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In RSA Encrypting working in Java but not working with encrypting from c#?

RSA encryption and decryption work well in java side with modulus and exponent as below:
Java RSA Modulus and Exponent:
String nModulusPublic = "AJ+L/dVL9jnRX6IM87H8x2fR24t6wpzBDV7bcgPWblegR0LNK91z/OSX+lSLUgHSKJ9to/Eo8OMsREpNoJlEzI0=";
String eExponentPublic = "AQAB";
String eExponentPrivate = "AIpmE5C9TiAlgYG/Hn5dOlTS9FFv8fWseX65eZPepOUY4ivxN0lOZ+MsugZd03wmKvnxBuCGu5nv2qrUBTPzjcE=";
Java Public and Private Key Generators:
static PublicKey GetPublicKey(String publicKString, String publicExponentStr) throws Exception {
byte[] modulusBytes = Base64.getDecoder().decode(publicKString);
byte[] exponentBytes = Base64.getDecoder().decode(publicExponentStr);
BigInteger modulus = new BigInteger(1, modulusBytes);
BigInteger exponent = new BigInteger(1, exponentBytes);
RSAPublicKeySpec rsaPubKey = new RSAPublicKeySpec(modulus, exponent);
KeyFactory fact = KeyFactory.getInstance("RSA");
PublicKey pubKey = fact.generatePublic(rsaPubKey);
return pubKey;
}
static PrivateKey GetPrivateKey(String nModulusPublic, String eExponentPrivate) throws Exception {
byte[] modulusBytes = Base64.getDecoder().decode(nModulusPublic);
byte[] exponentBytes = Base64.getDecoder().decode(eExponentPrivate);
BigInteger modulus = new BigInteger(1, modulusBytes);
BigInteger exponent = new BigInteger(1, exponentBytes);
RSAPrivateKeySpec privSpec = new RSAPrivateKeySpec(modulus, exponent);
KeyFactory fact = KeyFactory.getInstance("RSA");
PrivateKey privKey = fact.generatePrivate(privSpec);
return privKey;
}
I use nModulusPublic and eExponentPublic in c# to encrypting and decrypting in Java but doesn't work.
Worked on RSA.Encrypt(textBytes, true); parameters in c# function change it to false and RSAEncryptionPadding.Pkcs1 but doesn't' work. When I use the result of the c# Encrypt function in java to decrypt it always encounter with this error :
javax.crypto.IllegalBlockSizeException: Data must not be longer than 64 bytes
C# encrypt function:
static string Encrypt(string text)
{
string outputB64 = string.Empty;
byte[] textBytes = Encoding.UTF8.GetBytes(text);
RSAParameters result = new RSAParameters()
{
Modulus = Convert.FromBase64String(nModulusPublic),
Exponent = Convert.FromBase64String(eExponentPublic)
};
using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())
{
RSA.ImportParameters(result);
byte[] encryptedData = RSA.Encrypt(textBytes, true);
outputB64 = Convert.ToBase64String(encryptedData);
}
return outputB64;
}
Extra Information, Java Encrypt and Decrypt main functions:
static String Decrypt(String encodedString,PrivateKey privKey) {
try {
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.DECRYPT_MODE, privKey);
byte[] decrypted = cipher.doFinal(Base64.getDecoder().decode(encodedString));
return new String(decrypted, "UTF-8");
} catch (Exception err) {
return err.fillInStackTrace().toString();
}
}
static String Encrypt(String encodedString,PublicKey pubKey) {
try {
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pubKey);
byte[] plainBytes = new String(encodedString).getBytes("UTF-8");
byte[] cipherData = cipher.doFinal(plainBytes);
String encryptedString = Base64.getEncoder().encodeToString(cipherData);
return encryptedString;
} catch (Exception err) {
return err.fillInStackTrace().toString();
}
}
Update:
I was working on it and found that java encrypts function and c# have two different types, Java result always ended with "==" but c# function have one "=" at the end, It seems this is the problem.
