openpgp: Reset the hash algorithm when creating a SignatureBuilder.

  - When creating a `SignatureBuilder` from a `Signature`, reset the
    hash algorithm to `HashAlgorithm::default()`.

  - This ensures that updating an outdated signature won't use an
    outdated hash algorithm.

  - Fixes #609.

1 files changed, 9 insertions, 0 deletions

diff --git a/openpgp/src/packet/signature.rs b/openpgp/src/packet/signature.rs
index 50f84dd2..f083bffc 100644
--- a/openpgp/src/packet/signature.rs
+++ b/openpgp/src/packet/signature.rs
@@ -302,6 +302,13 @@ impl SignatureFields {
 ///
 ///   [`From`]: https://doc.rust-lang.org/stable/std/convert/trait.From.html
 ///
+/// When converting a `Signature` to a `SignatureBuilder`, the hash
+/// algorithm is reset to the default hash algorithm
+/// (`HashAlgorithm::Default()`).  This ensures that a recommended
+/// hash algorithm is used even when an old signature is used as a
+/// template, which is often the case when updating self signatures,
+/// and binding signatures.
+///
 /// According to [Section 5.2.3.4 of RFC 4880], `Signatures` must
 /// include a [`Signature Creation Time`] subpacket.  Since this
 /// should usually be set to the current time, and is easy to forget
@@ -1592,6 +1599,8 @@ impl From<Signature4> for SignatureBuilder {
     fn from(sig: Signature4) -> Self {
         let mut fields = sig.fields;
 
+        fields.hash_algo = HashAlgorithm::default();
+
         let creation_time = fields.signature_creation_time();
 
         fields.hashed_area_mut().remove_all(SubpacketTag::SignatureCreationTime);