openpgp: Fix verifying cleartext signed messages with multiple sigs.

  - We implement the cleartext signature framework by transforming the
    message on the fly to a signed message, then using our parsing
    framework as usual.  However, we need to tweak the behavior
    slightly.

  - Notably, our CSF transformation yields just one OPS packet per
    encountered 'Hash' algorithm header, and it cannot know how many
    signatures are in fact following.  Therefore, the message will not
    be well-formed according to the grammar.  But, since we created the
    message structure during the transformation, we know it is good,
    even if it is a little out of spec.

  - This patch tweaks the streaming verifier's behavior to accommodate
    this.

1 files changed, 6 insertions, 0 deletions

diff --git a/openpgp/src/parse.rs b/openpgp/src/parse.rs
index 3e5eead6..eaa10d29 100644
--- a/openpgp/src/parse.rs
+++ b/openpgp/src/parse.rs
@@ -4922,6 +4922,12 @@ impl <'a> PacketParser<'a> {
     pub fn take_map(&mut self) -> Option<map::Map> {
         self.map.take()
     }
+
+    /// Checks if we are processing a signed message using the
+    /// Cleartext Signature Framework.
+    pub(crate) fn processing_csf_message(&self) -> bool {
+        Cookie::processing_csf_message(&self.reader)
+    }
 }
 
 /// This interface allows a caller to read the content of a