C# Encrypt function result:
AJiltxqa1/8HU20XZlKJsJvclQ8PyQetpWdbCOpbqrXVg0q
/v4x5tXLxbzGKbO5InvKkib7tDQp+9BU0SYbZLv0=
Java Encrypt function result:
RlarFQBo2mcCWjidQ5l7ho2EOG6KGQWpR3ByXXHsGo6+HRQzmO4v7
TUTMdfB9wjI3aO6quruSReitrWu7QF9Vw==

On C# encrypt function you give the parameter 'true':
byte[] encryptedData = RSA.Encrypt(textBytes, true);
This means that C# is NOT using the PKCS1Padding but the OEAPPadding.
Simply change on Java-side in your Decrypt-method (and in your ENCRYPT-method as well :-) the line
// change:
//Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
// to
Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWITHSHA-1ANDMGF1PADDING");
I tested it with your keypair and it works like expected.
Edit:
On C#-side I run this code:
string plaintext = "The quick brown fox";
string encryptedDataBase64So = Encrypt(plaintext);
Console.WriteLine("encrypted SO : " + encryptedDataBase64So);
Console output:
encrypted SO : ZLylMsqcqbuDM7DprrmqIU8c8Q79fPXHudOY4INCNAo+iU7Oor3mZ8i+PP5PjtDkifqAXKYT8ON/ia9WjEFqRQ==
On Java-side I took the base64-String as input:
String fromCsharp = "Ew3nTEQuOX1tWfRNJEERa75A1o2bn6+HurVPYzGzA7kt+HAZAMdXKNACY2emvU6Bf42i8zpBO89lqvzuxNmRIw==";
String decryptedtext = DecryptWorking(fromCsharp, privateKey);
System.out.println("\ndecrypted from c#: " + decryptedtext);
Console output:
decrypted from c#: The quick brown fox
BTW: this is the PublicKey on C#-side that I generated from the PublicKey on Java-side and used it like this as source for the RSA-Encryption:
var publicKey = "<RSAKeyValue><Modulus>n4v91Uv2OdFfogzzsfzHZ9Hbi3rCnMENXttyA9ZuV6BHQs0r3XP85Jf6VItSAdIon22j8Sjw4yxESk2gmUTMjQ==</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>";
RSA.FromXmlString(publicKey);

Besides the padding error described in detail in Michael Fehr's answer, there is another issue which is actually responsible for the error message IllegalBlockSizeException: Data must not be longer than 64 bytes. The inconsistent padding would throw a BadPaddingException if the IllegalBlockSizeException would not be thrown first.
The posted ciphertext has a leading 0 byte and is therefore 65 bytes in size:
Base64: AJiltxqa1/8HU20XZlKJsJvclQ8PyQetpWdbCOpbqrXVg0q/v4x5tXLxbzGKbO5InvKkib7tDQp+9BU0SYbZLv0=
Hex: 0098a5b71a9ad7ff07536d17665289b09bdc950f0fc907ada5675b08ea5baab5d5834abfbf8c79b572f16f318a6cee489ef2a489beed0d0a7ef415344986d92efd
If you try to decrypt this ciphertext on the Java side, you will get the posted error: IllegalBlockSizeException: Data must not be longer than 64 bytes.
Why does the C# code produce a too long ciphertext? This is because of the modulus, which also has a leading 0 byte and therefore a length of 65 bytes:
Base64: AJ+L/dVL9jnRX6IM87H8x2fR24t6wpzBDV7bcgPWblegR0LNK91z/OSX+lSLUgHSKJ9to/Eo8OMsREpNoJlEzI0=
Hex: 009f8bfdd54bf639d15fa20cf3b1fcc767d1db8b7ac29cc10d5edb7203d66e57a04742cd2bdd73fce497fa548b5201d2289f6da3f128f0e32c444a4da09944cc8d
The modulus was derived with BigInteger.toByteArray() (see this question, Update section), which returns the two's-complement representation and places a leading 0 byte in front if the frontmost byte has a value larger than 0x7f.
The leading 0 byte in the modulus results in a ciphertext generated by the C# code, which also has a leading 0 byte and thus an invalid length of 65 bytes. This does not make much sense and might be a bug.
