RSA Encryption disparity between Android and Java environments

Firstly, apologies for the amount of code I’m about to post. I’m trying to use the RSA public key from my Java application to encrypt a message in an Android app, and then send the ciphertext back to a Java environment for decryption, but upon attempting to decrypt I always get this error:

javax.crypto.BadPaddingException: Decryption error
    at sun.security.rsa.RSAPadding.unpadV15(RSAPadding.java:380)
    at sun.security.rsa.RSAPadding.unpad(RSAPadding.java:291)
    at com.sun.crypto.provider.RSACipher.doFinal(RSACipher.java:356)
    at com.sun.crypto.provider.RSACipher.engineDoFinal(RSACipher.java:382)
    at javax.crypto.Cipher.doFinal(Cipher.java:2087)
    ...

The ciphertext does contain the correct number of bytes (512), so it’s confusing to see a “bad padding” exception. Other similar posts on SO have suggested using “RSA/ECB/PKCS1Padding” as the algorithm, but this does not work.

Annoyingly, encryption and decryption in the Android environment (using Base64.URL_SAFE as the ‘base64Type’) works just fine, I just can’t seem to get the initial encryption working with the public key produced via Java.

I have extracted the bare minimum code into examples, as shown below:

Android Code

private void exampleMethod(){
    String messageString = "Why does this not work in Android?";

    String serverPubKey = "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAoApIBIna77xq4j+M2RmyIhsB++41NHcY4KIPfX4VP4ADnkO+7ejbs4le/twrPtYGESVPF9czSMB5bzmTBZtq0jC8oT/6wiDIBlSuzo4fBrGociBIuaOjyG/j3ZhpcWpWPXuzER+ehuQ+8hZkMuJdK9IodqPR+5jmCef4rXoKObwS02LYQ1co5dEmtZVQRmmeYaVnWibd/s1d4KKGvSzXap3YBTf8peH5UGIQrLOTqvX0bo34xFxmj5U0H3xudnnwuVAlQlj9KiHPPABuwNtm1buRKJb5HZhSCveyT/2YAOmQqGrVN/nALtlZyTDZNs//Vp1zb9exSuG0t5xFc+pn4QIDAQAB";

    String encryptedMessageString = getUrlEncodedCipherText(messageString, serverPubKey, Base64.NO_WRAP);
    /**
     * CipherText is ALWAYS the same and does not decrypt: DA_-RpCki-mjF6tSwiP2IhuW2UfPzZC7A9oVTTNptjT73HtROiQZvUC0Z2veJ5VwVx4toolvLErQmKKoQlqELSD756bu8ohEQwgJ4Xsu-3tXv-uEi5a9a_u19WnNLIF7tayDUhFeD2RzNvW895y1v-D30TvQRskNCFJfnjaytr_vmcVv8HrXURCmG6AMltaqdN72zh8p6VkKcjXSLiCApH957GXSqJCRzxbaQwf8X5EJfn8CQrPDGbE3gdhc2_hFwXQNIdxPxrOLtVbaFp9i_4GRWXJ6E2jHttV2bDv_uSVIz3OBzh7EkJiCnl3c904sH8QZae8c3SQyrTxVL7EpIA,,
     */
}

public static String getUrlEncodedCipherText(String plainText, String pubKey, int base64Type){
    try {
        final PublicKey publicKey = loadPublicKey(pubKey, base64Type);
        final byte[] cipherBytes = encrypt(plainText, publicKey);
        String cipherText = base64Encode(cipherBytes, base64Type);
        String urlEncodedCipherText = urlEncode(cipherText);
        return urlEncodedCipherText;
    }
    catch (Exception e){
        e.printStackTrace();
        return null;
    }
}

public static final String ALGORITHM = "RSA";

public static PublicKey loadPublicKey(String stored, int base64Type) throws GeneralSecurityException {
    String pubKey = stored.replace(BEGIN_PUBLIC_KEY, "");
    pubKey = pubKey.replace(END_PUBLIC_KEY, "");

    byte[] data = Base64.decode(pubKey, base64Type);
    X509EncodedKeySpec spec = new X509EncodedKeySpec(data);
    KeyFactory fact = KeyFactory.getInstance(ALGORITHM);
    PublicKey pub = fact.generatePublic(spec);
    return pub;
}

public static byte[] encrypt(String text, PublicKey key) {
    byte[] cipherText = null;
    try {
        Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.ENCRYPT_MODE, key);
        cipherText = cipher.doFinal(text.getBytes());
    }
    catch (Exception e) {
        e.printStackTrace();
    }
    return cipherText;
}

public static String base64Encode(byte[] cipherBytes, int base64Type){
    byte[] base64Cipher = Base64.encode(cipherBytes, base64Type);
    return new String(base64Cipher);
}

public static String urlEncode(String text){
    return text.replace("+", "-").replace("/", "_").replace("=", ",");
}

Java Code

private void exampleMethod(){
    String pubKey = "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAoApIBIna77xq4j+M2RmyIhsB++41NHcY4KIPfX4VP4ADnkO+7ejbs4le/twrPtYGESVPF9czSMB5bzmTBZtq0jC8oT/6wiDIBlSuzo4fBrGociBIuaOjyG/j3ZhpcWpWPXuzER+ehuQ+8hZkMuJdK9IodqPR+5jmCef4rXoKObwS02LYQ1co5dEmtZVQRmmeYaVnWibd/s1d4KKGvSzXap3YBTf8peH5UGIQrLOTqvX0bo34xFxmj5U0H3xudnnwuVAlQlj9KiHPPABuwNtm1buRKJb5HZhSCveyT/2YAOmQqGrVN/nALtlZyTDZNs//Vp1zb9exSuG0t5xFc+pn4QIDAQAB";
    String privKey = "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";

    String message = "Why does this not work in Android?";
    String encryptedMessage = getUrlEncodedCipherText(message, pubKey);

    try {
        byte[] base64Decoded = Base64.decodeBase64(encryptedMessage.getBytes(Charset.forName("UTF-8")));
        String decryptedMessage = decrypt(base64Decoded, loadPrivateKey(privKey));
        System.out.println("decryptedMessage: " + decryptedMessage);
        /**
         * This works! Ciphertext always comes out different, as expected, and decodes successfully.
         */
    }
    catch (Exception e){
        e.printStackTrace();
    }
}

public static String getUrlEncodedCipherText(String plainText, String pubKey){
    try {
        final PublicKey publicKey = loadPublicKey(pubKey);
        final byte[] cipherBytes = encrypt(plainText, publicKey);
        String cipherText = base64Encode(cipherBytes);
        String urlEncodedCipherText = urlEncode(cipherText);
        return urlEncodedCipherText;
    }
    catch (Exception e){
        e.printStackTrace();
        return null;
    }
}

public static final String ALGORITHM = "RSA";

public static PublicKey loadPublicKey(String stored) throws GeneralSecurityException {
    String pubKey = stored.replace(BEGIN_PUBLIC_KEY, "");
    pubKey = pubKey.replace(END_PUBLIC_KEY, "");

    byte[] data = Base64.decodeBase64(pubKey);
    X509EncodedKeySpec spec = new X509EncodedKeySpec(data);
    KeyFactory fact = KeyFactory.getInstance(ALGORITHM);
    PublicKey pub = fact.generatePublic(spec);
    return pub;
}

public static byte[] encrypt(String text, PublicKey key) {
    byte[] cipherText = null;
    try {
        Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.ENCRYPT_MODE, key);
        cipherText = cipher.doFinal(text.getBytes());
    }
    catch (Exception e) {
        e.printStackTrace();
    }
    return cipherText;
}

public static String decrypt(byte[] encrypted, PrivateKey key) {
    byte[] decryptedText = null;
    try {
        final Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.DECRYPT_MODE, key);
        decryptedText = cipher.doFinal(encrypted);
    }
    catch (Exception e) {
        e.printStackTrace();
        return null;
    }
    return new String(decryptedText);
}

public static String base64Encode(byte[] cipherBytes){
    byte[] base64Cipher = Base64.encodeBase64(cipherBytes);
    return new String(base64Cipher);
}

public static String urlEncode(String text){
    return text.replace("+", "-").replace("/", "_").replace("=", ",");
}

I’m aware that the problem has to be to do with difference in the way Android and Java interpret the RSA algorithm, and/or differences with the Base64 encode/decode, but I’m stumped. Any assistance greatly appreciated.

Answer

Solved it! The issue was that I was using the same ALGORITHM String for Cipher, KeyFactory etc. I split this into two different Strings, and the full solution for my problem is…

Android Code

private void exampleMethod(){
    String messageString = "This actually works in Java AND Android!";

    String serverPubKey = "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAjS7T3WJ+VLucnUP5WYryeg+hhOjZZl5VxwvJAgo4GrXaXdernTPtmXnOSUlbhd928QRCip7D3rLwJNvGIwhPa6coA+YQnj+aHQC02AvCJP/9jpeNmm5MASZfYFXrdmOrMhAPpDZ4rUk1mqtvpwBkYmW3VbMtG336wT1bAIKPHCZuI2n6glupJvs8gK0NuIoAPRlxiQmQD7NCcRx1Et4JmqOMIRC+HqdGv9GGqC/0PB0Fv6LXi8GdzJQPMdoRLR0rvVykNeIzmcimejoIVjI78XUZeB1hF7p55h6W4C4Xm/PrnzKuXw4lBVehZtRhyIvNO62G/eNEZ3tup1/m+vkzHQIDAQAB";

