Locally store the signatures for oci-images archives

On air-gapped environements, it's now possible to load signatures
generated by `cosign save` commands. The signatures embedded in this
format will be converted to the one used by `cosign download signature`.

dangerzone/container_utils.py

@@ -155,6 +155,9 @@ def load_image_tarball_file(tarball_path: str) -> None:
 
 
 def tag_image_by_digest(digest: str, tag: str) -> None:
+    """Tag a container image by digest.
+    The sha256: prefix should be omitted from the digest.
+    """
     image_id = get_image_id_by_digest(digest)
     cmd = [get_runtime(), "tag", image_id, tag]
     log.debug(" ".join(cmd))
@@ -162,11 +165,14 @@ def tag_image_by_digest(digest: str, tag: str) -> None:
 
 
 def get_image_id_by_digest(digest: str) -> str:
+    """Get an image ID from a digest.
+    The sha256: prefix should be omitted from the digest.
+    """
     cmd = [
         get_runtime(),
         "images",
         "-f",
-        f"digest={digest}",
+        f"digest=sha256:{digest}",
         "--format",
         "{{.Id}}",
     ]
@@ -174,7 +180,9 @@ def get_image_id_by_digest(digest: str) -> str:
     process = subprocess.run(
         cmd, startupinfo=get_subprocess_startupinfo(), check=True, capture_output=True
     )
-    return process.stdout.decode().strip()
+    breakpoint()
+    # In case we have multiple lines, we only want the first one.
+    return process.stdout.decode().strip().split("\n")[0]
 
 
 def container_pull(image: str) -> bool:

dangerzone/updater/errors.py

@@ -22,6 +22,10 @@ class SignatureVerificationError(SignatureError):
     pass
 
 
+class SignatureExtractionError(SignatureError):
+    pass
+
+
 class SignaturesFolderDoesNotExist(SignatureError):
     pass
 

dangerzone/updater/signatures.py

@@ -3,7 +3,7 @@ import platform
 import re
 import subprocess
 import tarfile
-from base64 import b64decode
+from base64 import b64decode, b64encode
 from hashlib import sha256
 from io import BytesIO
 from pathlib import Path
@@ -156,6 +156,10 @@ def upgrade_container_image(image: str, manifest_hash: str, pubkey: str) -> bool
     return runtime.container_pull(image)
 
 
+def _get_blob(tmpdir: str, hash: str) -> Path:
+    return Path(tmpdir) / "blobs" / "sha256" / hash.replace("sha256:", "")
+
+
 def upgrade_container_image_airgapped(
     container_tar: str, pubkey: str, image_name: str
 ) -> bool:
@@ -166,8 +170,19 @@ def upgrade_container_image_airgapped(
     Right now, the archive is extracted and reconstructed, requiring some space
     on the filesystem.
     """
+
     # XXX Use a memory buffer instead of the filesystem
     with TemporaryDirectory() as tmpdir:
+
+        def _get_signature_filename(manifests: List[Dict]) -> Path:
+            for manifest in manifests:
+                if (
+                    manifest["annotations"].get("kind")
+                    == "dev.cosignproject.cosign/sigs"
+                ):
+                    return _get_blob(tmpdir, manifest["digest"])
+            raise errors.SignatureExtractionError()
+
         with tarfile.open(container_tar, "r") as archive:
             archive.extractall(tmpdir)
 
@@ -179,14 +194,19 @@ def upgrade_container_image_airgapped(
         # Remove the signatures from the archive.
         with open(Path(tmpdir) / "index.json") as f:
             index_json = json.load(f)
-            index_json["manifests"] = [
-                manifest
-                for manifest in index_json["manifests"]
-                if manifest["annotations"].get("kind")
-                != "dev.cosignproject.cosign/sigs"
-            ]
 
-        image_digest = index_json["manifests"][0].get("digest")
+        signature_filename = _get_signature_filename(index_json["manifests"])
+
+        index_json["manifests"] = [
+            manifest
+            for manifest in index_json["manifests"]
+            if manifest["annotations"].get("kind") != "dev.cosignproject.cosign/sigs"
+        ]
+
+        with open(signature_filename, "rb") as f:
+            signatures = convert_oci_images_signatures(json.load(f), tmpdir)
+
+        image_digest = index_json["manifests"][0].get("digest").replace("sha256:", "")
 
         # Write the new index.json to the temp folder
         with open(Path(tmpdir) / "index.json", "w") as f:
@@ -202,15 +222,34 @@ def upgrade_container_image_airgapped(
             runtime.load_image_tarball_file(temporary_tar.name)
             runtime.tag_image_by_digest(image_digest, image_name)
 
-    # XXX Convert the signatures to the expected format
-
-    # At this point, the signatures are verified
-    # We store the signatures just now to avoid storing unverified signatures
-    # store_signatures(signatures, image_hash, pubkey)
-
+    store_signatures(signatures, image_digest, pubkey)
     return True
 
 
+def convert_oci_images_signatures(
+    signatures_manifest: List[Dict], tmpdir: str
+) -> List[Dict]:
+    def _to_cosign_signature(layer: Dict) -> Dict:
+        signature = layer["annotations"]["dev.cosignproject.cosign/signature"]
+        bundle = json.loads(layer["annotations"]["dev.sigstore.cosign/bundle"])
+        payload_body = json.loads(b64decode(bundle["Payload"]["body"]))
+
+        payload_location = _get_blob(tmpdir, layer["digest"])
+        with open(payload_location, "rb") as f:
+            payload_b64 = b64encode(f.read()).decode()
+
+        return {
+            "Base64Signature": payload_body["spec"]["signature"]["content"],
+            "Payload": payload_b64,
+            "Cert": None,
+            "Chain": None,
+            "rekorBundle": bundle,
+            "RFC3161Timestamp": None,
+        }
+
+    return [_to_cosign_signature(layer) for layer in signatures_manifest["layers"]]
+
+
 def get_file_hash(file: Optional[str] = None, content: Optional[bytes] = None) -> str:
     """Get the sha256 hash of a file or content"""
     if not file and not content:
@@ -268,7 +307,9 @@ def store_signatures(signatures: list[Dict], image_hash: str, pubkey: str) -> No
         raise errors.InvalidSignatures("Signatures do not share the same image hash")
 
     if f"sha256:{image_hash}" != hashes[0]:
-        raise errors.SignatureMismatch("Signatures do not match the given image hash")
+        raise errors.SignatureMismatch(
+            f"Signatures do not match the given image hash ({image_hash}, {hashes[0]})"
+        )
 
     pubkey_signatures = SIGNATURES_PATH / get_file_hash(pubkey)
     pubkey_signatures.mkdir(exist_ok=True)