To solve the problem the 0 byte in the modulus should be removed for the C# code, resulting in the following Base64 encoded modulus (which will produce 64 bytes ciphertexts):
n4v91Uv2OdFfogzzsfzHZ9Hbi3rCnMENXttyA9ZuV6BHQs0r3XP85Jf6VItSAdIon22j8Sjw4yxESk2gmUTMjQ==
Alternatively the 0 byte in the ciphertext can be removed, resulting in the following ciphertext (Base64 encoded):
mKW3GprX/wdTbRdmUomwm9yVDw/JB62lZ1sI6luqtdWDSr+/jHm1cvFvMYps7kie8qSJvu0NCn70FTRJhtku/Q==
which can now be successfully decrypted by the Java code to the plaintext (if a consistent padding is applied, see Michael Fehr's answer):
Davood

Rebuild of a RSA Private Key from modulus & exponent fails

I'm trying to rebuild a RSA keypair from modulus & private/public exponents. The conversion works correct for the public key but fails to private key when comparing the encoded private keys.
When using this rebuild private/public keypair for encryption it works (!) in Java, but when using the rebuild keypair in PHP, the decryption part fails (encryption is working), so it seems to me that the rebuild private key is something different to the "original" private key.
Just for info: using the "original" keypair everything is working fine in PHP.
So my question: how can I retrieve the "original" private key from (BigInteger) modulus & private exponent?
Edit: see my final edit at the end
My sample code shows the equality of public key vs. rebuild one and that the private keys are different:
Rebuilding of a RSA PrivateKey from modulus & exponent
privateKey equals rebuild: false
publicKey equals rebuild: true
code:
import java.math.BigInteger;
import java.security.*;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.RSAPrivateKeySpec;
import java.security.spec.RSAPublicKeySpec;
import java.util.Arrays;
public class RebuildRSAPrivateKey {
public static void main(String[] args) throws NoSuchAlgorithmException, InvalidKeySpecException {
System.out.println("Rebuilding of a RSA PrivateKey from modulus & exponent");
// rsa key generation
KeyPairGenerator kpGen = KeyPairGenerator.getInstance("RSA");
//kpGen.initialize(2048, new SecureRandom());
kpGen.initialize(2048, new SecureRandom());
KeyPair keyPair = kpGen.generateKeyPair();
// private key
PrivateKey privateKey = keyPair.getPrivate();
// get modulus & exponent
RSAPrivateKey rsaPrivateKey = (RSAPrivateKey) privateKey;
BigInteger modulus = rsaPrivateKey.getModulus();
BigInteger privateExponent = rsaPrivateKey.getPrivateExponent();
// rebuild the private key
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
RSAPrivateKeySpec rsaPrivateKeySpec = new RSAPrivateKeySpec(modulus, privateExponent);
PrivateKey privateKeyRebuild = keyFactory.generatePrivate(rsaPrivateKeySpec);
System.out.println("privateKey equals rebuild: " + Arrays.equals(privateKey.getEncoded(), privateKeyRebuild.getEncoded()));
// public key
PublicKey publicKey = keyPair.getPublic();
// get modulus & exponent
RSAPublicKey rsaPublicKey = (RSAPublicKey) publicKey;
BigInteger modulusPub = rsaPublicKey.getModulus();
BigInteger publicExponent = rsaPublicKey.getPublicExponent();
// rebuild the public key
KeyFactory keyFactoryPub = KeyFactory.getInstance("RSA");
RSAPublicKeySpec rsaPublicKeySpec = new RSAPublicKeySpec(modulusPub, publicExponent);
PublicKey publicKeyRebuild = keyFactory.generatePublic(rsaPublicKeySpec);
System.out.println("publicKey equals rebuild: " + Arrays.equals(publicKey.getEncoded(), publicKeyRebuild.getEncoded()));
}
}
Edit: The following programs will show that a RSA private/public keypair derived from encoded keys can get restored and the
encryption and decryption works in Java and PHP. The keys are insecure RSA 512 bit keys and Base64 decoded.
The same keys are then derived from modulus and private/public exponents and the en-/decryption works in Java but not in PHP.
