GCPSigner import

pyproject.toml

@@ -44,7 +44,7 @@ Issues = "https://github.com/secure-systems-lab/securesystemslib/issues"
 
 [project.optional-dependencies]
 crypto = ["cryptography>=37.0.0"]
-gcpkms = ["google-cloud-kms"]
+gcpkms = ["google-cloud-kms", "cryptography>=37.0.0"]
 pynacl = ["pynacl>1.2.0"]
 PySPX = ["PySPX==0.5.0"]
 asn1 = ["asn1crypto"]

securesystemslib/signer/_gcp_signer.py

@@ -1,19 +1,26 @@
 """Signer implementation for Google Cloud KMS"""
 
 import logging
-from typing import Optional
+from typing import Optional, Tuple
 from urllib import parse
 
 import securesystemslib.hash as sslib_hash
 from securesystemslib import exceptions
+from securesystemslib.keys import _get_keyid
 from securesystemslib.signer._key import Key
-from securesystemslib.signer._signer import SecretsHandler, Signature, Signer
+from securesystemslib.signer._signer import (
+    SecretsHandler,
+    Signature,
+    Signer,
+    SSlibKey,
+)
 
 logger = logging.getLogger(__name__)
 
 GCP_IMPORT_ERROR = None
 try:
     from google.cloud import kms
+    from google.cloud.kms_v1.types import CryptoKeyVersion
 except ImportError:
     GCP_IMPORT_ERROR = (
         "google-cloud-kms library required to sign with Google Cloud keys."
@@ -29,9 +36,14 @@ class GCPSigner(Signer):
     The signer uses "ambient" credentials: typically environment var
     GOOGLE_APPLICATION_CREDENTIALS that points to a file with valid
     credentials. These will be found by google.cloud.kms, see
-    https://cloud.google.com/docs/authentication/getting-started
-    (and https://github.com/google-github-actions/auth for the relevant
-    GitHub action).
+    https://cloud.google.com/docs/authentication/getting-started.
+    Some practical authentication options include:
+    * GitHub Action: https://github.com/google-github-actions/auth
+    * gcloud CLI: https://cloud.google.com/sdk/gcloud
+
+    The specific permissions that GCPSigner needs are:
+    * roles/cloudkms.signer for sign()
+    * roles/cloudkms.publicKeyViewer for import()
 
     Arguments:
         gcp_keyid: Fully qualified GCP KMS key name, like
@@ -71,6 +83,79 @@ def from_priv_key_uri(
 
         return cls(uri.path, public_key)
 
+    @classmethod
+    def import_(cls, gcp_keyid: str) -> Tuple[str, Key]:
+        """Load key and signer details from KMS
+
+        Returns the private key uri and the public key. This method should only
+        be called once per key: the uri and Key should be stored for later use.
+        """
+        if GCP_IMPORT_ERROR:
+            raise exceptions.UnsupportedLibraryError(GCP_IMPORT_ERROR)
+
+        client = kms.KeyManagementServiceClient()
+        request = {"name": gcp_keyid}
+        kms_pubkey = client.get_public_key(request)
+        try:
+            keytype, scheme = cls._get_keytype_and_scheme(kms_pubkey.algorithm)
+        except KeyError as e:
+            raise exceptions.UnsupportedAlgorithmError(
+                f"{kms_pubkey.algorithm} is not a supported signing algorithm"
+            ) from e
+
+        keyval = {"public": kms_pubkey.pem}
+        keyid = _get_keyid(keytype, scheme, keyval)
+        public_key = SSlibKey(keyid, keytype, scheme, keyval)
+
+        return f"{cls.SCHEME}:{gcp_keyid}", public_key
+
+    @staticmethod
+    def _get_keytype_and_scheme(algorithm: int) -> Tuple[str, str]:
+        """Return keytype and scheme for the KMS algorithm enum"""
+        keytypes_and_schemes = {
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.EC_SIGN_P256_SHA256: (
+                "ecdsa",
+                "ecdsa-sha2-nistp256",
+            ),
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.EC_SIGN_P384_SHA384: (
+                "ecdsa",
+                "ecdsa-sha2-nistp384",
+            ),
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.RSA_SIGN_PSS_2048_SHA256: (
+                "rsa",
+                "rsassa-pss-sha256",
+            ),
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.RSA_SIGN_PSS_3072_SHA256: (
+                "rsa",
+                "rsassa-pss-sha256",
+            ),
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.RSA_SIGN_PSS_4096_SHA256: (
+                "rsa",
+                "rsassa-pss-sha256",
+            ),
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.RSA_SIGN_PSS_4096_SHA512: (
+                "rsa",
+                "rsassa-pss-sha512",
+            ),
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.RSA_SIGN_PKCS1_2048_SHA256: (
+                "rsa",
+                "rsa-pkcs1v15-sha256",
+            ),
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.RSA_SIGN_PKCS1_3072_SHA256: (
+                "rsa",
+                "rsa-pkcs1v15-sha256",
+            ),
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.RSA_SIGN_PKCS1_4096_SHA256: (
+                "rsa",
+                "rsa-pkcs1v15-sha256",
+            ),
+            CryptoKeyVersion.CryptoKeyVersionAlgorithm.RSA_SIGN_PKCS1_4096_SHA512: (
+                "rsa",
+                "rsa-pkcs1v15-sha512",
+            ),
+        }
+        return keytypes_and_schemes[algorithm]
+
     @staticmethod
     def _get_hash_algorithm(public_key: Key) -> str:
         """Helper function to return payload hash algorithm used for this key"""

