fix: ensure ecdh shared key width is 32 bytes (#295)

* fix: ensure that ECDH shared keys are 256-bits, even with leading zeros

* tests: update tests to ensure keys are 32 bytes

Author: HashEngineering

core/src/main/java/org/bitcoinj/crypto/KeyCrypterECDH.java

@@ -23,6 +23,7 @@
 import org.bouncycastle.crypto.params.ECPrivateKeyParameters;
 import org.bouncycastle.crypto.params.ECPublicKeyParameters;
 import org.bouncycastle.crypto.params.KeyParameter;
+import org.bouncycastle.util.BigIntegers;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -78,7 +79,8 @@ KeyParameter deriveKey(byte [] secretKey, byte [] publicKey) throws KeyCrypterEx
 
             Secp256k1ECDHAgreement agreement = new Secp256k1ECDHAgreement();
             agreement.init(prvkey);
-            byte[] password = agreement.calculateAgreement(pubKey).toByteArray();
+            BigInteger sharedSecret = agreement.calculateAgreement(pubKey);
+            byte[] password = BigIntegers.asUnsignedByteArray(32, sharedSecret);
             watch.stop();
 
             log.info("Deriving key took {} for a ECDH Key Exchange", watch);

core/src/test/java/org/bitcoinj/crypto/KeyCrypterAESCBCTest.java

@@ -38,7 +38,7 @@ public void setup() {
     @Test
     public void testEncryptionAndDecryption() {
         //Alice is sending to Bob
-        KeyCrypterECDH aliceKeyExchangeCrypter = new KeyCrypterECDH();
+        KeyCrypterAESCBC aliceKeyExchangeCrypter = new KeyCrypterAESCBC();
         KeyParameter aliceKeyParameter = aliceKeyExchangeCrypter.deriveKey(aliceKey);
 
         EncryptedData encryptedData = aliceKeyExchangeCrypter.encrypt(secret.getBytes(), aliceKeyParameter);

core/src/test/java/org/bitcoinj/crypto/KeyCrypterECDHTest.java

@@ -96,6 +96,7 @@ public void ecdhDecryptionTest() {
         KeyCrypterECDH aliceKeyExchangeCrypter = new KeyCrypterECDH();
 
         KeyParameter aliceKeyParameter = aliceKeyExchangeCrypter.deriveKey(alicePrivateKey, bobKeyPair.getPubKey());
+        assertEquals(32, aliceKeyParameter.getKeyLength());
 
         assertEquals("fbd27dbb9e7f471bf3de3704a35e884e37d35c676dc2cc8c3cc574c3962376d2", Utils.HEX.encode(aliceKeyParameter.getKey()));
         EncryptedData encryptedAliceData = aliceKeyExchangeCrypter.encrypt(secret.getBytes(),
@@ -116,6 +117,7 @@ public void ecdhDecryptionTest() {
 
         KeyCrypterECDH bobKeyExchangeCrypter = new KeyCrypterECDH();
         KeyParameter bobKeyParameter = bobKeyExchangeCrypter.deriveKey(bobKeyPair.getPrivKeyBytes(), alicePublicKey);
+        assertEquals(32, bobKeyParameter.getKeyLength());
 
         EncryptedData encryptedData = new EncryptedData(Arrays.copyOfRange(encryptedSecret, 0, 16),
                 Arrays.copyOfRange(encryptedSecret, 16, encryptedSecret.length));