That's why I'd like to get the "original" RSA keys from modulus and exponents, thanks for your kindly help.
Result of Java program:
Rebuilding of a RSA PrivateKey from modulus & exponent v4
privateKey Original Base64: MIIBVgIBADANBgkqhkiG9w0BAQEFAASCAUAwggE8AgEAAkEA2wFgcni89ijJ/uijQkzCGF4JiUB1+mEJ48u4Lk0vxB7ym3/FCvOEnN2H7FLUzsGvXRhFriLBiSJlg2tOhV5eiwIDAQABAkEAkDpf4gNRrms+W/mpSshyKsoDTbh9+d5ePP601QlQI79lrsjdy2GLgk4RV1XmwYinM9Sk8G+ssyXTYHdby6A2wQIhAPcRtl6tub6PFiIE1jcuIkib/HzAdRYHZx3ZdzRTYDetAiEA4uv43xpGl5N8yG27Kv0DkRoOlr4Ch6oM24hLVw7ClhcCIFgdRAo+MQlqJH2bdf6WAHoez4x6YwepOjhmD2Jk/eK9AiEAtHgI6J5EEB56+gfS+CBa6tZ3Tcl1x6ElMp8Vk/ooJScCIQDUa3LUkcc58yjJYq8ZNQC/86+HIzd5MldTwg5buR1lpw==
privateKey Rebuild Base64: MIIBVgIBADANBgkqhkiG9w0BAQEFAASCAUAwggE8AgEAAkEA2wFgcni89ijJ/uijQkzCGF4JiUB1+mEJ48u4Lk0vxB7ym3/FCvOEnN2H7FLUzsGvXRhFriLBiSJlg2tOhV5eiwIDAQABAkEAkDpf4gNRrms+W/mpSshyKsoDTbh9+d5ePP601QlQI79lrsjdy2GLgk4RV1XmwYinM9Sk8G+ssyXTYHdby6A2wQIhAPcRtl6tub6PFiIE1jcuIkib/HzAdRYHZx3ZdzRTYDetAiEA4uv43xpGl5N8yG27Kv0DkRoOlr4Ch6oM24hLVw7ClhcCIFgdRAo+MQlqJH2bdf6WAHoez4x6YwepOjhmD2Jk/eK9AiEAtHgI6J5EEB56+gfS+CBa6tZ3Tcl1x6ElMp8Vk/ooJScCIQDUa3LUkcc58yjJYq8ZNQC/86+HIzd5MldTwg5buR1lpw==
publicKey Base64: MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBANsBYHJ4vPYoyf7oo0JMwhheCYlAdfphCePLuC5NL8Qe8pt/xQrzhJzdh+xS1M7Br10YRa4iwYkiZYNrToVeXosCAwEAAQ==
generate private & public key via modulus and private/public exponent
privateKey Modulus Base64: MIGzAgEAMA0GCSqGSIb3DQEBAQUABIGeMIGbAgEAAkEA2wFgcni89ijJ/uijQkzCGF4JiUB1+mEJ48u4Lk0vxB7ym3/FCvOEnN2H7FLUzsGvXRhFriLBiSJlg2tOhV5eiwIBAAJBAJA6X+IDUa5rPlv5qUrIcirKA024ffneXjz+tNUJUCO/Za7I3cthi4JOEVdV5sGIpzPUpPBvrLMl02B3W8ugNsECAQACAQACAQACAQACAQA=
publicKey Modulus Base64: MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBANsBYHJ4vPYoyf7oo0JMwhheCYlAdfphCePLuC5NL8Qe8pt/xQrzhJzdh+xS1M7Br10YRa4iwYkiZYNrToVeXosCAwEAAQ==
en-/decryption with original keys
ciphertext Original : fvFPRZ5B2GMgv9aXQjyQsxnRHK2wotfXlLV+zGea1E3nsZC6RMn+LQMOe9yvZ8IcaG2F/8wWv2NkNmBX4wuxaw==
decryptedtext Original: this is the message to encrypt
en-/decryption with keys from modulus & exponent
ciphertext Modulus : o0tB4xQIwQRFDSsWj1WgWHexXnJOp9jeBymFPJvy+xZBvfJay2yR0XZEy+0VwaedxdTf9CoyKVvgCbn2HCohSQ==
decryptedtext Modulus : this is the message to encrypt
Result of PHP program:
php version: 7.4.6 openssl version: OpenSSL 1.1.1g 21 Apr 2020
plaintext: this is the message to encrypt
