Improve error messages when cryptography isn't installed (#846)

* Improve error messages when cryptography isn't installed

* Add test

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

2 files changed, 17 insertions, 6 deletions

diff --git a/jwt/api_jwk.py b/jwt/api_jwk.py
index ef8cce8..fdcde21 100644
--- a/jwt/api_jwk.py
+++ b/jwt/api_jwk.py
@@ -4,7 +4,7 @@ import json
 import time
 from typing import Any, Optional
 
-from .algorithms import get_default_algorithms
+from .algorithms import get_default_algorithms, has_crypto, requires_cryptography
 from .exceptions import InvalidKeyError, PyJWKError, PyJWKSetError
 from .types import JWKDict
 
@@ -49,10 +49,13 @@ class PyJWK:
             else:
                 raise InvalidKeyError(f"Unsupported kty: {kty}")
 
+        if not has_crypto and algorithm in requires_cryptography:
+            raise PyJWKError(f"{algorithm} requires 'cryptography' to be installed.")
+
         self.Algorithm = self._algorithms.get(algorithm)
 
         if not self.Algorithm:
-            raise PyJWKError(f"Unable to find a algorithm for key: {self._jwk_data}")
+            raise PyJWKError(f"Unable to find an algorithm for key: {self._jwk_data}")
 
         self.key = self.Algorithm.from_jwk(self._jwk_data)
 
@@ -66,11 +69,11 @@ class PyJWK:
         return PyJWK.from_dict(obj, algorithm)
 
     @property
-    def key_type(self) -> str:
+    def key_type(self) -> Optional[str]:
         return self._jwk_data.get("kty", None)
 
     @property
-    def key_id(self) -> str:
+    def key_id(self) -> Optional[str]:
         return self._jwk_data.get("kid", None)
 
     @property
@@ -96,7 +99,9 @@ class PyJWKSet:
                 continue
 
         if len(self.keys) == 0:
-            raise PyJWKSetError("The JWK Set did not contain any usable keys")
+            raise PyJWKSetError(
+                "The JWK Set did not contain any usable keys. Perhaps 'cryptography' is not installed?"
+            )
 
     @staticmethod
     def from_dict(obj: dict[str, Any]) -> "PyJWKSet":
diff --git a/tests/test_api_jwk.py b/tests/test_api_jwk.py
index 3f40fc5..f04f575 100644
--- a/tests/test_api_jwk.py
+++ b/tests/test_api_jwk.py
@@ -6,7 +6,7 @@ from jwt.algorithms import has_crypto
 from jwt.api_jwk import PyJWK, PyJWKSet
 from jwt.exceptions import InvalidKeyError, PyJWKError, PyJWKSetError
 
-from .utils import crypto_required, key_path
+from .utils import crypto_required, key_path, no_crypto_required
 
 if has_crypto:
     from jwt.algorithms import ECAlgorithm, HMACAlgorithm, OKPAlgorithm, RSAAlgorithm
@@ -207,6 +207,12 @@ class TestPyJWK:
         with pytest.raises(InvalidKeyError):
             PyJWK.from_dict(v)
 
+    @no_crypto_required
+    def test_missing_crypto_library_good_error_message(self):
+        with pytest.raises(PyJWKError) as exc:
+            PyJWK({"kty": "dummy"}, algorithm="RS256")
+            assert "cryptography" in str(exc.value)
+
 
 @crypto_required
 class TestPyJWKSet: