Converting EC PublicKey Hex String to PublicKey

Question

While performing Elliptic Curve cryptography with secp256k1 curve, I noticed that while the code and test cases compile on the Android Studio IDE they do not compile on the android device since the curve isn't defined in the jre/jdk that the mobile device uses. Changing the curve to a prime256v1 I seem to be facing difficulties in converting a hex string of the publicKey to PublicKey object.

Given the hex string of a PublicKey.getEncoded() which is in a database. I want an android client to convert the byte[] from converting the hex string into a PublicKey object. I am converting the byte[] using X509EncodedKeySpec() as follows:

public static PublicKey getPublicKey(byte[] pk) throws NoSuchAlgorithmException, InvalidKeySpecException {
    EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(pk);
    KeyFactory kf = KeyFactory.getInstance("EC");
    PublicKey pub = kf.generatePublic(publicKeySpec);
    return pub;
}

The conversion from a hex string to a byte[] happens as follows:

public static byte[] hexStringToByteArray(String hexString) {
    byte[] bytes = new byte[hexString.length() / 2];

    for(int i = 0; i < hexString.length(); i += 2){
        String sub = hexString.substring(i, i + 2);
        Integer intVal = Integer.parseInt(sub, 16);
        bytes[i / 2] = intVal.byteValue();
        String hex = "".format("0x%x", bytes[i / 2]);
    }
    return bytes;
}

The conversion from a byte[] to Hex string is as follows:

public static String convertBytesToHex(byte[] bytes) {
    char[] hexChars = new char[bytes.length * 2];
    for ( int j = 0; j < bytes.length; j++ ) {
        int v = bytes[j] & 0xFF;
        hexChars[j * 2] = hexArray[v >>> 4];
        hexChars[j * 2 + 1] = hexArray[v & 0x0F];
    }
    return new String(hexChars).toLowerCase();
}

However when I run this on the android app (7.0, API 24) I get the following System Error

W/System.err: java.security.spec.InvalidKeySpecException: java.lang.RuntimeException: error:0c0000b9:ASN.1 encoding routines:OPENSSL_internal:WRONG_TAG
                  at com.android.org.conscrypt.OpenSSLKey.getPublicKey(OpenSSLKey.java:295)
                  at com.android.org.conscrypt.OpenSSLECKeyFactory.engineGeneratePublic(OpenSSLECKeyFactory.java:47)
                  at java.security.KeyFactory.generatePublic(KeyFactory.java:357)

What is the recommended approach for converting a Hex string to a PublicKey for EC instance on an android device.

Here's sample code that you can execute:

ECDSA.java

public class ECDSA {

    public static KeyPair generateKeyPair() throws NoSuchAlgorithmException, InvalidAlgorithmParameterException {
        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("EC");
        ECGenParameterSpec ecSpec = new ECGenParameterSpec("prime256v1");
        SecureRandom random = SecureRandom.getInstance("SHA1PRNG");
        keyGen.initialize(ecSpec, random);
        KeyPair pair = keyGen.generateKeyPair();
        return pair;
    }

    public static PublicKey getPublicKey(byte[] pk) throws NoSuchAlgorithmException, InvalidKeySpecException {
        EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(pk);
        KeyFactory kf = KeyFactory.getInstance("EC");
        PublicKey pub = kf.generatePublic(publicKeySpec);
        return pub;
    }

    public static PrivateKey getPrivateKey(byte[] privk) throws NoSuchAlgorithmException, InvalidKeySpecException {
        EncodedKeySpec privateKeySpec = new X509EncodedKeySpec(privk);
        KeyFactory kf = KeyFactory.getInstance("EC");
        PrivateKey privateKey = kf.generatePrivate(privateKeySpec);
        return privateKey;
    }
}

MainActivity.java

public class MainActivity extends AppCompatActivity {

    KeyPair keyPair = ECDSA.generateKeyPair();
    PublicKey publicKey = keyPair.getPublic();
    PrivateKey privateKey = keyPair.getPrivate();
    // Converting byte[] to Hex
    publicKeyHex = convertBytesToHex(publicKey.getEncoded());
    privateKeyHex = convertBytesToHex(privateKey.getEncoded());
    // Trying to convert Hex to PublicKey/PrivateKey objects
    PublicKey pkReconstructed = ECDSA.getPublicKey(hexStringToByteArray(publicKeyHex));
    PrivateKey skReconstructed = ECDSA.getPrivateKey(hexStringToByteArray(privateKeyHex));
    // This throws an error when running on an android device
    // because there seems to be some library mismatch with 
    // java.security.* vs conscrypt.OpenSSL.* on android.
}

Answer 1

Finally we get a real MCVE and we can now see that you are not using the correct class for encoded private keys. X509EncodedKeySpec is only for public keys. From the javadocs (emphasis mine):

This class represents the ASN.1 encoding of a public key, encoded according to the ASN.1 type SubjectPublicKeyInfo.

For private keys, the correct encoding is usually a PKCS8EncodedKeySpec. The encoding can be determined by examining the output of Key.getFormat(). Therefore, change your method getPrivateKey of ECDSA to

public static PrivateKey getPrivateKey(byte[] privk) throws NoSuchAlgorithmException, InvalidKeySpecException {
        EncodedKeySpec privateKeySpec = new PKCS8EncodedKeySpec(privk);
        KeyFactory kf = KeyFactory.getInstance("EC");
        PrivateKey privateKey = kf.generatePrivate(privateKeySpec);
        return privateKey;
    }