rsa encryption with original keys
priBase64:MIIBVgIBADANBgkqhkiG9w0BAQEFAASCAUAwggE8AgEAAkEA2wFgcni89ijJ/uijQkzCGF4JiUB1+mEJ48u4Lk0vxB7ym3/FCvOEnN2H7FLUzsGvXRhFriLBiSJlg2tOhV5eiwIDAQABAkEAkDpf4gNRrms+W/mpSshyKsoDTbh9+d5ePP601QlQI79lrsjdy2GLgk4RV1XmwYinM9Sk8G+ssyXTYHdby6A2wQIhAPcRtl6tub6PFiIE1jcuIkib/HzAdRYHZx3ZdzRTYDetAiEA4uv43xpGl5N8yG27Kv0DkRoOlr4Ch6oM24hLVw7ClhcCIFgdRAo+MQlqJH2bdf6WAHoez4x6YwepOjhmD2Jk/eK9AiEAtHgI6J5EEB56+gfS+CBa6tZ3Tcl1x6ElMp8Vk/ooJScCIQDUa3LUkcc58yjJYq8ZNQC/86+HIzd5MldTwg5buR1lpw==
pubBase64:MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBANsBYHJ4vPYoyf7oo0JMwhheCYlAdfphCePLuC5NL8Qe8pt/xQrzhJzdh+xS1M7Br10YRa4iwYkiZYNrToVeXosCAwEAAQ==
ciphertext Base64:WmvVwqf2EHQc0yb6L4pVJ0/23pNW4QsBun3SNvYE8p/sEk+1GQSYxYpbY/mLbSGF2Lb1P5g5er+z7dWxHmodNA==
decryptedtext: this is the message to encrypt
rsa encryption with keys created via modulus & exponents
priBase64:MIGzAgEAMA0GCSqGSIb3DQEBAQUABIGeMIGbAgEAAkEA2wFgcni89ijJ/uijQkzCGF4JiUB1+mEJ48u4Lk0vxB7ym3/FCvOEnN2H7FLUzsGvXRhFriLBiSJlg2tOhV5eiwIBAAJBAJA6X+IDUa5rPlv5qUrIcirKA024ffneXjz+tNUJUCO/Za7I3cthi4JOEVdV5sGIpzPUpPBvrLMl02B3W8ugNsECAQACAQACAQACAQACAQA=
pubBase64:MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBANsBYHJ4vPYoyf7oo0JMwhheCYlAdfphCePLuC5NL8Qe8pt/xQrzhJzdh+xS1M7Br10YRa4iwYkiZYNrToVeXosCAwEAAQ==
ciphertext Base64:kqn8aZpvfpPzr3u2NBX/XmnlFweEvOm+Qu4l2wiUSQCjA0hutQ10mbLaO55oCox7GixvMgb3VtoDBJ8hfW1zbQ==
Cannot Decrypt error:0407109F:rsa routines:RSA_padding_check_PKCS1_type_2:pkcs decoding error
decryptedtext:
decrypt error: error:0909006C:PEM routines:get_name:no start line
Source Java:
import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import java.io.IOException;
import java.math.BigInteger;
import java.security.*;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;
import java.security.spec.*;
import java.util.Base64;
public class RebuildRSAPrivateKey4 {
public static void main(String[] args) throws NoSuchAlgorithmException, InvalidKeySpecException, IllegalBlockSizeException, InvalidKeyException, BadPaddingException, NoSuchPaddingException, IOException {
System.out.println("Rebuilding of a RSA PrivateKey from modulus & exponent v4");
// rsa key generation
KeyPairGenerator kpGen = KeyPairGenerator.getInstance("RSA");
//kpGen.initialize(2048, new SecureRandom());
kpGen.initialize(512, new SecureRandom()); // don't use 512 bit keys as they are insecure !!
