1 files changed, 31 insertions, 0 deletions

diff --git a/src/mailman_pgp/utils/pgp.py b/src/mailman_pgp/utils/pgp.py
index 621aa02..1dfdc5e 100644
--- a/src/mailman_pgp/utils/pgp.py
+++ b/src/mailman_pgp/utils/pgp.py
@@ -18,6 +18,7 @@
 """Miscellaneous PGP utilities."""
 from pgpy import PGPKey, PGPSignature
 from pgpy.constants import SignatureType
+from pgpy.errors import PGPError
 from pgpy.packet import Packet, Signature
 from pgpy.types import Armorable
 from public import public
@@ -97,3 +98,33 @@ def revoc_from_blob(blob):
     sig = PGPSignature()
     sig |= p
     return sig
+
+
+@public
+def key_usable(key, flags_required):
+    """
+
+    :param key:
+    :type key: pgpy.PGPKey
+    :param flags_required:
+    :return:
+    :rtype: bool
+    """
+    if key.is_expired:
+        return False
+    primary_revocs = (sig for sig in key.self_signatures if
+                      sig.sigtype is SignatureType.KeyRevocation)
+    for revoc in primary_revocs:
+        try:
+            verified = key.verify(key, revoc)
+        except PGPError:
+            continue
+        if bool(verified):
+            return False
+    usage_flags = key.usage_flags()
+    for subkey in key.subkeys.values():
+        usage_flags |= subkey.usage_flags()
+
+    if flags_required not in usage_flags:
+        return False
+    return True