securesystemslib/signer/_hsm_signer.py

@@ -11,6 +11,7 @@
 
 from securesystemslib import KEY_TYPE_ECDSA
 from securesystemslib.exceptions import UnsupportedLibraryError
+from securesystemslib.keys import _get_keyid
 from securesystemslib.signer._key import Key, SSlibKey
 from securesystemslib.signer._signature import Signature
 from securesystemslib.signer._signer import SecretsHandler, Signer
@@ -191,13 +192,14 @@ def _find_key_values(
         return ECDomainParameters.load(bytes(params)), bytes(point)
 
     @classmethod
-    def pubkey_from_hsm(
-        cls, sslib_keyid: str, hsm_keyid: Optional[int] = None
-    ) -> SSlibKey:
-        """Export public key from HSM.
+    def import_(cls, hsm_keyid: Optional[int] = None) -> Tuple[str, SSlibKey]:
+        """Import public key and signer details from HSM.
+
+        Returns a private key URI (for Signer.from_priv_key_uri()) and a public
+        key. import_() should be called once and the returned URI and public
+        key should be stored for later use.
 
         Arguments:
-            sslib_keyid: Key identifier that is unique within the metadata it is used in.
             hsm_keyid: Key identifier on the token. Default is 2 (meaning PIV key slot 9c).
 
         Raises:
@@ -240,12 +242,12 @@ def pubkey_from_hsm(
             .decode()
         )
 
-        return SSlibKey(
-            sslib_keyid,
-            KEY_TYPE_ECDSA,
-            _SCHEME_FOR_CURVE[curve],
-            {"public": public_pem},
-        )
+        keyval = {"public": public_pem}
+        scheme = _SCHEME_FOR_CURVE[curve]
+        keyid = _get_keyid(KEY_TYPE_ECDSA, scheme, keyval)
+        key = SSlibKey(keyid, KEY_TYPE_ECDSA, scheme, keyval)
+
+        return "hsm:", key
 
     @classmethod
     def from_priv_key_uri(

tests/check_kms_signers.py

@@ -18,13 +18,25 @@
 import unittest
 
 from securesystemslib.exceptions import UnverifiedSignatureError
-from securesystemslib.signer import Key, Signer
+from securesystemslib.signer import GCPSigner, Key, Signer
 
 
 class TestKMSKeys(unittest.TestCase):
     """Test that KMS keys can be used to sign."""
 
