+**session-key**
+
+ When **notmuch-show(1)** or **nomtuch-reply** encounters a message
+ with an encrypted part and ``--decrypt`` is set, if notmuch finds a
+ ``session-key`` property associated with the message, it will try
+ that stashed session key for decryption.
+
+ Using a stashed session key with "notmuch show" will speed up
+ rendering of long encrypted threads. It also allows the user to
+ destroy the secret part of any expired encryption-capable subkey
+ while still being able to read any retained messages for which
+ they have stashed the session key. This enables truly deletable
+ e-mail, since (once the session key and asymmetric subkey are both
+ destroyed) there are no keys left that can be used to decrypt any
+ copy of the original message previously stored by an adversary.
+
+ However, access to the stashed session key for an encrypted message
+ permits full byte-for-byte reconstruction of the cleartext
+ message. This includes attachments, cryptographic signatures, and
+ other material that cannot be reconstructed from the index alone.
+
+ The session key should be in the ASCII text form produced by
+ GnuPG. For OpenPGP, that consists of a decimal representation of
+ the hash algorithm used (identified by number from RFC 4880,
+ e.g. 9 means AES-256) followed by a colon, followed by a
+ hexadecimal representation of the algorithm-specific key. For
+ example, an AES-128 key might be stashed in a notmuch property as:
+ ``session-key=7:14B16AF65536C28AF209828DFE34C9E0``.
+