openpgp: Add a way to disable automatic hashing.

  - When encountering a one-pass-signature packet, the packet parser
    will, by default, start hashing later packets using the hash
    algorithm specified in the packet.  In some cases, this is not
    needed, and hashing will incur a non-trivial overhead.

  - See #1034.

1 files changed, 26 insertions, 0 deletions

diff --git a/openpgp/src/parse/packet_parser_builder.rs b/openpgp/src/parse/packet_parser_builder.rs
index 006f7db7..52ac1027 100644
--- a/openpgp/src/parse/packet_parser_builder.rs
+++ b/openpgp/src/parse/packet_parser_builder.rs
@@ -364,6 +364,32 @@ impl<'a> PacketParserBuilder<'a> {
         self
     }
 
+    /// Controls automatic hashing.
+    ///
+    /// When encountering a [`OnePassSig`] packet, the packet parser
+    /// will, by default, start hashing later packets using the hash
+    /// algorithm specified in the packet.  In some cases, this is not
+    /// needed, and hashing will incur a non-trivial overhead.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # fn main() -> sequoia_openpgp::Result<()> {
+    /// # use sequoia_openpgp as openpgp;
+    /// # use openpgp::parse::{Parse, PacketParserBuilder};
+    /// #
+    /// let message_data = b"\xcb\x12t\x00\x00\x00\x00\x00Hello world.";
+    /// let pp = PacketParserBuilder::from_bytes(message_data)?
+    ///     .automatic_hashing(false) // Disable automatic hashing.
+    ///     .build()?
+    ///     .expect("One packet, not EOF");
+    /// # Ok(()) }
+    /// ```
+    pub fn automatic_hashing(mut self, enable: bool) -> Self {
+        self.settings.automatic_hashing = enable;
+        self
+    }
+
     /// Controls transparent transformation of messages using the
     /// cleartext signature framework into signed messages.
     ///