keyfork-shard: traitify functionality

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

@@ -7,50 +7,33 @@ use std::{
     process::ExitCode,
 };
 
-use keyfork_shard::openpgp::{combine, discover_certs, openpgp::Cert, parse_messages};
+use keyfork_shard::{Format, openpgp::OpenPGP};
 
 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, Vec<Cert>)> {
+) -> Result<(File, Option<PathBuf>)> {
     let key_discovery = key_discovery.map(PathBuf::from);
     key_discovery.as_ref().map(std::fs::metadata).transpose()?;
 
-    // Load certs from path
-    let certs = key_discovery
-        .map(discover_certs)
-        .transpose()?
-        .unwrap_or(vec![]);
-
-    Ok((File::open(shard)?, certs))
+    Ok((File::open(shard)?, key_discovery))
 }
 
 fn run() -> Result<()> {
     let mut args = env::args();
     let program_name = args.next().expect("program name");
     let args = args.collect::<Vec<_>>();
-    let (messages_file, cert_list) = match args.as_slice() {
+    let (messages_file, key_discovery) = match args.as_slice() {
         [shard, key_discovery] => validate(shard, Some(key_discovery))?,
         [shard] => validate(shard, None)?,
         _ => panic!("Usage: {program_name} <shard> [key_discovery]"),
     };
 
-    let mut encrypted_messages = parse_messages(messages_file)?;
+    let openpgp = OpenPGP;
 
-    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,
-    )?;
+    let bytes = openpgp.decrypt_all_shards_to_secret(key_discovery, messages_file)?;
 
     print!("{}", smex::encode(&bytes));
 

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

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

crates/keyfork-shard/src/lib.rs

@@ -1,10 +1,13 @@
 #![doc = include_str!("../README.md")]
 
-use std::io::{stdin, stdout, Write};
+use std::{
+    io::{stdin, stdout, Read, Write},
+    path::Path,
+};
 
 use aes_gcm::{
-    aead::{Aead, AeadCore, OsRng},
-    Aes256Gcm, KeyInit,
+    aead::{consts::U12, Aead, AeadCore, OsRng},
+    Aes256Gcm, KeyInit, Nonce,
 };
 use hkdf::Hkdf;
 use keyfork_mnemonic_util::{Mnemonic, Wordlist};
@@ -16,9 +19,279 @@ 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 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 the public key recipients of shards.
+    type PublicKeyData;
+
+    /// A type encapsulating the private key recipients of shards.
+    type PrivateKeyData;
+
+    /// A type representing the parsed, but encrypted, Shard data.
+    type ShardData;
+
+    /// A type representing a Signer derived from the secret.
+    type Signer;
+
+    /// Parse the public key data from a readable type.
+    ///
+    /// # Errors
+    /// The method may return an error if private key data could not be properly parsed from the
+    /// path.
+    /// occurred while parsing the public key data.
+    fn parse_public_key_data(
+        &self,
+        key_data_path: impl AsRef<Path>,
+    ) -> Result<Self::PublicKeyData, Self::Error>;
+
+    /// Parse the private key data from a readable type. The private key may not be accessible (it
+    /// may be hardware only, such as a smartcard), for which this method may return None.
+    ///
+    /// # Errors
+    /// The method may return an error if private key data could not be properly parsed from the
+    /// path.
+    fn parse_private_key_data(
+        &self,
+        key_data_path: impl AsRef<Path>,
+    ) -> Result<Self::PrivateKeyData, 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<Self::ShardData, 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,
+        shard_data: Self::ShardData,
+        shard_file: impl Write,
+    ) -> Result<(), Self::Error>;
+
+    /// Derive a Signer from the secret.
+    ///
+    /// # Errors
+    /// This function may return an error if a Signer could not be properly created.
+    fn derive_signer(&self, secret: &[u8]) -> Result<Self::Signer, Self::Error>;
+
+    /// Encrypt multiple shares to public keys.
+    ///
+    /// # Errors
+    /// The method may return an error if the share could not be encrypted to a public key or if
+    /// the ShardData could not be created.
+    fn generate_shard_data(
+        &self,
+        shares: &[Share],
+        signer: &Self::Signer,
+        public_keys: Self::PublicKeyData,
+    ) -> Result<Self::ShardData, 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>,
+        shard_data: Self::ShardData,
+    ) -> 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>,
+        shard_data: Self::ShardData,
+    ) -> 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_data_path: Option<impl AsRef<Path>>,
+        reader: impl Read + Send + Sync,
+    ) -> Result<Vec<u8>, Box<dyn std::error::Error>> {
+        let private_keys = private_key_data_path
+            .map(|p| self.parse_private_key_data(p))
+            .transpose()?;
+        let shard_data = self.parse_shard_file(reader)?;
+        let (shares, threshold) = self.decrypt_all_shards(private_keys, shard_data)?;
+
+        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_data_path: Option<impl AsRef<Path>>,
+        reader: impl Read + Send + Sync,
+    ) -> Result<(), Box<dyn std::error::Error>> {
+        let mut pm = Terminal::new(stdin(), stdout())?;
+        let wordlist = Wordlist::default();
+
+        // parse input
+        let private_keys = private_key_data_path
+            .map(|p| self.parse_private_key_data(p))
+            .transpose()?;
+        let shard_data = 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(
+                "Press enter, then present QR code to camera".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(
+                    "Unable to detect QR code, falling back to text".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("Their words: ", &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)
+            }
+        };
+
+        // create our shared key
+        let our_key = EphemeralSecret::random();
+        let our_pubkey_mnemonic =
+            Mnemonic::from_entropy(PublicKey::from(&our_key).as_bytes(), Default::default())?;
+        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, shard_data)?;
+        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_entropy(&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::Lowest) {
+                pm.prompt_message(PromptMessage::Data(qrcode))?;
+            }
+        }
+
+        pm.prompt_message(PromptMessage::Text(format!(
+            "Our words: {our_pubkey_mnemonic} {payload_mnemonic}"
+        )))?;
+
+        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

