Refuse a signature made when its key was not valid (closes #209)

verifySignature lets a key that has expired since verify a signature,
because a manifest outlives its key, but it checked the key only as of
now. It accepted a signature dated after its key expired, after the
key's self-signature or the signing subkey's binding signature expired,
or before the key was made. It now verifies a second time with the
library's clock set to the signature's creation time and refuses the
signature if the key or subkey was not valid then. docs/FORMAT.md
states the rule.

Judgement call: the tests make the refused signatures with the
library's packet API, because openpgp.DetachSign will not sign with a
key that is not valid at its clock's time.

Model: opus-5-5

docs/FORMAT.md

@@ -175,12 +175,15 @@ secret key or subkey, since checking the self-signatures of a DSA key or the
 numbers of a secret key can take hours when those numbers are very large.
 
 A signature is good only if the key that made it was valid at the signature's
-creation time, meaning the key had been made by then and neither it nor its
-self-signature had expired. When a subkey made the signature, the same must hold
-of the primary key, and of the subkey and its binding signature. A key that has
-expired since does not make the signature bad, because a manifest outlives its
-key. A signature past its own expiry time is not good, whether or not the key
-has expired too.
+creation time. The primary key is judged by the newest self-signature on its
+primary user ID, and a subkey by its newest binding signature. By the
+signature's creation time the primary key and that self-signature must have been
+made and neither may have expired, and when a subkey made the signature the same
+holds of the subkey and its binding signature. A signature made before that
+self-signature or binding signature is not good, even if an older one made the
+key valid at that time. A key that has expired since does not make the signature
+bad, because a manifest outlives its key. A signature past its own expiry time
+is not good, whether or not the key has expired too.
 
 ## Deterministic Serialization
 

mfer/openpgp.go

@@ -288,7 +288,10 @@ func verifySignature(data, signature, pubKey []byte) (string, error) {
 	// gives, which above is now. Verifying again as of the signature's
 	// creation time refuses a signature made before the key or subkey that
 	// made it was made, after it expired, or after its self-signature or
-	// binding signature expired.
+	// binding signature expired. The library judges the primary key by the
+	// newest self-signature on its primary user ID and a subkey by its
+	// newest binding signature, so this also refuses a signature made
+	// before either of those, even if an older one was valid then.
 	signedAt := &packet.Config{
 		Time: func() time.Time { return sig.CreationTime },
 	}