openpgp: Fix subtraction with overflow.

  - When the decryptor returns a short session key without reporting
    an error, an overflow occurs.  Avoid this by using saturating
    subtraction.  Then, the resulting range is empty, and an error
    will be reported just a few lines down.

  - Fixes #913.

1 files changed, 1 insertions, 1 deletions

diff --git a/openpgp/src/packet/pkesk.rs b/openpgp/src/packet/pkesk.rs
index bf4464f9..ac0b7932 100644
--- a/openpgp/src/packet/pkesk.rs
+++ b/openpgp/src/packet/pkesk.rs
@@ -155,7 +155,7 @@ impl PKESK3 {
             None
         };
         let plain = decryptor.decrypt(&self.esk, plaintext_len)?;
-        let key_rgn = 1..(plain.len() - 2);
+        let key_rgn = 1..plain.len().saturating_sub(2);
         let sym_algo: SymmetricAlgorithm = plain[0].into();
         let mut key: SessionKey = vec![0u8; sym_algo.key_size()?].into();