Encrypt in java with public key and decrypt in C# with private key RSA

Question

I have an public key that was generated in C# in the back end.

I want to encrypt the message in Android / Java, but when I encrypt the message, the encrypted message is not same as in C#.

I tried encrypting with this specification for the RSA algorithm:

‫‪KeyExchangeAlgorithm‬‬ ==> ‫‪RSA-PKCS1-KeyEx‬‬
‫‪KeySize‬‬ ==>2048 
‫‪RSAEncryptionPadding‬‬ ==> ‫‪false‬‬

This is specification that the back end expects me; below is my implement algorithm on Android.

String encoded = "";

byte[] encrypted = null;
try {

    byte[] publicBytes = Base64.decode(PUBLIC_KEY,Base64.DEFAULT);
    byte[] exponent = Base64.decode("AQAB",Base64.DEFAULT);
    BigInteger key = new BigInteger(1,publicBytes);
    BigInteger expo = new BigInteger(1,exponent);
    RSAPublicKeySpec keySpec = new RSAPublicKeySpec(key,expo);
    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    PublicKey pubKey = keyFactory.generatePublic(keySpec);

    Cipher cipher = Cipher.getInstance("RSA/ECB/NoPadding");

    //Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1PADDING");
    cipher.init(Cipher.ENCRYPT_MODE, pubKey);
    encrypted = cipher.doFinal(txt.getBytes());
    encoded = Base64.encodeToString(encrypted, Base64.DEFAULT);
}
catch (Exception e) {
    e.printStackTrace();
    Log.e("3771",""+e.getMessage());
}
return encoded;

The default padding in java and c# are different. You need to sue the same padding. See following : stackoverflow.com/questions/30849594/… — jdweng
– jdweng, Commented Jan 7, 2020 at 17:45
thanks, but in my case sha is not used, i want algorithm like this ‫‪KeyExchangeAlgorithm‬‬ ==> ‫‪RSA-PKCS1-KeyEx‬‬ ‫‪KeySize‬‬ ==>2048 ‫‪RSAEncryptionPadding‬‬ ==> ‫‪false‬‬ — dariush
– dariush, Commented Jan 7, 2020 at 17:51
You need to use the same public and private key when encrypting and decrypting. — jdweng
– jdweng, Commented Jan 7, 2020 at 18:01
key are same result is different, and i just use public key for encrypt — dariush
– dariush, Commented Jan 7, 2020 at 18:13

Maarten Bodewes · Accepted Answer · 2020-01-09 02:49:22Z

1

RSA uses random padding, so the signature is supposed to be different every time. Use PKCS#1 v1.5 padding ("PKCS1Padding" for historical reasons) and the test is in verification, not in encrypting the same message again.

answered Jan 9, 2020 at 2:49

Maarten Bodewes

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