@@ -56,7 +56,7 @@ use smartcard::SmartcardManager;
 const SHARD_METADATA_VERSION: u8 = 1;
 const SHARD_METADATA_OFFSET: usize = 2;
 
-use super::{InvalidData, SharksError, HUNK_VERSION};
+use super::{Format, InvalidData, SharksError, HUNK_VERSION};
 
 // 256 bit share is 49 bytes + some amount of hunk bytes, gives us reasonable padding
 const ENC_LEN: u8 = 4 * 16;
@@ -156,6 +156,18 @@ impl EncryptedMessage {
         }
     }
 
+    /// Serialize all contents of the message to a writer.
+    ///
+    /// # Errors
+    /// The function may error for any condition in Sequoia's Serialize trait.
+    pub fn serialize(&self, o: &mut dyn std::io::Write) -> openpgp::Result<()> {
+        for pkesk in &self.pkesks {
+            pkesk.serialize(o)?;
+        }
+        self.message.serialize(o)?;
+        Ok(())
+    }
+
     /// Decrypt the message with a Sequoia policy and decryptor.
     ///
     /// This method creates a container containing the packets and passes the serialized container
@@ -200,12 +212,275 @@ impl EncryptedMessage {
     }
 }
 
+///
+pub struct OpenPGP;
+
+impl OpenPGP {
+    /// 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)
+        }
+    }
+}
+
+impl Format for OpenPGP {
+    type Error = Error;
+    type PublicKeyData = Vec<Cert>;
+    type PrivateKeyData = Vec<Cert>;
+    type ShardData = Vec<EncryptedMessage>;
+    type Signer = openpgp::crypto::KeyPair;
+
+    fn parse_public_key_data(
+        &self,
+        key_data_path: impl AsRef<Path>,
+    ) -> std::result::Result<Self::PublicKeyData, Self::Error> {
+        Self::discover_certs(key_data_path)
+    }
+
+    fn parse_private_key_data(
+        &self,
+        key_data_path: impl AsRef<Path>,
+    ) -> std::result::Result<Self::PrivateKeyData, Self::Error> {
+        Self::discover_certs(key_data_path)
+    }
+
+    fn parse_shard_file(
+        &self,
+        shard_file: impl Read + Send + Sync,
+    ) -> Result<Self::ShardData, Self::Error> {
+        let mut pkesks = Vec::new();
+        let mut encrypted_messages = vec![];
+
+        for packet in PacketPile::from_reader(shard_file)
+            .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)
+    }
+
+    fn derive_signer(&self, secret: &[u8]) -> Result<Self::Signer, Self::Error> {
+        let userid = UserID::from("keyfork-sss");
+        let kdr = DerivationRequest::new(
+            DerivationAlgorithm::Ed25519,
+            &DerivationPath::from_str("m/7366512'/0'")?,
+        )
+        .derive_with_master_seed(secret.to_vec())?;
+        let derived_cert = keyfork_derive_openpgp::derive(
+            kdr,
+            &[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)?;
+
+        Ok(signing_key)
+    }
+
+    fn format_shard_file(
+        &self,
+        shard_data: Self::ShardData,
+        shard_file: impl Write,
+    ) -> Result<(), Self::Error> {
+        let mut writer = Writer::new(shard_file, Kind::Message).map_err(Error::SequoiaIo)?;
+        for message in shard_data {
