cob: Parametrize `Change` with single signature

Signed-off-by: Alexis Sellier <alexis@radicle.xyz>

radicle-cob/src/backend/git/change.rs

@@ -64,6 +64,10 @@ pub mod error {
         NoChange(Oid),
         #[error("the 'change' found at '{0}' was not a blob")]
         ChangeNotBlob(Oid),
+        #[error("the 'change' found at '{0}' was not signed")]
+        ChangeNotSigned(Oid),
+        #[error("the 'change' found at '{0}' has more than one signature")]
+        TooManySignatures(Oid),
         #[error(transparent)]
         AuthorTrailer(#[from] trailers::error::InvalidAuthorTrailer),
         #[error(transparent)]
@@ -82,7 +86,7 @@ impl change::Storage for git2::Repository {
     type ObjectId = Oid;
     type Author = Oid;
     type Resource = Oid;
-    type Signatures = Signatures;
+    type Signatures = Signature;
 
     fn create<Signer>(
         &self,
@@ -127,7 +131,7 @@ impl change::Storage for git2::Repository {
         Ok(Change {
             id,
             revision: revision.into(),
-            signatures: signature.into(),
+            signature,
             author,
             resource,
             manifest,
@@ -138,7 +142,15 @@ impl change::Storage for git2::Repository {
     fn load(&self, id: Self::ObjectId) -> Result<Change, Self::LoadError> {
         let commit = Commit::read(self, id.into())?;
         let (author, resource) = parse_trailers(commit.trailers())?;
-        let signatures = Signatures::try_from(&commit)?;
+        let mut signatures = Signatures::try_from(&commit)?
+            .into_iter()
+            .collect::<Vec<_>>();
+        let Some(signature) = signatures.pop() else {
+            return Err(error::Load::ChangeNotSigned(id));
+        };
+        if !signatures.is_empty() {
+            return Err(error::Load::TooManySignatures(id));
+        }
 
         let tree = self.find_tree(commit.tree())?;
         let manifest = load_manifest(self, &tree)?;
@@ -147,7 +159,7 @@ impl change::Storage for git2::Repository {
         Ok(Change {
             id,
             revision: tree.id().into(),
-            signatures,
+            signature: signature.into(),
             author,
             resource,
             manifest,

radicle-cob/src/change.rs

@@ -8,9 +8,9 @@ use git_ext::Oid;
 pub mod store;
 pub use store::{Create, Storage};
 
-use crate::signatures::Signatures;
+use crate::signatures::Signature;
 
 /// A single change in the change graph. The layout of changes in the repository
 /// is specified in the RFC (docs/rfc/0662-collaborative-objects.adoc)
 /// under "Change Commits".
-pub type Change = store::Change<Oid, Oid, Oid, Signatures>;
+pub type Change = store::Change<Oid, Oid, Oid, Signature>;

radicle-cob/src/change/store.rs

@@ -54,14 +54,14 @@ pub struct Create<Id> {
 }
 
 #[derive(Clone, Debug)]
-pub struct Change<Author, Resource, Id, Signatures> {
+pub struct Change<Author, Resource, Id, Signature> {
     /// The content address of the `Change` itself.
     pub id: Id,
     /// The content address of the tree of the `Change`.
     pub revision: Id,
-    /// The cryptographic signatures and their public keys of the
+    /// The cryptographic signature(s) and their public keys of the
     /// authors.
-    pub signatures: Signatures,
+    pub signature: Signature,
     /// The author of this change. The `Author` is expected to be a
     /// content address to look up the identity of the author.
     pub author: Option<Author>,
@@ -111,12 +111,21 @@ where
     Id: AsRef<[u8]>,
 {
     pub fn valid_signatures(&self) -> bool {
-        self.signatures
+        self.signature
             .iter()
             .all(|(key, sig)| key.verify(self.revision.as_ref(), sig).is_ok())
     }
 }
 
+impl<A, R, Id> Change<A, R, Id, signatures::Signature>
+where
+    Id: AsRef<[u8]>,
+{
+    pub fn valid_signatures(&self) -> bool {
+        self.signature.verify(self.revision.as_ref())
+    }
+}
+
 #[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct Manifest {
     /// The name given to the type of collaborative object.

radicle-cob/src/change_graph.rs

@@ -15,7 +15,7 @@ use petgraph::{
 };
 
 use crate::{
-    change, object, signatures::Signatures, Change, CollaborativeObject, ObjectId, TypeName,
+    change, object, signatures::Signature, Change, CollaborativeObject, ObjectId, TypeName,
 };
 
 mod evaluation;
@@ -37,7 +37,7 @@ impl ChangeGraph {
         oid: &ObjectId,
     ) -> Option<ChangeGraph>
     where
-        S: change::Storage<ObjectId = Oid, Author = Oid, Resource = Oid, Signatures = Signatures>,
+        S: change::Storage<ObjectId = Oid, Author = Oid, Resource = Oid, Signatures = Signature>,
     {
         log::info!("loading object '{}' '{}'", typename, oid);
         let mut builder = GraphBuilder::default();

radicle-cob/src/lib.rs

@@ -97,7 +97,7 @@ pub use history::{Contents, Entry, History, HistoryType};
 mod pruning_fold;
 
 pub mod signatures;
-use signatures::Signatures;
+use signatures::Signature;
 
 pub mod type_name;
 pub use type_name::TypeName;
@@ -134,7 +134,7 @@ where
             ObjectId = git_ext::Oid,
             Author = git_ext::Oid,
             Resource = git_ext::Oid,
-            Signatures = Signatures,
+            Signatures = Signature,
         >,
 {
 }

radicle-cob/src/signatures.rs

@@ -24,6 +24,12 @@ pub struct Signature {
     sig: crypto::Signature,
 }
 
+impl Signature {
+    pub fn verify(&self, payload: &[u8]) -> bool {
+        self.key.verify(payload, &self.sig).is_ok()
+    }
+}
+
 impl From<Signature> for ExtendedSignature {
     fn from(sig: Signature) -> Self {
         Self::new(sig.key, sig.sig)