Relax the self-signature requirement.

  - When we verify a data signature, we canonicalize the signer's
    certificate to look as it did at the time of the data signature.

  - When exporting a certificate, GnuPG strips old self signatures.

  - This means that when a certificate's expiration time is extended,
    say, we are no longer able to verify old data signatures, because
    the certificate is not considered to be valid as of the data
    signature's creation time!

  - Relax this requirement.  Also allow a certificate, if it can be
    canonicalized as of the current time.

  - Fixes #50.

src/lib.rs

@@ -802,19 +802,10 @@ fn pgp_verify_signature(key: Option<&PgpDigParams>,
         // time.
         let p = &*P.read().unwrap();
         let vc = cert.with_policy(p, sig_time)
-            .or_else(|err| {
-                legacy = true;
-                add_lint!(
-                    Some(err),
-                    "Certificate {} invalid: policy violation",
-                    cert.keyid());
-                cert.with_policy(NP, sig_time)
-            })
             .or_else(|err| {
                 // Try again, but use the current time as a reference
-                // time.  This is necessary if older self-signatures
-                // are stripped.
-                legacy = true;
+                // time.  It is quite comment for old self-signatures
+                // to be stripped.
                 match cert.with_policy(p, None) {
                     Ok(vc) => {
                         add_lint!(
@@ -833,6 +824,14 @@ fn pgp_verify_signature(key: Option<&PgpDigParams>,
                         Err(err2)
                     }
                 }
+            })
+            .or_else(|err| {
+                legacy = true;
+                add_lint!(
+                    Some(err),
+                    "Certificate {} invalid: policy violation",
+                    cert.keyid());
+                cert.with_policy(NP, sig_time)
             })?;
 
         if let Err(err) = vc.alive() {