openpgp: Add a RustCrypto backend.

  - This adds a cryptographic backend based on the RustCrypto crates.
    The backend is marked as experimental, as the RustCrypto crates'
    authors state that they have not been audited and may not perform
    computations in constant time.  Nevertheless, it may be useful in
    certain environments, e.g. WebAssembly.

  - The backend implements RSA, EdDSA and ECDH over Curve25519, IDEA,
    3DES, CAST5, Blowfish, AES, Twofish, EAX, MD5, SHA1, RipeMD160, and
    the SHA2 family.

  - Notably missing are DSA, ElGamal, and ECDSA and ECDH over the NIST
    curves.

  - See #333.

1 files changed, 64 insertions, 0 deletions

diff --git a/openpgp/src/crypto/backend/rust.rs b/openpgp/src/crypto/backend/rust.rs
new file mode 100644
index 00000000..a661c14e
--- /dev/null
+++ b/openpgp/src/crypto/backend/rust.rs
@@ -0,0 +1,64 @@
+//! Implementation of Sequoia crypto API using pure Rust cryptographic
+//! libraries.
+
+use crate::types::*;
+
+pub mod aead;
+pub mod asymmetric;
+pub mod ecdh;
+pub mod hash;
+pub mod symmetric;
+
+/// Fills the given buffer with random data.
+///
+/// Fills the given buffer with random data produced by a
+/// cryptographically secure pseudorandom number generator (CSPRNG).
+/// The output may be used as session keys or to derive long-term
+/// cryptographic keys from.
+pub fn random<B: AsMut<[u8]>>(mut buf: B) {
+    use rand::rngs::OsRng;
+    use rand::RngCore;
+
+    OsRng.fill_bytes(buf.as_mut())
+}
+
+impl PublicKeyAlgorithm {
+    pub(crate) fn is_supported_by_backend(&self) -> bool {
+        use PublicKeyAlgorithm::*;
+        #[allow(deprecated)]
+        match &self {
+            RSAEncryptSign | RSAEncrypt | RSASign | ECDH | EdDSA
+                => true,
+            DSA | ECDSA
+                => false,
+            ElGamalEncrypt | ElGamalEncryptSign | Private(_) | Unknown(_)
+                => false,
+        }
+    }
+}
+
+impl Curve {
+    pub(crate) fn is_supported_by_backend(&self) -> bool {
+        use self::Curve::*;
+        match &self {
+            NistP256 | NistP384 | NistP521
+                => false,
+            Ed25519 | Cv25519
+                => true,
+            BrainpoolP256 | BrainpoolP512 | Unknown(_)
+                => false,
+        }
+    }
+}
+
+impl AEADAlgorithm {
+    pub(crate) fn is_supported_by_backend(&self) -> bool {
+        use self::AEADAlgorithm::*;
+        match &self {
+            EAX
+                => true,
+            OCB | Private(_) | Unknown(_)
+                => false,
+        }
+    }
+}