8 files changed, 73 insertions, 26 deletions
diff --git a/openpgp/src/crypto/backend.rs b/openpgp/src/crypto/backend.rs
index 2aecfb69..03c142ea 100644
--- a/openpgp/src/crypto/backend.rs
+++ b/openpgp/src/crypto/backend.rs
@@ -1,6 +1,7 @@
 //! Concrete implementation of the crypto primitives used by the rest of the
 //! crypto API.
 
+pub(crate) mod interface;
 pub(crate) mod sha1cd;
 
 // Nettle is the default backend, but on Windows targets we instead
@@ -25,6 +26,12 @@ mod nettle;
                       feature = "crypto-botan2",
                       feature = "crypto-rust")))))]
 pub use self::nettle::*;
+#[cfg(all(feature = "crypto-nettle",
+          not(all(feature = "__implicit-crypto-backend-for-tests",
+                  any(feature = "crypto-openssl",
+                      feature = "crypto-botan",
+                      feature = "crypto-rust")))))]
+pub use self::nettle::Backend;
 
 // Nettle is the default backend, but on Windows targets we instead
 // enable CNG for running the tests in non-leaf crates that depend on
@@ -50,18 +57,31 @@ mod cng;
                       feature = "crypto-botan2",
                       feature = "crypto-rust")))))]
 pub use self::cng::*;
+#[cfg(all(feature = "crypto-cng",
+          not(all(feature = "__implicit-crypto-backend-for-tests",
+                  any(feature = "crypto-nettle",
+                      feature = "crypto-openssl",
+                      feature = "crypto-botan",
+                      feature = "crypto-rust")))))]
+pub use self::cng::Backend;
 
 #[cfg(feature = "crypto-rust")]
 mod rust;
 #[cfg(feature = "crypto-rust")]
 pub use self::rust::*;
+#[cfg(feature = "crypto-rust")]
+pub use self::rust::Backend;
 
 #[cfg(feature = "crypto-openssl")]
 mod openssl;
 #[cfg(feature = "crypto-openssl")]
 pub use self::openssl::*;
+#[cfg(feature = "crypto-openssl")]
+pub use self::openssl::Backend;
 
 #[cfg(any(feature = "crypto-botan", feature = "crypto-botan2"))]
 mod botan;
 #[cfg(any(feature = "crypto-botan", feature = "crypto-botan2"))]
 pub use self::botan::*;
+#[cfg(feature = "crypto-botan")]
+pub use self::botan::Backend;
diff --git a/openpgp/src/crypto/backend/botan.rs b/openpgp/src/crypto/backend/botan.rs
index 975d6196..04951225 100644
--- a/openpgp/src/crypto/backend/botan.rs
+++ b/openpgp/src/crypto/backend/botan.rs
@@ -8,9 +8,12 @@ pub mod ecdh;
 pub mod hash;
 pub mod symmetric;
 
-/// Returns a short, human-readable description of the backend.
-pub fn backend() -> String {
-    "Botan".to_string()
+pub struct Backend(());
+
+impl super::interface::Backend for Backend {
+    fn backend() -> String {
+        "Botan".to_string()
+    }
 }
 
 /// Fills the given buffer with random data.
diff --git a/openpgp/src/crypto/backend/cng.rs b/openpgp/src/crypto/backend/cng.rs
index 3ded3f5b..96a4c2c0 100644
--- a/openpgp/src/crypto/backend/cng.rs
+++ b/openpgp/src/crypto/backend/cng.rs
@@ -10,10 +10,13 @@ pub mod ecdh;
 pub mod hash;
 pub mod symmetric;
 
-/// Returns a short, human-readable description of the backend.
-pub fn backend() -> String {
-    // XXX: can we include features and the version?
-    "Windows CNG".to_string()
+pub struct Backend(());
+
+impl super::interface::Backend for Backend {
+    fn backend() -> String {
+        // XXX: can we include features and the version?
+        "Windows CNG".to_string()
+    }
 }
 
