fixes #8.

spec/sodium/sign/secret_key_spec.cr

@@ -1,5 +1,6 @@
 require "../../spec_helper"
 require "../../../src/sodium/sign/secret_key"
+require "../../../src/sodium/crypto_box/secret_key"
 
 detached_test_vectors = [
   {
@@ -71,6 +72,12 @@ describe Sodium::Sign::SecretKey do
     end
   end
 
+  it "to_curve25519" do
+    message = "foo"
+    sskey = Sodium::Sign::SecretKey.new
+    cskey = sskey.to_curve25519
+  end
+
   it "RbNaCl detached test vectors" do
     detached_test_vectors.each do |vec|
       seckey, plaintext, signature = sign_from_vec vec

src/sodium/crypto_box/secret_key.cr

@@ -55,6 +55,22 @@ class Sodium::CryptoBox
       end
     end
 
+    # Use existing secret and public keys.
+    #
+    # Takes ownership of an existing key in a SecureBuffer.
+    # Recomputes the public key from a secret key if missing.
+    def initialize(@sbuf : SecureBuffer, pkey : Bytes? = nil)
+      raise ArgumentError.new("Secret key must be #{KEY_SIZE} bytes, got #{@sbuf.bytesize}") if @sbuf.bytesize != KEY_SIZE
+      if pk = pkey
+        @public_key = PublicKey.new pk
+      else
+        @public_key = PublicKey.new
+        if LibSodium.crypto_scalarmult_base(@public_key.to_slice, self.to_slice) != 0
+          raise Sodium::Error.new("crypto_scalarmult_base")
+        end
+      end
+    end
+
     # Use existing secret and public keys.
     #
     # Copies secret key to a SecureBuffer.

src/sodium/sign/secret_key.cr

@@ -83,5 +83,12 @@ module Sodium
       end
       sig
     end
+
+    def to_curve25519 : CryptoBox::SecretKey
+      key = SecureBuffer.new CryptoBox::SecretKey::KEY_SIZE
+      LibSodium.crypto_sign_ed25519_sk_to_curve25519 key.to_slice, @sbuf.to_slice
+      key.readonly
+      CryptoBox::SecretKey.new key
+    end
   end
 end