Validate curve key during DecodePubCurveKey and add test to reject non-curve keys.

Author: mtmk

NATS.NKeys.Tests/NKeysTest.cs

@@ -307,16 +307,10 @@ public void DecodePubCurveKey_does_not_include_crc_bytes()
         var raw = new byte[rawLen];
         Base32.FromBase32(publicKey.ToCharArray(), raw);
 
-        // CRC is the last 2 bytes (indices 33 and 34)
-        var crcByte1 = raw[33];
-        var crcByte2 = raw[34];
-
+        // Decoded key should be exactly raw[1..33) — the 32-byte key without prefix or CRC
         var decoded = KeyPair.DecodePubCurveKey(publicKey);
-
-        // The decoded key must not contain CRC bytes
-        // If it were 33 bytes, the last byte would be crcByte1
         Assert.Equal(32, decoded.Length);
-        Assert.NotEqual(crcByte1, decoded[31]); // would match if off-by-one bug existed
+        Assert.Equal(raw.AsSpan().Slice(1, 32).ToArray(), decoded);
     }
 
     [Fact]
@@ -331,6 +325,15 @@ public void DecodePubCurveKey_round_trips_with_seal_open()
         Assert.Equal(message, opened);
     }
 
+    [Fact]
+    public void DecodePubCurveKey_rejects_non_curve_key()
+    {
+        // A valid User public key has the same decoded length but wrong prefix
+        var kp = KeyPair.CreatePair(PrefixByte.User);
+        var ex = Assert.Throws<NKeysException>(() => KeyPair.DecodePubCurveKey(kp.GetPublicKey()));
+        Assert.Equal("Not a valid curve key", ex.Message);
+    }
+
     [Fact]
     public void DecodePubCurveKey_rejects_too_short()
     {

NATS.NKeys/KeyPair.cs

@@ -320,6 +320,11 @@ internal static void DecodePubCurveKey(string key, Span<byte> pub)
 
         Base32.FromBase32(keySpan, buf);
 
+        if (buf[0] != (byte)PrefixByte.Curve)
+        {
+            throw new NKeysException("Not a valid curve key");
+        }
+
         var crc = (ushort)(buf[CurveDecodeLen - 2] | buf[CurveDecodeLen - 1] << 8);
         if (crc != Crc16.Checksum(buf.Slice(0, CurveDecodeLen - 2)))
         {