Initial xkeys support

.editorconfig

@@ -8,7 +8,10 @@ trim_trailing_whitespace = true
 insert_final_newline = true
 end_of_line = lf
 
-[src/NATS.Client.Core/NaCl/**.cs]
+[NATS.NKeys/NaCl/**.cs]
+generated_code = true
+
+[NATS.NKeys/X25519/**.cs]
 generated_code = true
 
 [*.cs]

NATS.NKeys.Tests/NKeysTest.cs

@@ -20,6 +20,25 @@ public static IEnumerable<object[]> PrefixData
         }
     }
 
+    [Fact]
+    public void XKey_from_seed()
+    {
+        var kp = KeyPair.FromSeed("SXALBV3GJABONHXVSTRWL2NXYVUF5LY66CGVF4INX2XBMMVYT2KCZXRXWA".ToCharArray());
+        Assert.Equal("XBKXUQXILUXDDHTWEDECINN24IUYFQAYG737MB5PMEAVMUMHCIWRA3UD", kp.GetPublicKey());
+        var exception = Assert.Throws<NKeysException>(() => kp.Sign(default, default));
+        Assert.Equal("Invalid curve key operation", exception.Message);
+    }
+
+    [Fact]
+    public void XKey_create()
+    {
+        var kp = KeyPair.CreatePair(PrefixByte.Curve, new FixedRng());
+        Assert.Equal("SXAD4F52S2XAJTJ3TGDJ4VXQVW7TU35XJUSVKF25ZRXIWCIUK6NLANRHVY", kp.GetSeed());
+        Assert.Equal("XDS6PE7IMMUA7XXUZLWQIPUCRS3J2IGXVMI3ZEOWX7LY4IMBPH2XECZM", kp.GetPublicKey());
+        var exception = Assert.Throws<NKeysException>(() => kp.Sign(default, default));
+        Assert.Equal("Invalid curve key operation", exception.Message);
+    }
+
     [MemberData(nameof(PrefixData))]
     [Theory]
     public void Create_key_pair(PrefixByte prefix, char initial)

NATS.NKeys/KeyPair.cs

@@ -5,6 +5,7 @@
 using System.Security.Cryptography;
 using NATS.NKeys.Internal;
 using NATS.NKeys.NaCl;
+using X25519;
 
 namespace NATS.NKeys;
 
@@ -63,7 +64,17 @@ public static KeyPair FromSeed(ReadOnlySpan<char> encodedSeed)
         var sk = new ArraySegment<byte>(new byte[64]);
 
         seedSpan.CopyTo(seed.Array);
-        Ed25519.KeyPairFromSeed(pk, sk, seed);
+
+        if (type == PrefixByte.Curve)
+        {
+            var publicKey = Curve25519.ScalarMultiplication(seed.Array, Curve25519.Basepoint);
+            publicKey.AsSpan().CopyTo(pk.Array);
+        }
+        else
+        {
+            Ed25519.KeyPairFromSeed(pk, sk, seed);
+        }
+
         return new KeyPair(type, seed.Array!, sk.Array!, pk.Array!);
     }
 
@@ -93,7 +104,15 @@ public static KeyPair CreatePair(PrefixByte prefix, RandomNumberGenerator rng)
         var seed = new byte[32];
         rng.GetBytes(seed);
         var sk = Ed25519.ExpandedPrivateKeyFromSeed(seed);
-        var pk = Ed25519.PublicKeyFromSeed(seed);
+        byte[] pk;
+        if (prefix == PrefixByte.Curve)
+        {
+            pk = Curve25519.ScalarMultiplication(seed, Curve25519.Basepoint);
+        }
+        else
+        {
+            pk = Ed25519.PublicKeyFromSeed(seed);
+        }
 
         return new KeyPair(prefix, seed, sk, pk);
     }
@@ -124,6 +143,9 @@ public string GetSeed()
     /// <exception cref="NKeysException">Thrown if the private key is not valid or there is an error during the signing process.</exception>
     public void Sign(ReadOnlyMemory<byte> message, Memory<byte> signature)
     {
+        if (_type == PrefixByte.Curve)
+            ThrowInvalidCurveKeyOperationException();
+
         if (_sk.Length == 0)
             ThrowNoSecretKeyException();
 
@@ -272,6 +294,8 @@ private static void DecodeSeed(ReadOnlySpan<byte> raw, out ReadOnlySpan<byte> bu
             return PrefixByte.Account;
         case NKeysConstants.PrefixByteUser:
             return PrefixByte.User;
+        case NKeysConstants.PrefixByteCurve:
+            return PrefixByte.Curve;
         }
 
         return null;
@@ -308,4 +332,7 @@ or NKeysConstants.PrefixByteUser
 
     [MethodImpl(MethodImplOptions.NoInlining)]
     private static void ThrowNoSecretKeyException() => throw new NKeysException("No secret key");
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void ThrowInvalidCurveKeyOperationException() => throw new NKeysException("Invalid curve key operation");
 }

NATS.NKeys/PrefixByte.cs

@@ -36,4 +36,10 @@ public enum PrefixByte : byte
     /// </summary>
     /// <remarks>When encoded as base32, it encodes to 'U...'</remarks>
     User = NKeysConstants.PrefixByteUser,
+
+    /// <summary>
+    /// The version byte used for encoded CurveKeys (X25519).
+    /// </summary>
+    /// <remarks>When encoded as base32, it encodes to 'X...'</remarks>
+    Curve = NKeysConstants.PrefixByteCurve,
 }

NATS.NKeys/PublicAPI.Unshipped.txt

@@ -11,6 +11,7 @@ NATS.NKeys.NKeysException.NKeysException(string! message) -> void
 NATS.NKeys.PrefixByte
 NATS.NKeys.PrefixByte.Account = 0 -> NATS.NKeys.PrefixByte
 NATS.NKeys.PrefixByte.Cluster = 16 -> NATS.NKeys.PrefixByte
+NATS.NKeys.PrefixByte.Curve = 184 -> NATS.NKeys.PrefixByte
 NATS.NKeys.PrefixByte.Operator = 112 -> NATS.NKeys.PrefixByte
 NATS.NKeys.PrefixByte.Server = 104 -> NATS.NKeys.PrefixByte
 NATS.NKeys.PrefixByte.User = 160 -> NATS.NKeys.PrefixByte