openpgp: Change the `decrypt` proxy in the decryption helper.

  - Returning rich errors from this function may compromise secret key
    material due to Bleichenbacher-style attacks.  Change the API to
    prevent this.

  - Hat tip to Hanno Böck.

  - Fixes #507.

2 files changed, 18 insertions, 12 deletions

diff --git a/openpgp-ffi/include/sequoia/openpgp/types.h b/openpgp-ffi/include/sequoia/openpgp/types.h
index b77e4251..216c69cb 100644
--- a/openpgp-ffi/include/sequoia/openpgp/types.h
+++ b/openpgp-ffi/include/sequoia/openpgp/types.h
@@ -1,6 +1,8 @@
 #ifndef SEQUOIA_OPENPGP_TYPES_H
 #define SEQUOIA_OPENPGP_TYPES_H
 
+#include <stdbool.h>
+
 /*/
 /// Holds a session key.
 ///
@@ -524,7 +526,7 @@ typedef pgp_status_t (*pgp_decryptor_get_certs_cb_t) (void *,
     pgp_cert_t **, size_t *,
     void (**free)(void *));
 
-typedef pgp_status_t (pgp_decryptor_do_decrypt_cb_t) (
+typedef bool (pgp_decryptor_do_decrypt_cb_t) (
     void *,
     uint8_t,
     pgp_session_key_t);
diff --git a/openpgp-ffi/src/parse/stream.rs b/openpgp-ffi/src/parse/stream.rs
index 9c900a76..cce302ad 100644
--- a/openpgp-ffi/src/parse/stream.rs
+++ b/openpgp-ffi/src/parse/stream.rs
@@ -416,7 +416,7 @@ type DecryptCallback = fn(*mut HelperCookie,
                           u8, // XXX SymmetricAlgorithm
                           extern "C" fn (*mut c_void, u8,
                                               *const crypto::SessionKey)
-                                              -> Status,
+                                              -> bool,
                           *mut c_void,
                           *mut Maybe<super::super::fingerprint::Fingerprint>)
                           -> Status;
@@ -837,7 +837,7 @@ impl DecryptionHelper for DHelper {
                   sym_algo: Option<SymmetricAlgorithm>,
                   mut decrypt: D)
                   -> openpgp::Result<Option<openpgp::Fingerprint>>
-        where D: FnMut(SymmetricAlgorithm, &SessionKey) -> openpgp::Result<()>
+        where D: FnMut(SymmetricAlgorithm, &SessionKey) -> bool
     {
         let mut identity: Maybe<super::super::fingerprint::Fingerprint> = None;
 
@@ -860,12 +860,11 @@ impl DecryptionHelper for DHelper {
 
         extern "C" fn trampoline<D>(data: *mut c_void, algo: u8,
                                          sk: *const crypto::SessionKey)
-                                         -> Status
-            where D: FnMut(SymmetricAlgorithm, &SessionKey)
-                           -> openpgp::Result<()>
+                                         -> bool
+            where D: FnMut(SymmetricAlgorithm, &SessionKey) -> bool
         {
             let closure: &mut D = unsafe { &mut *(data as *mut D) };
-            (*closure)(algo.into(), sk.ref_raw()).into()
+            (*closure)(algo.into(), sk.ref_raw())
         }
 
         let result = (self.decrypt_cb)(
@@ -954,7 +953,6 @@ impl DecryptionHelper for DHelper {
 ///             void *decrypt_cookie,
 ///             pgp_fingerprint_t *identity_out)
 /// {
-///   pgp_status_t rc;
 ///   pgp_error_t err;
 ///   struct decrypt_cookie *cookie = cookie_opaque;
 ///
@@ -989,18 +987,24 @@ impl DecryptionHelper for DHelper {
 ///     if (pgp_pkesk_decrypt (&err,
 ///                            pkesk, key, &algo,
 ///                            session_key, &session_key_len)) {
-///       error (1, 0, "pgp_pkesk_decrypt: %s", pgp_error_to_string (err));
+///       error (0, 0, "pgp_pkesk_decrypt: %s", pgp_error_to_string (err));
+///       pgp_key_free (key);
+///       pgp_key_amalgamation_free (ka);
+///       continue;
 ///     }
 ///     pgp_key_free (key);
 ///     pgp_key_amalgamation_free (ka);
 ///
 ///     pgp_session_key_t sk = pgp_session_key_from_bytes (session_key,
 ///                                                        session_key_len);
-///     rc = decrypt (decrypt_cookie, algo, sk);
-///     pgp_session_key_free (sk);
+///     if (! decrypt (decrypt_cookie, algo, sk)) {
+///       pgp_session_key_free (sk);
+///       continue;
+///     }
 ///
+///     pgp_session_key_free (sk);
 ///     *identity_out = pgp_cert_fingerprint (cookie->key);
-///     return rc;
+///     return PGP_STATUS_SUCCESS;
 ///   }
 ///
 ///   return PGP_STATUS_UNKNOWN_ERROR;