openpgp: Rework the DecryptionHelper trait.

  - Simplify the protocol by removing the iteration.  Instead, the
    callee gets a proxy for PacketParser::decrypt() that she can use
    to decrypt the message.  If successful, the session key can be
    cached without involving the DecryptionHelper trait.  This also
    allows us to dump session keys.

  - Fixes #219.

1 files changed, 15 insertions, 19 deletions

diff --git a/openpgp/examples/generate-encrypt-decrypt.rs b/openpgp/examples/generate-encrypt-decrypt.rs
index 369e82ba..88f245fb 100644
--- a/openpgp/examples/generate-encrypt-decrypt.rs
+++ b/openpgp/examples/generate-encrypt-decrypt.rs
@@ -3,9 +3,10 @@
 use std::io::{self, Write};
 
 extern crate sequoia_openpgp as openpgp;
+use openpgp::crypto::SessionKey;
+use openpgp::constants::SymmetricAlgorithm;
 use openpgp::serialize::stream::*;
 use openpgp::parse::stream::*;
-use openpgp::packet::key::SecretKey;
 
 const MESSAGE: &'static str = "дружба";
 
@@ -100,10 +101,12 @@ impl<'a> VerificationHelper for Helper<'a> {
 }
 
 impl<'a> DecryptionHelper for Helper<'a> {
-    fn get_secret(&mut self,
-                  _pkesks: &[&openpgp::packet::PKESK],
-                  _skesks: &[&openpgp::packet::SKESK])
-                  -> openpgp::Result<Option<Secret>>
+    fn decrypt<D>(&mut self,
+                  pkesks: &[openpgp::packet::PKESK],
+                  _skesks: &[openpgp::packet::SKESK],
+                  mut decrypt: D)
+                  -> openpgp::Result<Option<openpgp::Fingerprint>>
+        where D: FnMut(SymmetricAlgorithm, &SessionKey) -> openpgp::Result<()>
     {
         // The encryption key is the first and only subkey.
         let key = self.secret.subkeys().nth(0)
@@ -111,19 +114,12 @@ impl<'a> DecryptionHelper for Helper<'a> {
             .unwrap();
 
         // The secret key is not encrypted.
-        let secret =
-            if let Some(SecretKey::Unencrypted {
-                ref mpis,
-            }) = key.secret() {
-                mpis.clone()
-            } else {
-                unreachable!()
-            };
-
-        Ok(Some(Secret::Asymmetric {
-            identity: self.secret.fingerprint(),
-            key: key,
-            secret: secret,
-        }))
+        let pair = key.into_keypair().unwrap();
+
+        pkesks[0].decrypt(pair.public(), pair.secret())
+            .and_then(|(algo, session_key)| decrypt(algo, &session_key))
+            .map(|_| None)
+        // XXX: In production code, return the Fingerprint of the
+        // recipient's TPK here
     }
 }