Added Ed25519 to X25519 key conversion & tests

core/src/main/java/org/bouncycastle/math/ec/rfc8032/Ed25519.java

@@ -1373,4 +1373,62 @@ public static boolean verifyPrehash(byte[] sig, int sigOff, byte[] pk, int pkOff
 
         return implVerify(sig, sigOff, pk, pkOff, ctx, phflag, m, 0, m.length);
     }
+
+    private static int[] obtainYFromPublicKey(byte[] ed25519PublicKey)
+    {
+        PointAffine pA = new PointAffine();
+
+        boolean result = decodePointVar(ed25519PublicKey, 0, true, pA);
+        if (!result)
+            return null;
+
+        return pA.y;
+    }
+
+    public static byte[] toX25519PublicKey(byte[] ed25519PublicKey)
+    {
+        int[] one = new int[X25519Field.SIZE];
+        X25519Field.one(one);
+
+        int[] y = obtainYFromPublicKey(ed25519PublicKey);
+        if (y == null)
+            return null;
+
+        int[] oneMinusY = new int[X25519Field.SIZE];
+        X25519Field.sub(one, y, oneMinusY);
+
+        int[] onePlusY = new int[X25519Field.SIZE];
+        X25519Field.add(one, y, onePlusY);
+
+        int[] oneMinusYInverted = new int[X25519Field.SIZE];
+        X25519Field.inv(oneMinusY, oneMinusYInverted);
+
+        int[] u = new int[X25519Field.SIZE];
+        X25519Field.mul(onePlusY, oneMinusYInverted, u);
+
+        X25519Field.normalize(u);
+
+        byte[] x25519PublicKey = new byte[X25519.SCALAR_SIZE];
+        X25519Field.encode(u, x25519PublicKey, 0);
+
+        return x25519PublicKey;
+    }
+
+    public static byte[] toX25519PrivateKey(byte[] ed25519PrivateKey)
+    {
+        Digest d = Ed25519.createPrehash();
+        byte[] h = new byte[d.getDigestSize()];
+
+        d.update(ed25519PrivateKey, 0, ed25519PrivateKey.length);
+        d.doFinal(h, 0);
+
+        byte[] s = new byte[X25519.SCALAR_SIZE];
+
+        System.arraycopy(h, 0, s, 0, X25519.SCALAR_SIZE);
+        s[0] &= 0xF8;
+        s[X25519.SCALAR_SIZE - 1] &= 0x7F;
+        s[X25519.SCALAR_SIZE - 1] |= 0x40;
+
+        return s;
+    }
 }

core/src/test/java/org/bouncycastle/math/ec/rfc8032/test/Ed25519Test.java

@@ -4,6 +4,8 @@
 
 import junit.framework.TestCase;
 import org.bouncycastle.crypto.Digest;
+import org.bouncycastle.crypto.params.Ed25519PrivateKeyParameters;
+import org.bouncycastle.crypto.params.X25519PrivateKeyParameters;
 import org.bouncycastle.math.ec.rfc8032.Ed25519;
 import org.bouncycastle.util.Arrays;
 import org.bouncycastle.util.encoders.Hex;
@@ -494,4 +496,43 @@ private static void checkEd25519phVector(String sSK, String sPK, String sM, Stri
             assertFalse(text, shouldNotVerify);
         }
     }
+
+//    @Test
+    public void testEd25519ToX25519()
+    {
+        checkEd25519ToX25519Vector("be5bf46a933c8703fa48d0c4075c8fe35fb5f2358778c62008d7265ea6eb0858", "188dedb57fb265624e370e214eba35799cd17897f1d44663530606a2ed5cb57f", "2038f67c3fcfc38429819229d4c874d1f22540ab1349949a766cca0846363f28", "Ed25519 to X25519 vector #1");
+        checkEd25519ToX25519Vector("7013fabacfa4bd6eafb75e9d2d426a1f956ccd9acb19b615d3041d3e0b3000e6", "c0418dcd2fc1da92d6fb07c2ae4e0e4ddd71819533326047deab1c8c882e806f", "ec3e66a867e1f383dbcda7084569ffced6af071e85cb20791523347c59ec3459", "Ed25519 to X25519 vector #2");
+        // This vector checks proper normalization of 'u' (X25519 public key) when converting from Ed25519 public key
+        checkEd25519ToX25519Vector("f81fe2c27e3884dfa6c3a288f37d0ff5699ddade04b6c7dbc379c68a7e8129a0", "e0ee579cf0e094f9aa2c2f87caf8a2e48843fca000325b45400189991c684564", "4d8f5ab537e51507965ed841c35cb896ef6c474f789188cd3dd86dfb769ac661", "Ed25519 to X25519 vector #3");
+    }
+
+    private void checkEd25519ToX25519Vector(String ed25519SK, String x25519SK, String x25519PK, String text)
+    {
+        byte[] esk = Hex.decode(ed25519SK);
+        byte[] xsk = Hex.decode(x25519SK);
+        byte[] xpk = Hex.decode(x25519PK);
+
+        // Check Ed25519 secret key converts to expected X25519 secret key
+        {
+            byte[] converted = Ed25519.toX25519PrivateKey(esk);
+            assertTrue(text, Arrays.areEqual(xsk, converted));
+        }
+
+        // Derive X25519 public key from X25519 secret key and check
+        {
+            X25519PrivateKeyParameters x25519PrivateKeyParams = new X25519PrivateKeyParameters(xsk, 0);
+            byte[] derived = x25519PrivateKeyParams.generatePublicKey().getEncoded();
+            assertTrue(text, Arrays.areEqual(xpk, derived));
+        }
+
+        // Derive Ed25519 public key from Ed25519 secret key,
+        // then convert Ed25519 public key to X25519 public key and check
+        {
+            Ed25519PrivateKeyParameters ed25519PrivateKeyParams = new Ed25519PrivateKeyParameters(esk, 0);
+            byte[] derived = ed25519PrivateKeyParams.generatePublicKey().getEncoded();
+
+            byte[] converted = Ed25519.toX25519PublicKey(derived);
+            assertTrue(text, Arrays.areEqual(xpk, derived));
+        }
+    }
 }