如何在 android 中使用 (RSA/ECB/PKCS1Padding)

Question

About a year ago, I wrote an application for Android and used a class in it RSA In this class, there was the following code snippet and the application worked

Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding")

But when I re-entered the application code, I did not open the new encrypted information to change the private key until I changed the above code line to the following code line.

Cipher cipher = Cipher.getInstance(keyFactory.getAlgorithm());

The problem is that if I replace the above code snippet in class RSA it is no longer possible to open previously encrypted information (with the same keys as before). And I see the following error

javax.crypto.BadPaddingException: error:04000084:RSA routines:OPENSSL_internal:PKCS_DECODING_ERROR

RSA decryption

public static byte[] decryptByPrivateKey(byte[] data, String key)
            throws Exception {

        byte[] keyBytes = decryptBASE64(key);


        PKCS8EncodedKeySpec pkcs8KeySpec = new PKCS8EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance(KEY_ALGORITHM);
        Key privateKey = keyFactory.generatePrivate(pkcs8KeySpec);

        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        
        // Cipher cipher = Cipher.getInstance(keyFactory.getAlgorithm());


        cipher.init(Cipher.DECRYPT_MODE, privateKey);

        return cipher.doFinal(data);
    }

Answer 1

RSA key pairs can be used within different RSA based schemes, such as PKCS#1 and OAEP padding for encryption, and PKCS#1 and PSS padding for signing. However, there is only one key pair generation possible, which is simply denoted "RSA" .

If only "RSA" is used as input string it will use the defaults set for the specific cryptography provider, which is - in this case - the first provider that implements RSA using keys in software. Apparently that's different on Android from PKCS#1 padding (assuming that you still use the original list of providers, of course). One stupid thing in Java is that you cannot programmatically find out which defaults are used; getAlgorithm() ususally just returns the string you've provided earlier. The only thing you can do is to get the provider using getProvider() and then lookup the defaults...

I would never go for any defaults (except for SecureRandom defaults) as it is unspecified which defaults will be used for Java. Always specify the algorithm in full; your earlier string was fine.

Answer 2

My function

 private fun getEncryptCodeWord(publicKey:String, codeWord:String):String{
        try{
            val publicBytes = Base64.decode(publicKey, Base64.NO_WRAP)
            val keySpec = X509EncodedKeySpec(publicBytes)
            val keyFactory = KeyFactory.getInstance("RSA")
            val pubKey = keyFactory.generatePublic(keySpec)
            val encryptCodeWord = Cipher.getInstance("RSA/ECB/PKCS1Padding")
                .apply { init(Cipher.ENCRYPT_MODE, pubKey) }
                .doFinal(codeWord.toByteArray())
            return Base64.encodeToString(encryptCodeWord, Base64.NO_WRAP)
        }
        catch (ex:Exception){
            Crash.recordException(ex)
            Crash.setKey("error_get_encrypt_code_word",ex.message)
        }
        return codeWord
    }

and for RSA/ECB/OAEPWithSHA-256AndMGF1Padding

private fun getEncryptCodeWord(publicKey:String,codeWord:String):String{
    try{
        val publicBytes = Base64.decode(publicKey, Base64.NO_WRAP)
        val keySpec = X509EncodedKeySpec(publicBytes)
        val keyFactory = KeyFactory.getInstance("RSA")
        val pubKey = keyFactory.generatePublic(keySpec)
        val sp = OAEPParameterSpec("SHA-256", "MGF1", MGF1ParameterSpec("SHA-1"), PSource.PSpecified.DEFAULT)
        val encrypt = Cipher.getInstance("RSA/ECB/OAEPWithSHA-256AndMGF1Padding")
        encrypt.init(Cipher.ENCRYPT_MODE, pubKey, sp)
        val encryptCodeWord = encrypt.doFinal(codeWord.toByteArray())
        return Base64.encodeToString(encryptCodeWord, Base64.NO_WRAP)
    }
    catch (ex:Exception){
        Crash.recordException(ex)
        Crash.setKey("error_get_encrypt_code_word",ex.message)
    }
    return codeWord
}