-    def test_gcp(self):
+    pubkey = Key.from_dict(
+        "218611b80052667026c221f8774249b0f6b8b310d30a5c45a3b878aa3a02f39e",
+        {
+            "keytype": "ecdsa",
+            "scheme": "ecdsa-sha2-nistp256",
+            "keyval": {
+                "public": "-----BEGIN PUBLIC KEY-----\nMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE/ptvrXYuUc2ZaKssHhtg/IKNbO1X\ncDWlbKqLNpaK62MKdOwDz1qlp5AGHZkTY9tO09iq1F16SvVot1BQ9FJ2dw==\n-----END PUBLIC KEY-----\n"
+            },
+        },
+    )
+    gcp_id = "projects/python-tuf-kms/locations/global/keyRings/securesystemslib-tests/cryptoKeys/ecdsa-sha2-nistp256/cryptoKeyVersions/1"
+
+    def test_gcp_sign(self):
         """Test that GCP KMS key works for signing
 
         NOTE: The KMS account is setup to only accept requests from the
@@ -35,25 +47,27 @@ def test_gcp(self):
         """
 
         data = "data".encode("utf-8")
-        pubkey = Key.from_dict(
-            "abcd",
-            {
-                "keyid": "abcd",
-                "keytype": "ecdsa",
-                "scheme": "ecdsa-sha2-nistp256",
-                "keyval": {
-                    "public": "-----BEGIN PUBLIC KEY-----\nMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE/ptvrXYuUc2ZaKssHhtg/IKNbO1X\ncDWlbKqLNpaK62MKdOwDz1qlp5AGHZkTY9tO09iq1F16SvVot1BQ9FJ2dw==\n-----END PUBLIC KEY-----\n"
-                },
-            },
-        )
-        gcp_id = "projects/python-tuf-kms/locations/global/keyRings/securesystemslib-tests/cryptoKeys/ecdsa-sha2-nistp256/cryptoKeyVersions/1"
 
-        signer = Signer.from_priv_key_uri(f"gcpkms:{gcp_id}", pubkey)
+        signer = Signer.from_priv_key_uri(f"gcpkms:{self.gcp_id}", self.pubkey)
         sig = signer.sign(data)
 
-        pubkey.verify_signature(sig, data)
+        self.pubkey.verify_signature(sig, data)
         with self.assertRaises(UnverifiedSignatureError):
-            pubkey.verify_signature(sig, b"NOT DATA")
+            self.pubkey.verify_signature(sig, b"NOT DATA")
+
+    def test_gcp_import(self):
+        """Test that GCP KMS key can be imported
+
+        NOTE: The KMS account is setup to only accept requests from the
+        Securesystemslib GitHub Action environment: test cannot pass elsewhere.
+
+        In case of problems with KMS account, please file an issue and
+        assign @jku.
+        """
+
+        uri, key = GCPSigner.import_(self.gcp_id)
+        self.assertEqual(key, self.pubkey)
+        self.assertEqual(uri, f"gcpkms:{self.gcp_id}")
 
 
 if __name__ == "__main__":

tests/test_hsm_signer.py

@@ -51,7 +51,6 @@ class TestHSM(unittest.TestCase):
     See .github/workflows/hsm.yml for how this can be done on Linux, macOS and Windows.
     """
 
-    sslib_keyid = "a" * 64  # Mock SSlibKey conform sha256 hex digest keyid
     hsm_keyid = 1
     hsm_keyid_default = 2
     hsm_user_pin = "123456"
@@ -139,7 +138,7 @@ def test_hsm(self):
         """Test HSM key export and signing."""
 
         for hsm_keyid in [self.hsm_keyid, self.hsm_keyid_default]:
-            key = HSMSigner.pubkey_from_hsm(self.sslib_keyid, hsm_keyid)
+            _, key = HSMSigner.import_(hsm_keyid)
             signer = HSMSigner(hsm_keyid, key, lambda sec: self.hsm_user_pin)
             sig = signer.sign(b"DATA")
             key.verify_signature(sig, b"DATA")
@@ -150,11 +149,9 @@ def test_hsm(self):
     def test_hsm_uri(self):
         """Test HSM default key export and signing from URI."""
 
-        key = HSMSigner.pubkey_from_hsm(
-            self.sslib_keyid, self.hsm_keyid_default
-        )
+        uri, key = HSMSigner.import_(self.hsm_keyid_default)
         signer = Signer.from_priv_key_uri(
-            "hsm:", key, lambda sec: self.hsm_user_pin
+            uri, key, lambda sec: self.hsm_user_pin
         )
         sig = signer.sign(b"DATA")
         key.verify_signature(sig, b"DATA")