)) => o.combine(
-                        key_discovery.as_deref(),
+                        key_discovery.as_ref(),
                         shard_content.as_bytes(),
                         &mut stdout,
                     ),

crates/keyfork/src/cli/wizard.rs

@@ -15,8 +15,6 @@ use keyfork_prompt::{
     Message, PromptHandler, Terminal,
 };
 
-use keyfork_shard::{Format, openpgp::OpenPGP};
-
 #[derive(thiserror::Error, Debug)]
 #[error("Invalid PIN length: {0}")]
 pub struct PinLength(usize);
@@ -165,13 +163,11 @@ fn generate_shard_secret(
         certs.push(cert);
     }
 
-    let opgp = OpenPGP;
-
     if let Some(output_file) = output_file {
         let output = File::create(output_file)?;
-        opgp.shard_and_encrypt(threshold, certs.len() as u8, &seed, &certs[..], output)?;
+        keyfork_shard::openpgp::split(threshold, certs, &seed, output)?;
     } else {
-        opgp.shard_and_encrypt(threshold, certs.len() as u8, &seed, &certs[..], std::io::stdout())?;
+        keyfork_shard::openpgp::split(threshold, certs, &seed, std::io::stdout())?;
     }
     Ok(())
 }