    String encryptedMessageString = getUrlEncodedCipherText(messageString, serverPubKey, Base64.NO_WRAP);
    System.out.println("encryptedMessageString: " + encryptedMessageString);
    /**
     * This works! Ciphertext always comes out different, as expected, and decodes successfully when fed into Java application.
     */
}

public static String getUrlEncodedCipherText(String plainText, String pubKey, int base64Type){
    try {
        final PublicKey publicKey = loadPublicKey(pubKey, base64Type);
        final byte[] cipherBytes = encrypt(plainText, publicKey);
        String cipherText = base64Encode(cipherBytes, base64Type);
        String urlEncodedCipherText = urlEncode(cipherText);
        return urlEncodedCipherText;
    }
    catch (Exception e){
        e.printStackTrace();
        return null;
    }
}

public static final String ALGORITHM = "RSA";
public static final String CIPHER_ALGORITHM = "RSA/ECB/PKCS1Padding";

public static PublicKey loadPublicKey(String stored, int base64Type) throws GeneralSecurityException {
    String pubKey = stored.replace(BEGIN_PUBLIC_KEY, "");
    pubKey = pubKey.replace(END_PUBLIC_KEY, "");

    byte[] data = Base64.decode(pubKey, base64Type);
    X509EncodedKeySpec spec = new X509EncodedKeySpec(data);
    KeyFactory fact = KeyFactory.getInstance(ALGORITHM);
    PublicKey pub = fact.generatePublic(spec);
    return pub;
}

public static byte[] encrypt(String text, PublicKey key) {
    byte[] cipherText = null;
    try {
        Cipher cipher = Cipher.getInstance(CIPHER_ALGORITHM);
        cipher.init(Cipher.ENCRYPT_MODE, key);
        cipherText = cipher.doFinal(text.getBytes());
    }
    catch (Exception e) {
        e.printStackTrace();
    }
    return cipherText;
}

public static String base64Encode(byte[] cipherBytes, int base64Type){
    byte[] base64Cipher = Base64.encode(cipherBytes, base64Type);
    return new String(base64Cipher);
}

public static String urlEncode(String text){
    return text.replace("+", "-").replace("/", "_").replace("=", ",");
}

Java Code

private void exampleMethod(){
    String pubKey = "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAjS7T3WJ+VLucnUP5WYryeg+hhOjZZl5VxwvJAgo4GrXaXdernTPtmXnOSUlbhd928QRCip7D3rLwJNvGIwhPa6coA+YQnj+aHQC02AvCJP/9jpeNmm5MASZfYFXrdmOrMhAPpDZ4rUk1mqtvpwBkYmW3VbMtG336wT1bAIKPHCZuI2n6glupJvs8gK0NuIoAPRlxiQmQD7NCcRx1Et4JmqOMIRC+HqdGv9GGqC/0PB0Fv6LXi8GdzJQPMdoRLR0rvVykNeIzmcimejoIVjI78XUZeB1hF7p55h6W4C4Xm/PrnzKuXw4lBVehZtRhyIvNO62G/eNEZ3tup1/m+vkzHQIDAQAB";
    String privKey = "MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQCNLtPdYn5Uu5ydQ/lZivJ6D6GE6NlmXlXHC8kCCjgatdpd16udM+2Zec5JSVuF33bxBEKKnsPesvAk28YjCE9rpygD5hCeP5odALTYC8Ik//2Ol42abkwBJl9gVet2Y6syEA+kNnitSTWaq2+nAGRiZbdVsy0bffrBPVsAgo8cJm4jafqCW6km+zyArQ24igA9GXGJCZAPs0JxHHUS3gmao4whEL4ep0a/0YaoL/Q8HQW/oteLwZ3MlA8x2hEtHSu9XKQ14jOZyKZ6OghWMjvxdRl4HWEXunnmHpbgLheb8+ufMq5fDiUFV6Fm1GHIi807rYb940Rne26nX+b6+TMdAgMBAAECggEAOHrlUwmWFHvBmcCZvlKx0lbyfONSJXvTwP9b+K7x5u2dYDFpfEhL4zwxZGwuaw4M3TqhDCeboDnhjD1HtLgcXarPfU/KkiRLmRKxRkTd9ENcwnCqu38odMVPHpEA06nn0O1P9Je+C0TgZvHyhtLHVf3vLB+0Ce2KJUhQYZHZgp7dBt0jgsGlEvQTtc1q3UtElNYbcPdLhg16Dcvw93+C+CWizUxHKl2VAxYBz5fGBxa+Cc6xbj4OGd0mvK9Z+71cX8sgp+WIdSV3a2SZrwYq8XxdtJJfdOwPc6LE56Ul9y3fE/biG8DM8ysu7g4C2K1tjKBTq1Rzz4NBCaGwkWQwGQKBgQDCX5nOiHYi0OYoE3WEVYVuEaf/YljurMiU4xYsy3bXzI42S+iKfVHz+/tvTrXSE0n6AdJnDHyP7mncFZodaurYR90S9VW3P+aOGoysmOmBRWW30xyIZ1g5+PJ9xPzoBfyc/AGHO5w5roY7Gg5myUC+UbucFU+9/JHUVoGV8yFz+wKBgQC58f8iY37sXf/sguv0YHpUV1suPWT90kxC25eRQz0lHWIIw4gfGXI7Tf2vtx7hUPK1z3X9lPiTsqmZIRKPrvykD8djmpfZEOzZlLxxhHUZMZppx19xSsgHaespCZir68aytBWn7FOVpehJP7MWaJurg03V8Cv0NSPU5JhN/j8xxwKBgQCkb6I78oAWtilvz/6EJKGL244HZJkd2bibFH4HCV4R9HB/CLrCpoB1a0BsCueQwFa+FVp9aTfbv/N4iCHoLzJcJHeneTu5mmqe32ERBw/MF/yUhhnGX79o0+25brQSSjZKTHuyf0CMH9RZHZL/a9TE7XpM8k6SyKBKRaC9TYGIjwKBgDZG10x4coIYZi3pgWqSBuM7pJf4v9P/YNdbNgDm+aAt1YHtYXyCdv+solJ6R39Jm69KYYylwXGLg0n5h2t9jq1tFayTYaOw9xIEAoW4Pl4eRo597fQp+f2AA261KGV2q0danb+okjVqekV3XJU778S+zSeXtZzRLiZkm7iYcGXdAoGAUEbRAsopG+blDBmuC5Fhr7Grof9lw/zRgfl516n9oujOmDY8Sl4F7jHkgKzOL1CpvMbRswr6G3tmpJMYGNy0HVX4n4QAiIvEraLUtLlh7LotUiU8zzjniK7HPANoOsQxEi6iUR6jWRf0LKgckE9aaUJjvljj1Hr4PbFpny2gjD4=";

