keyfork-shard: derive cert from combined secret to ensure data integrity

keyfork-shard/src/openpgp.rs

@@ -20,7 +20,7 @@ use openpgp::{
         Marshal,
     },
     types::KeyFlags,
-    KeyID, PacketPile
+    KeyID, PacketPile,
 };
 pub use sequoia_openpgp as openpgp;
 use sharks::{Share, Sharks};
@@ -234,6 +234,31 @@ pub fn combine(
         .map_err(|e| WrappedError(e.to_string()))?;
     let secret = Sharks(threshold).recover(&shares)?;
 
+    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,
+    )?;
+
+    // NOTE: Signatures on certs will be different. Compare fingerprints instead.
+    if Some(derived_cert.fingerprint()) != keyring.root_cert().map(Cert::fingerprint) {
+        return Err(WrappedError(format!(
+            "Derived {} != expected {}",
+            derived_cert.fingerprint(),
+            keyring
+                .root_cert()
+                .expect("cert was previously set")
+                .fingerprint()
+        ))
+        .into());
+    }
+
     output.write_all(smex::encode(&secret).as_bytes())?;
 
     Ok(())

keyfork-shard/src/openpgp/keyring.rs

@@ -47,6 +47,10 @@ impl Keyring {
         cert
     }
 
+    pub fn root_cert(&self) -> Option<&Cert> {
+        self.root.as_ref()
+    }
+
     pub fn get_cert_for_primary_keyid<'a>(&'a self, keyid: &KeyID) -> Option<&'a Cert> {
         self.full_certs.iter().find(|cert| &cert.keyid() == keyid)
     }