subkey binding signature with no usage flags and/or a critical notation

Werner Koch wk at
Fri Mar 15 15:52:16 CET 2013

On Thu, 14 Mar 2013 17:05, dkg at said:
> On 03/13/2013 06:27 PM, David Shaw wrote:
>> Yes. Having no flags set at all is treated as if there is no subpacket present.  This may not be the best behavior.
> yeah, i think this needs fixing.

What about this untested patch against master:

commit 8f8f3984e82a025cf1384132a419f67f39c7e07d 
Author: Werner Koch <wk at>
Date:   Fri Mar 15 15:46:03 2013 +0100

    gpg: Distinguish between missing and cleared key flags.
    * include/cipher.h (PUBKEY_USAGE_NONE): New.
    * g10/getkey.c (parse_key_usage): Set new flag.
    We do not want to use the default capabilities (derived from the
    algorithm) if any key flags are given in a signature.  Thus if key
    flags are used in any way, the default key capabilities are never
    This allows to create a key with key flags set to all zero so it can't
    be used.  This better reflects common sense.

	Modified g10/getkey.c
diff --git a/g10/getkey.c b/g10/getkey.c
index 9294273..8cc5601 100644
--- a/g10/getkey.c
+++ b/g10/getkey.c
@@ -1276,13 +1276,19 @@ parse_key_usage (PKT_signature * sig)
       if (flags)
 	key_usage |= PUBKEY_USAGE_UNKNOWN;
+      if (!key_usage)
+	key_usage |= PUBKEY_USAGE_NONE;
+  else if (p) /* Key flags of length zero.  */
+    key_usage |= PUBKEY_USAGE_NONE;
   /* We set PUBKEY_USAGE_UNKNOWN to indicate that this key has a
      capability that we do not handle.  This serves to distinguish
      between a zero key usage which we handle as the default
      capabilities for that algorithm, and a usage that we do not
-     handle. */
+     handle.  Likewise we use PUBKEY_USAGE_NONE to indicate that
+     key_flags have been given but they do not specify any usage.  */
   return key_usage;
	Modified include/cipher.h
diff --git a/include/cipher.h b/include/cipher.h
index 191e197..557ab70 100644
--- a/include/cipher.h
+++ b/include/cipher.h
@@ -54,9 +54,14 @@
 #define PUBKEY_USAGE_SIG     GCRY_PK_USAGE_SIGN  /* Good for signatures. */
 #define PUBKEY_USAGE_ENC     GCRY_PK_USAGE_ENCR  /* Good for encryption. */
-#define PUBKEY_USAGE_CERT    GCRY_PK_USAGE_CERT  /* Also good to certify keys. */
+#define PUBKEY_USAGE_CERT    GCRY_PK_USAGE_CERT  /* Also good to certify keys.*/
 #define PUBKEY_USAGE_AUTH    GCRY_PK_USAGE_AUTH  /* Good for authentication. */
 #define PUBKEY_USAGE_UNKNOWN GCRY_PK_USAGE_UNKN  /* Unknown usage flag. */
+#define PUBKEY_USAGE_NONE    256                 /* No usage given. */
+# error Please choose another value for PUBKEY_USAGE_NONE
 #define DIGEST_ALGO_MD5       /*  1 */ GCRY_MD_MD5
 #define DIGEST_ALGO_SHA1      /*  2 */ GCRY_MD_SHA1

Die Gedanken sind frei.  Ausnahmen regelt ein Bundesgesetz.

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