    String message = "This actually works in Java AND Android!";
    String encryptedMessage = getUrlEncodedCipherText(message, pubKey);

    try {
        byte[] base64Decoded = Base64.decodeBase64(encryptedMessage.getBytes(Charset.forName("UTF-8")));
        String decryptedMessage = decrypt(base64Decoded, loadPrivateKey(privKey));
        System.out.println("decryptedMessage: " + decryptedMessage);
        /**
         * This works! Ciphertext always comes out different, as expected, and decodes successfully.
         */
    }
    catch (Exception e){
        e.printStackTrace();
    }
}

public static String getUrlEncodedCipherText(String plainText, String pubKey){
    try {
        final PublicKey publicKey = loadPublicKey(pubKey);
        final byte[] cipherBytes = encrypt(plainText, publicKey);
        String cipherText = base64Encode(cipherBytes);
        String urlEncodedCipherText = urlEncode(cipherText);
        return urlEncodedCipherText;
    }
    catch (Exception e){
        e.printStackTrace();
        return null;
    }
}

public static final String ALGORITHM = "RSA";
public static final String CIPHER_ALGORITHM = "RSA/ECB/PKCS1Padding";

public static PublicKey loadPublicKey(String stored) throws GeneralSecurityException {
    String pubKey = stored.replace(BEGIN_PUBLIC_KEY, "");
    pubKey = pubKey.replace(END_PUBLIC_KEY, "");

    byte[] data = Base64.decodeBase64(pubKey);
    X509EncodedKeySpec spec = new X509EncodedKeySpec(data);
    KeyFactory fact = KeyFactory.getInstance(ALGORITHM);
    PublicKey pub = fact.generatePublic(spec);
    return pub;
}

public static byte[] encrypt(String text, PublicKey key) {
    byte[] cipherText = null;
    try {
        Cipher cipher = Cipher.getInstance(CIPHER_ALGORITHM);
        cipher.init(Cipher.ENCRYPT_MODE, key);
        cipherText = cipher.doFinal(text.getBytes());
    }
    catch (Exception e) {
        e.printStackTrace();
    }
    return cipherText;
}

public static String decrypt(byte[] encrypted, PrivateKey key) {
    byte[] decryptedText = null;
    try {
        final Cipher cipher = Cipher.getInstance(CIPHER_ALGORITHM);
        cipher.init(Cipher.DECRYPT_MODE, key);
        decryptedText = cipher.doFinal(encrypted);
    }
    catch (Exception e) {
        e.printStackTrace();
        return null;
    }
    return new String(decryptedText);
}

public static String base64Encode(byte[] cipherBytes){
    byte[] base64Cipher = Base64.encodeBase64(cipherBytes);
    return new String(base64Cipher);
}

public static String urlEncode(String text){
    return text.replace("+", "-").replace("/", "_").replace("=", ",");
}