+            message.serialize(&mut writer).map_err(Error::Sequoia)?;
+        }
+        writer.finalize().map_err(Error::SequoiaIo)?;
+        Ok(())
+    }
+
+    fn generate_shard_data(
+        &self,
+        shares: &[Share],
+        signer: &Self::Signer,
+        public_keys: Self::PublicKeyData,
+    ) -> std::result::Result<Self::ShardData, Self::Error> {
+        let policy = StandardPolicy::new();
+        let mut total_recipients = vec![];
+        let mut messages = vec![];
+
+        for (share, cert) in shares.iter().zip(public_keys) {
+            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, signer.clone())
+                .build()
+                .map_err(Error::Sequoia)?;
+            let mut message = LiteralWriter::new(message)
+                .build()
+                .map_err(Error::Sequoia)?;
+            // NOTE: This shouldn't be an alloc, but it's a minor alloc, so it's fine.
+            message
+                .write_all(&Vec::from(share))
+                .map_err(Error::SequoiaIo)?;
+            message.finalize().map_err(Error::Sequoia)?;
+
+            messages.push(message_output);
+        }
+
+        // A little bit of back and forth, we're going to parse the messages just to serialize them
+        // later.
+        let message = messages.into_iter().flatten().collect::<Vec<_>>();
+        let data = self.parse_shard_file(message.as_slice())?;
+        Ok(data)
+    }
+
+    fn decrypt_all_shards(
+        &self,
+        private_keys: Option<Self::PrivateKeyData>,
+        mut shard_data: Self::ShardData,
+    ) -> 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 metadata = shard_data.remove(0);
+        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(shard_data)
+            .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>,
+        mut shard_data: Self::ShardData,
+    ) -> 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 metadata = shard_data.remove(0);
+        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(shard_data)
+            .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");
+    }
+}
+
 /// 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.
+#[deprecated]
 pub fn discover_certs(path: impl AsRef<Path>) -> Result<Vec<Cert>> {
     let path = path.as_ref();
 
@@ -238,6 +513,7 @@ pub fn discover_certs(path: impl AsRef<Path>) -> Result<Vec<Cert>> {
 /// # 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.
+#[deprecated]
 pub fn parse_messages(reader: impl Read + Send + Sync) -> Result<VecDeque<EncryptedMessage>> {
     let mut pkesks = Vec::new();
     let mut encrypted_messages = VecDeque::new();
@@ -409,6 +685,7 @@ fn decrypt_metadata(
     })
 }
 
+#[deprecated]
 fn decrypt_one(
     messages: Vec<EncryptedMessage>,
     certs: &[Cert],
@@ -458,6 +735,8 @@ fn decrypt_one(
 /// 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`].
+#[deprecated]
+#[allow(deprecated)]
 pub fn decrypt(
     certs: &[Cert],
     metadata: &EncryptedMessage,