KeyPair keyPair = kpGen.generateKeyPair();
// privateKey Base64: MIIBVgIBADANBgkqhkiG9w0BAQEFAASCAUAwggE8AgEAAkEA2wFgcni89ijJ/uijQkzCGF4JiUB1+mEJ48u4Lk0vxB7ym3/FCvOEnN2H7FLUzsGvXRhFriLBiSJlg2tOhV5eiwIDAQABAkEAkDpf4gNRrms+W/mpSshyKsoDTbh9+d5ePP601QlQI79lrsjdy2GLgk4RV1XmwYinM9Sk8G+ssyXTYHdby6A2wQIhAPcRtl6tub6PFiIE1jcuIkib/HzAdRYHZx3ZdzRTYDetAiEA4uv43xpGl5N8yG27Kv0DkRoOlr4Ch6oM24hLVw7ClhcCIFgdRAo+MQlqJH2bdf6WAHoez4x6YwepOjhmD2Jk/eK9AiEAtHgI6J5EEB56+gfS+CBa6tZ3Tcl1x6ElMp8Vk/ooJScCIQDUa3LUkcc58yjJYq8ZNQC/86+HIzd5MldTwg5buR1lpw==
// publicKey Base64: MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBANsBYHJ4vPYoyf7oo0JMwhheCYlAdfphCePLuC5NL8Qe8pt/xQrzhJzdh+xS1M7Br10YRa4iwYkiZYNrToVeXosCAwEAAQ==
String privateKeyBase64 = "MIIBVgIBADANBgkqhkiG9w0BAQEFAASCAUAwggE8AgEAAkEA2wFgcni89ijJ/uijQkzCGF4JiUB1+mEJ48u4Lk0vxB7ym3/FCvOEnN2H7FLUzsGvXRhFriLBiSJlg2tOhV5eiwIDAQABAkEAkDpf4gNRrms+W/mpSshyKsoDTbh9+d5ePP601QlQI79lrsjdy2GLgk4RV1XmwYinM9Sk8G+ssyXTYHdby6A2wQIhAPcRtl6tub6PFiIE1jcuIkib/HzAdRYHZx3ZdzRTYDetAiEA4uv43xpGl5N8yG27Kv0DkRoOlr4Ch6oM24hLVw7ClhcCIFgdRAo+MQlqJH2bdf6WAHoez4x6YwepOjhmD2Jk/eK9AiEAtHgI6J5EEB56+gfS+CBa6tZ3Tcl1x6ElMp8Vk/ooJScCIQDUa3LUkcc58yjJYq8ZNQC/86+HIzd5MldTwg5buR1lpw==";
String publicKeyBase64 = "MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBANsBYHJ4vPYoyf7oo0JMwhheCYlAdfphCePLuC5NL8Qe8pt/xQrzhJzdh+xS1M7Br10YRa4iwYkiZYNrToVeXosCAwEAAQ==";
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
PKCS8EncodedKeySpec privateKeySpec = new PKCS8EncodedKeySpec(Base64.getDecoder().decode(privateKeyBase64));
PrivateKey privateKey = keyFactory.generatePrivate(privateKeySpec);
X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(Base64.getDecoder().decode(publicKeyBase64));
PublicKey publicKey = keyFactory.generatePublic(publicKeySpec);
System.out.println("privateKey Original Base64: " + privateKeyBase64);
System.out.println("privateKey Rebuild Base64: " + Base64.getEncoder().encodeToString(privateKey.getEncoded()));
System.out.println("publicKey Base64: " + publicKeyBase64);
// get modulus & private exponent via RSAPrivateKey
RSAPrivateKey rsaPrivateKey = (RSAPrivateKey) privateKey;
BigInteger modulus = rsaPrivateKey.getModulus();
BigInteger privateExponent = rsaPrivateKey.getPrivateExponent();
// rebuild the private key
RSAPrivateKeySpec rsaPrivateKeySpec = new RSAPrivateKeySpec(modulus, privateExponent);
PrivateKey privateKeyModulusExponent = keyFactory.generatePrivate(rsaPrivateKeySpec);
// public key
RSAPublicKey rsaPublicKey = (RSAPublicKey) publicKey;
BigInteger modulusPub = rsaPublicKey.getModulus();
BigInteger publicExponent = rsaPublicKey.getPublicExponent();
// rebuild the public key
RSAPublicKeySpec rsaPublicKeySpec = new RSAPublicKeySpec(modulusPub, publicExponent);
PublicKey publicKeyModulusExponent = keyFactory.generatePublic(rsaPublicKeySpec);
System.out.println("\ngenerate private & public key via modulus and private/public exponent");
System.out.println("privateKey Modulus Base64: " + Base64.getEncoder().encodeToString(privateKeyModulusExponent.getEncoded()));
