openpgp: Move `Key4::import_secret_cv25519` into common code.

  - Most of the logic in this function is the same across backends.

  - Introduce `Key4::derive_cv25519_public_key` that does
    backend-specific derivation.

  - Fixes https://gitlab.com/sequoia-pgp/sequoia/-/issues/958

1 files changed, 2 insertions, 32 deletions

diff --git a/openpgp/src/crypto/backend/botan/asymmetric.rs b/openpgp/src/crypto/backend/botan/asymmetric.rs
index bf9b53c6..51ed2edc 100644
--- a/openpgp/src/crypto/backend/botan/asymmetric.rs
+++ b/openpgp/src/crypto/backend/botan/asymmetric.rs
@@ -29,7 +29,6 @@ use crate::{
         Curve,
         HashAlgorithm,
         PublicKeyAlgorithm,
-        SymmetricAlgorithm,
     },
 };
 
@@ -392,40 +391,11 @@ impl<P: key::KeyParts, R: key::KeyRole> Key<P, R> {
 impl<R> Key4<SecretParts, R>
     where R: key::KeyRole,
 {
-    /// Creates a new OpenPGP secret key packet for an existing X25519 key.
-    ///
-    /// The ECDH key will use hash algorithm `hash` and symmetric
-    /// algorithm `sym`.  If one or both are `None` secure defaults
-    /// will be used.  The key will have it's creation date set to
-    /// `ctime` or the current time if `None` is given.
-    pub fn import_secret_cv25519<H, S, T>(private_key: &[u8],
-                                          hash: H, sym: S, ctime: T)
-        -> Result<Self> where H: Into<Option<HashAlgorithm>>,
-                              S: Into<Option<SymmetricAlgorithm>>,
-                              T: Into<Option<SystemTime>>
+    pub(crate) fn derive_cv25519_public_key(private_key: &Protected) -> Result<[u8; 32]>
     {
         let secret = Privkey::load_x25519(private_key)?;
-        let public = secret.pubkey()?.get_x25519_key()?;
-        let mut secret = secret.get_x25519_key()?;
-
-        // OpenPGP stores the secret in reverse order.
-        secret.reverse();
 
-        use crate::crypto::ecdh;
-        Self::with_secret(
-            ctime.into().unwrap_or_else(crate::now),
-            PublicKeyAlgorithm::ECDH,
-            mpi::PublicKey::ECDH {
-                curve: Curve::Cv25519,
-                hash: hash.into().unwrap_or_else(
-                    || ecdh::default_ecdh_kdf_hash(&Curve::Cv25519)),
-                sym: sym.into().unwrap_or_else(
-                    || ecdh::default_ecdh_kek_cipher(&Curve::Cv25519)),
-                q: MPI::new_compressed_point(&public),
-            },
-            mpi::SecretKeyMaterial::ECDH {
-                scalar: secret.into(),
-            }.into())
+        Ok(<[u8; 32]>::try_from(&secret.pubkey()?.get_x25519_key()?[..])?)
     }
 
     /// Creates a new OpenPGP secret key packet for an existing Ed25519 key.