openpgp: seal Aead trait.

- Seal the Aead trait so it cannot be implemented outside the openpgp
  crate.

- This way we can extend the trait without breaking the API
  compatibility.

- See #538.

3 files changed, 15 insertions, 1 deletions

diff --git a/openpgp/src/crypto/aead.rs b/openpgp/src/crypto/aead.rs
index d6b937b4..17153aa1 100644
--- a/openpgp/src/crypto/aead.rs
+++ b/openpgp/src/crypto/aead.rs
@@ -16,6 +16,7 @@ use crate::Error;
 use crate::Result;
 use crate::crypto::SessionKey;
 use crate::crypto::mem::secure_cmp;
+use crate::seal;
 use crate::parse::Cookie;
 
 /// Disables authentication checks.
@@ -33,7 +34,16 @@ pub(crate) fn chunk_size_usize(chunk_size: u64) -> Result<usize> {
 }
 
 /// An AEAD mode of operation.
-pub trait Aead {
+///
+/// # Sealed trait
+///
+/// This trait is [sealed] and cannot be implemented for types outside this crate.
+/// Therefore it can be extended in a non-breaking way.
+/// If you want to implement the trait inside the crate
+/// you also need to implement the `seal::Sealed` marker trait.
+///
+/// [sealed]: https://rust-lang.github.io/api-guidelines/future-proofing.html#sealed-traits-protect-against-downstream-implementations-c-sealed
+pub trait Aead : seal::Sealed {
     /// Adds associated data `ad`.
     fn update(&mut self, ad: &[u8]);
 
diff --git a/openpgp/src/crypto/backend/cng/aead.rs b/openpgp/src/crypto/backend/cng/aead.rs
index d0208221..d30073ca 100644
--- a/openpgp/src/crypto/backend/cng/aead.rs
+++ b/openpgp/src/crypto/backend/cng/aead.rs
@@ -2,6 +2,7 @@
 
 use crate::{Error, Result};
 use crate::crypto::aead::Aead;
+use crate::seal;
 use crate::types::{AEADAlgorithm, SymmetricAlgorithm};
 use super::symmetric::Ctr;
 
@@ -73,6 +74,7 @@ impl EaxAes {
 }
 
 
+impl seal::Sealed for EaxAes {}
 impl Aead for EaxAes {
     /// Adds associated data `ad`.
     fn update(&mut self, ad: &[u8]) {
diff --git a/openpgp/src/crypto/backend/nettle/aead.rs b/openpgp/src/crypto/backend/nettle/aead.rs
index 1614a20a..797dd040 100644
--- a/openpgp/src/crypto/backend/nettle/aead.rs
+++ b/openpgp/src/crypto/backend/nettle/aead.rs
@@ -4,8 +4,10 @@ use nettle::{aead, cipher};
 use crate::{Error, Result};
 
 use crate::crypto::aead::Aead;
+use crate::seal;
 use crate::types::{AEADAlgorithm, SymmetricAlgorithm};
 
+impl<T: nettle::aead::Aead> seal::Sealed for T {}
 impl<T: nettle::aead::Aead> Aead for T {
     fn update(&mut self, ad: &[u8]) {
         self.update(ad)