Raise friendly error when SHA1 signing is unavailable

.changes/next-release/bugfix-cloudfront-sign.json

@@ -0,0 +1,5 @@
+{
+  "category": "cloudfront",
+  "description": "Raise a friendly error message when SHA1 signing is unavailable on the platform while running ``aws cloudfront sign``.",
+  "type": "bugfix"
+}

awscli/customizations/cloudfront.py

@@ -19,6 +19,7 @@
 from botocore.utils import parse_to_aware_datetime
 
 from awscrt.crypto import RSA, RSASignatureAlgorithm
+from awscrt.exceptions import AwsCrtError
 
 from awscli.arguments import CustomArgument
 from awscli.customizations.commands import BasicCommand
@@ -306,7 +307,18 @@ def __init__(self, private_key):
         self.priv_key = RSA.new_private_key_from_pem_data(key_bytes)
 
     def sign(self, message):
-        return self.priv_key.sign(
-            RSASignatureAlgorithm.PKCS1_5_SHA1,
-            hashlib.sha1(message).digest()
-        )
+        try:
+            return self.priv_key.sign(
+                RSASignatureAlgorithm.PKCS1_5_SHA1,
+                hashlib.sha1(message).digest()
+            )
+        except (AwsCrtError, RuntimeError) as e:
+            if 'AWS_ERROR_CAL_UNSUPPORTED_ALGORITHM' in str(e):
+                raise RuntimeError(
+                    "Failed to sign the URL using the SHA1 hash algorithm: "
+                    f"{e} CloudFront signed URLs require RSA PKCS1 v1.5 "
+                    "signing with SHA1, which may be disabled by default on "
+                    "your platform. Enable SHA1 support in your system's "
+                    "crypto provider and try again."
+                ) from e
+            raise

tests/functional/cloudfront/test_sign.py

@@ -108,6 +108,43 @@ def test_custom_policy(self):
             self.run_cmd(cmdline)[0], 'http://example.com/hi', expected_params
         )
 
+    def test_raises_friendly_error_when_sha1_unavailable(self):
+        cmdline = (
+            self.prefix
+            + '--private-key file://'
+            + self.private_key_file
+            + ' --date-less-than 2016-1-1'
+        )
+
+        crt_error = RuntimeError(
+            '7174 (AWS_ERROR_CAL_UNSUPPORTED_ALGORITHM): The specified '
+            'algorithm is unsupported on this platform.'
+        )
+        with mock.patch(
+            'awscli.customizations.cloudfront.RSA.sign',
+            side_effect=crt_error,
+        ):
+            stdout, stderr, rc = self.run_cmd(cmdline, expected_rc=255)
+        self.assertNotEqual(rc, 0)
+        self.assertIn('SHA1', stderr)
+
+    def test_unrelated_signing_error_is_not_reported_as_sha1(self):
+        cmdline = (
+            self.prefix
+            + '--private-key file://'
+            + self.private_key_file
+            + ' --date-less-than 2016-1-1'
+        )
+        other_error = RuntimeError('some other signing failure')
+        with mock.patch(
+            'awscli.customizations.cloudfront.RSA.sign',
+            side_effect=other_error,
+        ):
+            stdout, stderr, rc = self.run_cmd(cmdline, expected_rc=255)
+        self.assertNotEqual(rc, 0)
+        self.assertNotIn('SHA1', stderr)
+        self.assertIn('some other signing failure', stderr)
+
 
 class TestSignPKCS8(BaseAWSCommandParamsTest):
     # A private key only for testing purpose.