add ed25519 to curve25519 conversion helpers

lib/rbnacl/signatures/ed25519/signing_key.rb

@@ -38,6 +38,10 @@ class SigningKey
                          :crypto_sign_ed25519_seed_keypair,
                          %i[pointer pointer pointer]
 
+        sodium_function :to_private_key,
+                        :crypto_sign_ed25519_sk_to_curve25519,
+                        %i[pointer pointer]
+
         attr_reader :verify_key
 
         # Generate a random SigningKey
@@ -123,6 +127,18 @@ def self.signature_bytes
         def signature_bytes
           Ed25519::SIGNATUREBYTES
         end
+
+        # Return a new curve25519 (x25519) private key converted from this key
+        #
+        # it's recommeneded to read https://libsodium.gitbook.io/doc/advanced/ed25519-curve25519
+        # as it encourages using distinct keys for signing and for encryption
+        #
+        # @return [RbNaCl::PrivateKey]
+        def to_curve25519_private_key
+          buffer = Util.zeros(Boxes::Curve25519XSalsa20Poly1305::PrivateKey::BYTES)
+          self.class.crypto_sign_ed25519_sk_to_curve25519(buffer, @signing_key)
+          Boxes::Curve25519XSalsa20Poly1305::PrivateKey.new(buffer)
+        end
       end
     end
   end

lib/rbnacl/signatures/ed25519/verify_key.rb

@@ -23,6 +23,10 @@ class VerifyKey
                          :crypto_sign_ed25519_open,
                          %i[pointer pointer pointer ulong_long pointer]
 
+        sodium_function :to_public_key,
+                        :crypto_sign_ed25519_pk_to_curve25519,
+                        %i[pointer pointer]
+
         # Create a new VerifyKey object from a public key.
         #
         # @param key [String] Ed25519 public key
@@ -99,6 +103,18 @@ def self.signature_bytes
         def signature_bytes
           Ed25519::SIGNATUREBYTES
         end
+
+        # Return a new curve25519 (x25519) public key converted from this key
+        #
+        # it's recommeneded to read https://libsodium.gitbook.io/doc/advanced/ed25519-curve25519
+        # as it encourages using distinct keys for signing and for encryption
+        #
+        # @return [RbNaCl::PublicKey]
+        def to_curve25519_public_key
+          buffer = Util.zeros(Boxes::Curve25519XSalsa20Poly1305::PublicKey::BYTES)
+          self.class.crypto_sign_ed25519_pk_to_curve25519(buffer, @key)
+          Boxes::Curve25519XSalsa20Poly1305::PublicKey.new(buffer)
+        end
       end
     end
   end

lib/rbnacl/test_vectors.rb

@@ -49,6 +49,8 @@ module RbNaCl
                     "376d7f3ac22ff372c18f613f2ae2e856af40",
     sign_signature: "6bd710a368c1249923fc7a1610747403040f0cc30815a00f9ff548a896bbda0b" \
                        "4eb2ca19ebcf917f0f34200a9edbad3901b64ab09cc5ef7b9bcc3c40c0ff7509",
+    sign_curve25519_private: "38e5cdf33bc9e13086f58a3fea86d574e85e7865cffa5e8c9335f200a41d036c",
+    sign_curve25519_public: "35488a98f7ec26ae27099809afb27587b198b1197b5bcb0dec41153db2bf9952",
 
     #
     # SHA256 test vectors

spec/rbnacl/signatures/ed25519/signing_key_spec.rb

@@ -6,6 +6,7 @@
   let(:signing_keypair) { vector :sign_keypair }
   let(:message)         { vector :sign_message }
   let(:signature)       { vector :sign_signature }
+  let(:curve25519_private_key) { vector :sign_curve25519_private }
 
   subject { described_class.new(signing_key) }
 
@@ -31,6 +32,12 @@
     expect(subject.keypair_bytes).to eq signing_keypair
   end
 
+  it "can be converted to curve25519 private key" do
+    sk = subject.to_curve25519_private_key
+    expect(sk).to be_a_kind_of RbNaCl::PrivateKey
+    expect(sk.to_s).to eq curve25519_private_key
+  end
+
   include_examples "key equality" do
     let(:key_bytes) { signing_key }
     let(:key)       { described_class.new(key_bytes) }

spec/rbnacl/signatures/ed25519/verify_key_spec.rb

@@ -7,6 +7,7 @@
 
   let(:message)     { vector :sign_message }
   let(:signature)   { vector :sign_signature }
+  let(:curve25519_public_key) { vector :sign_curve25519_public }
 
   let(:bad_signature) do
     sig = signature.dup
@@ -52,6 +53,12 @@
     expect(described_class.new(verify_key).to_s).to eq verify_key
   end
 
+  it "can be converted to curve25519 public key" do
+    pk = subject.to_curve25519_public_key
+    expect(pk).to be_a_kind_of RbNaCl::PublicKey
+    expect(pk.to_s).to eq curve25519_public_key
+  end
+
   include_examples "key equality" do
     let(:key_bytes) { verify_key }
     let(:key)       { described_class.new(verify_key) }