util/crypto: add information about the payload part

When walking the MIME tree, if we discover that we are at the
cryptographic payload, then we would like to record at least the
Subject header of the current MIME part.

In the future, we might want to record many other headers as well, but
for now we will stick with just the Subject.

See
https://dkg.fifthhorseman.net/blog/e-mail-cryptography.html#cryptographic-envelope
for more description of the Cryptographic Payload vs. the
Cryptographic Envelope.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

1 files changed, 5 insertions, 0 deletions

diff --git a/util/crypto.h b/util/crypto.h
index c6fa7f4b..fdbb5da5 100644
--- a/util/crypto.h
+++ b/util/crypto.h
@@ -59,6 +59,11 @@ typedef struct _notmuch_message_crypto {
      * is not part of the cryptographic envelope) */
     bool payload_encountered;
 
+    /* the value of any "Subject:" header in the cryptographic payload
+     * (the top level part within the crypto envelope), converted to
+     * UTF-8 */
+    char * payload_subject;
+
     /* if both signed and encrypted, was the signature encrypted? */
     bool signature_encrypted;
 } _notmuch_message_crypto_t;