 /// Fills the given buffer with random data.
diff --git a/openpgp/src/crypto/backend/interface.rs b/openpgp/src/crypto/backend/interface.rs
new file mode 100644
index 00000000..d0c8ffff
--- /dev/null
+++ b/openpgp/src/crypto/backend/interface.rs
@@ -0,0 +1,11 @@
+//! The crypto-backend abstraction.
+
+/// Abstracts over the cryptographic backends.
+pub trait Backend {
+    /// Returns a short, human-readable description of the backend.
+    ///
+    /// This starts with the name of the backend, possibly a version,
+    /// and any optional features that are available.  This is meant
+    /// for inclusion in version strings to improve bug reports.
+    fn backend() -> String;
+}
diff --git a/openpgp/src/crypto/backend/nettle.rs b/openpgp/src/crypto/backend/nettle.rs
index 7109fa3a..0800540f 100644
--- a/openpgp/src/crypto/backend/nettle.rs
+++ b/openpgp/src/crypto/backend/nettle.rs
@@ -10,15 +10,18 @@ pub mod ecdh;
 pub mod hash;
 pub mod symmetric;
 
-/// Returns a short, human-readable description of the backend.
-pub fn backend() -> String {
-    let (major, minor) = nettle::version();
-    format!(
-        "Nettle {}.{} (Cv448: {:?}, OCB: {:?})",
-        major, minor,
-        nettle::curve448::IS_SUPPORTED,
-        nettle::aead::OCB_IS_SUPPORTED,
-    )
+pub struct Backend(());
+
+impl super::interface::Backend for Backend {
+    fn backend() -> String {
+        let (major, minor) = nettle::version();
+        format!(
+            "Nettle {}.{} (Cv448: {:?}, OCB: {:?})",
+            major, minor,
+            nettle::curve448::IS_SUPPORTED,
+            nettle::aead::OCB_IS_SUPPORTED,
+        )
+    }
 }
 
 /// Fills the given buffer with random data.
diff --git a/openpgp/src/crypto/backend/openssl.rs b/openpgp/src/crypto/backend/openssl.rs
index 1c077cf2..e4f73418 100644
--- a/openpgp/src/crypto/backend/openssl.rs
+++ b/openpgp/src/crypto/backend/openssl.rs
@@ -9,9 +9,12 @@ pub mod ecdh;
 pub mod hash;
 pub mod symmetric;
 
-/// Returns a short, human-readable description of the backend.
-pub fn backend() -> String {
-    "OpenSSL".to_string()
+pub struct Backend(());
+
+impl super::interface::Backend for Backend {
+    fn backend() -> String {
+        "OpenSSL".to_string()
+    }
 }
 
 /// Fills the given buffer with random data.
diff --git a/openpgp/src/crypto/backend/rust.rs b/openpgp/src/crypto/backend/rust.rs
index 8dd85559..2a057569 100644
--- a/openpgp/src/crypto/backend/rust.rs
+++ b/openpgp/src/crypto/backend/rust.rs
@@ -12,6 +12,15 @@ pub mod ecdh;
 pub mod hash;
 pub mod symmetric;
 
+pub struct Backend(());
+
+impl super::interface::Backend for Backend {
+    fn backend() -> String {
+        // XXX: can we include features and the version?
+        "RustCrypto".to_string()
+    }
+}
+
 trait GenericArrayExt<T, N: ArrayLength<T>> {
     const LEN: usize;
 
@@ -44,12 +53,6 @@ impl<T, N: ArrayLength<T>> GenericArrayExt<T, N> for GenericArray<T, N> {
     const LEN: usize = N::USIZE;
 }
 
-/// Returns a short, human-readable description of the backend.
-pub fn backend() -> String {
-    // XXX: can we include features and the version?
-    "RustCrypto".to_string()
-}
-
 /// Fills the given buffer with random data.
 pub fn random(buf: &mut [u8]) {
     use rand07::rngs::OsRng;
diff --git a/openpgp/src/crypto/mod.rs b/openpgp/src/crypto/mod.rs
index 6dc2002d..12527b70 100644
--- a/openpgp/src/crypto/mod.rs
+++ b/openpgp/src/crypto/mod.rs
@@ -49,7 +49,8 @@ mod tests;
 /// any optional features that are available.  This is meant for
 /// inclusion in version strings to improve bug reports.
 pub fn backend() -> String {
-    backend::backend()
+    use backend::interface::Backend;
+    backend::Backend::backend()
 }
 
 /// Fills the given buffer with random data.