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"""Signer implementation for Google Cloud KMS"""
from __future__ import annotations
import hashlib
import logging
from urllib import parse
from securesystemslib import exceptions
from securesystemslib.signer._key import Key, SSlibKey
from securesystemslib.signer._signer import SecretsHandler, Signature, Signer
from securesystemslib.signer._utils import compute_default_keyid
logger = logging.getLogger(__name__)
GCP_IMPORT_ERROR = None
try:
from google.cloud import kms
from google.cloud.kms_v1.types import CryptoKeyVersion
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",
),
}
except ImportError:
GCP_IMPORT_ERROR = (
"google-cloud-kms library required to sign with Google Cloud keys."
)
class GCPSigner(Signer):
"""Google Cloud KMS Signer
This Signer uses Google Cloud KMS to sign: the payload is hashed locally,
but the signature is created on the KMS.
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.
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
projects/python-tuf-kms/locations/global/keyRings/securesystemslib-tests/cryptoKeys/ecdsa-sha2-nistp256/cryptoKeyVersions/1
public_key: The related public key instance
Raises:
UnsupportedAlgorithmError: The payload hash algorithm is unsupported.
UnsupportedLibraryError: google.cloud.kms was not found
Various errors from google.cloud modules: e.g.
google.auth.exceptions.DefaultCredentialsError if ambient
credentials are not found
"""
SCHEME = "gcpkms"
def __init__(self, gcp_keyid: str, public_key: SSlibKey):
if GCP_IMPORT_ERROR:
raise exceptions.UnsupportedLibraryError(GCP_IMPORT_ERROR)
if (public_key.keytype, public_key.scheme) not in KEYTYPES_AND_SCHEMES.values():
raise exceptions.UnsupportedAlgorithmError(
f"Unsupported key ({public_key.keytype}/{public_key.scheme}) "
f"in key {public_key.keyid}"
)
self.hash_algorithm = public_key.get_hash_algorithm_name()
self.gcp_keyid = gcp_keyid
self._public_key = public_key
self.client = kms.KeyManagementServiceClient()
@property
def public_key(self) -> SSlibKey:
return self._public_key
@classmethod
def from_priv_key_uri(
cls,
priv_key_uri: str,
public_key: Key,
secrets_handler: SecretsHandler | None = None,
) -> GCPSigner:
if not isinstance(public_key, SSlibKey):
raise ValueError(f"Expected SSlibKey for {priv_key_uri}")
uri = parse.urlparse(priv_key_uri)
if uri.scheme != cls.SCHEME:
raise ValueError(f"GCPSigner does not support {priv_key_uri}")
return cls(uri.path, public_key)
@classmethod
def import_(cls, gcp_keyid: str) -> tuple[str, SSlibKey]:
"""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 = KEYTYPES_AND_SCHEMES[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 = compute_default_keyid(keytype, scheme, keyval)
public_key = SSlibKey(keyid, keytype, scheme, keyval)
return f"{cls.SCHEME}:{gcp_keyid}", public_key
def sign(self, payload: bytes) -> Signature:
"""Signs payload with Google Cloud KMS.
Arguments:
payload: bytes to be signed.
Raises:
Various errors from google.cloud modules.
Returns:
Signature.
"""
# NOTE: request and response can contain CRC32C of the digest/sig:
# Verifying could be useful but would require another dependency...
hasher = hashlib.new(self.hash_algorithm)
hasher.update(payload)
digest = {self.hash_algorithm: hasher.digest()}
request = {"name": self.gcp_keyid, "digest": digest}
logger.debug("signing request %s", request)
response = self.client.asymmetric_sign(request)
logger.debug("signing response %s", response)
return Signature(self.public_key.keyid, response.signature.hex())
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