keyfork-shard: begin work on (re)provisioning shardholder keys

Cargo.lock

@@ -1829,6 +1829,7 @@ dependencies = [
  "card-backend-pcsc",
  "hkdf",
  "keyfork-derive-openpgp",
+ "keyfork-derive-util",
  "keyfork-mnemonic-util",
  "keyfork-prompt",
  "keyfork-qrcode",

crates/derive/keyfork-derive-openpgp/src/main.rs

@@ -78,9 +78,7 @@ fn validate(
     subkey_format: &str,
     default_userid: &str,
 ) -> Result<(DerivationPath, Vec<KeyType>, UserID), Box<dyn std::error::Error>> {
-    let mut pgp_u32 = [0u8; 4];
-    pgp_u32[1..].copy_from_slice(&"pgp".bytes().collect::<Vec<u8>>());
-    let index = DerivationIndex::new(u32::from_be_bytes(pgp_u32), true)?;
+    let index = DerivationIndex::new(u32::from_be_bytes(*b"\x00pgp"), true)?;
 
     let path = DerivationPath::from_str(path)?;
     assert_eq!(2, path.len(), "Expected path of m/{index}/account_id'");

crates/keyfork-shard/Cargo.toml

@@ -36,3 +36,4 @@ card-backend-pcsc = { version = "0.5.0", optional = true }
 openpgp-card-sequoia = { version = "0.2.0", optional = true, default-features = false }
 openpgp-card = { version = "0.4.0", optional = true }
 sequoia-openpgp = { version = "1.17.0", optional = true, default-features = false }
+keyfork-derive-util = { version = "0.1.0", path = "../derive/keyfork-derive-util", default-features = false }

crates/keyfork-shard/src/lib.rs

@@ -7,6 +7,7 @@ use aes_gcm::{
     Aes256Gcm, KeyInit, Nonce,
 };
 use hkdf::Hkdf;
+use keyfork_derive_util::{DerivationIndex, DerivationPath};
 use keyfork_mnemonic_util::{English, Mnemonic};
 use keyfork_prompt::{
     validators::{mnemonic::MnemonicSetValidator, Validator},
@@ -60,6 +61,52 @@ pub trait Format {
     /// A type representing the parsed, but encrypted, Shard data.
     type EncryptedData;
 
+    /// Provision hardware with a deterministic key based on a shardholder's DerivationIndex.
+    ///
+    /// The derivation path for provisioned shardholder keys is built using the following template:
+    /// `m / purpose ' / shard_index ' / shardholder_index '`.
+    ///
+    /// Purpose is defined by the Format, and can be a four-byte sequence transformed into a u32
+    /// using `u32::from_be_bytes(*purpose)`. For OpenPGP, for legacy reasons, this purpose is
+    /// "\x00pgp". The purpose can be _any_ sequence of four bytes so long as the _first_ byte is
+    /// not higher than 0x80 (meaning, all ASCII / 7-bit characters are allowed).
+    ///
+    /// The shard index is provided by Keyfork, and is equivalent to b"shrd".
+    ///
+    /// The shardholder index is how Keyfork is able to recreate keys for specific shardholders -
+    /// the only necessary information is which shardholder is not accounted for. Shardholders are
+    /// encouraged to mark hardware with the shardholder number so shardholders can verify their
+    /// index.
+    fn provision_shardholder_key(
+        &self,
+        derivation_path: DerivationPath,
+        seed: &[u8],
+    ) -> Result<(), Self::Error>;
+
+    /// Return a DerivationIndex for the Format.
+    ///
+    /// The derivation path for provisioned shardholder keys is built using the following template:
+    /// `m / purpose ' / shard_index ' / shardholder_index '`.
+    ///
+    /// Purpose is defined by the Format, and can be a four-byte sequence transformed into a u32
+    /// using `u32::from_be_bytes(*purpose)`. For OpenPGP, for legacy reasons, this purpose is
+    /// "\x00pgp". The purpose can be _any_ sequence of four bytes so long as the _first_ byte is
+    /// not higher than 0x80 (meaning, all ASCII / 7-bit characters are allowed).
+    fn purpose_derivation_index(&self) -> DerivationIndex;
+
+    /// Create a shardholder derivation path for the given format.
+    ///
+    /// The derivation path for provisioned shardholder keys is built using the following template:
+    /// `m / purpose ' / shard_index ' / shardholder_index '`.
+    fn create_derivation_path(&self, shardholder_index: DerivationIndex) -> DerivationPath {
+        let purpose = self.purpose_derivation_index();
+        let shard_index = DerivationIndex::new(u32::from_be_bytes(*b"shrd"), true).unwrap();
+        DerivationPath::default()
+            .chain_push(purpose)
+            .chain_push(shard_index)
+            .chain_push(shardholder_index)
+    }
+
     /// Derive a signer
     fn derive_signing_key(&self, seed: &[u8]) -> Self::SigningKey;
 

crates/keyfork-shard/src/openpgp.rs

@@ -8,7 +8,7 @@ use std::{
 };
 
 use keyfork_derive_openpgp::{
-    derive_util::{DerivationPath, VariableLengthSeed},
+    derive_util::{DerivationIndex, DerivationPath, VariableLengthSeed},
     XPrv,
 };
 use openpgp::{
@@ -216,6 +216,18 @@ impl Format for OpenPGP {
     type SigningKey = Cert;
     type EncryptedData = EncryptedMessage;
 
+    fn provision_shardholder_key(
+        &self,
+        derivation_path: DerivationPath,
+        seed: &[u8],
+    ) -> Result<(), Self::Error> {
+        todo!()
+    }
+
+    fn purpose_derivation_index(&self) -> DerivationIndex {
+        DerivationIndex::new(u32::from_be_bytes(*b"\x00pgp"), true).unwrap()
+    }
+
     /// Derive an OpenPGP Shard certificate from the given seed.
     fn derive_signing_key(&self, seed: &[u8]) -> Self::SigningKey {
         let seed = VariableLengthSeed::new(seed);