System.out.println("publicKey Modulus Base64: " + Base64.getEncoder().encodeToString(publicKeyModulusExponent.getEncoded()));
System.out.println("\nen-/decryption with original keys");
String plaintext = "this is the message to encrypt";
String ciphertextOriginal = encrypt(publicKey, plaintext);
String decryptedtextOriginal = decrypt(privateKey, ciphertextOriginal);
System.out.println("ciphertext Original : " + ciphertextOriginal);
System.out.println("decryptedtext Original: " + decryptedtextOriginal);
System.out.println("\nen-/decryption with keys from modulus & exponent");
String ciphertextModulus = encrypt(publicKeyModulusExponent, plaintext);
String decryptedtextModulus = decrypt(privateKeyModulusExponent, ciphertextOriginal);
System.out.println("ciphertext Modulus : " + ciphertextModulus);
System.out.println("decryptedtext Modulus : " + decryptedtextModulus);
}
private static String encrypt(PublicKey publicKey, String plaintext) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IOException, BadPaddingException, IllegalBlockSizeException {
String ciphertext = "";
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, publicKey);
byte[] ciphertextByte = cipher.doFinal(plaintext.getBytes("UTF8"));
ciphertext = Base64.getEncoder().encodeToString(ciphertextByte).replaceAll("\\r|\\n", "");
return ciphertext;
}
private static String decrypt(PrivateKey privateKey, String ciphertext) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, BadPaddingException, IllegalBlockSizeException {
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.DECRYPT_MODE, privateKey);
byte[] ciphertextByte = Base64.getDecoder().decode(ciphertext);
byte[] decryptedtextByte = cipher.doFinal(ciphertextByte);
return new String(decryptedtextByte);
}
private static String bytesToHex(byte[] bytes) {
StringBuffer result = new StringBuffer();
for (byte b : bytes) result.append(Integer.toString((b & 0xff) + 0x100, 16).substring(1));
return result.toString();
}
}
Source PHP:
<?php
function encrypt($publicKeyBase64, $plaintext){
$pub = base64_decode($publicKeyBase64);
// public key conversion der to pem
$pubPem = chunk_split(base64_encode($pub), 64, "\n");
$pubPem = "-----BEGIN PUBLIC KEY-----\n" . $pubPem . "-----END PUBLIC KEY-----\n";
$ublicKey = "";
$publicKey = openssl_get_publickey($pubPem);
if (!$publicKey) {
echo "Cannot get public key" . "<br>";
}
$ciphertext = "";
openssl_public_encrypt($plaintext, $ciphertext, $publicKey);
if (!empty($ciphertext)) {
openssl_free_key($publicKey);
//echo "Encryption OK!" . "<br>";
} else {
echo "Cannot Encrypt" . "<br>";
}
$ciphertextBase64 = base64_encode($ciphertext);
return $ciphertextBase64;
}
function decrypt($privateKeyBase64, $ciphertext){
$pri = base64_decode($privateKeyBase64);
// private key conversion der to pem
$priPem = chunk_split(base64_encode($pri), 64, "\n");
$priPem = "-----BEGIN PRIVATE KEY-----\n" . $priPem . "-----END PRIVATE KEY-----\n";
$privateKey = openssl_get_privatekey($priPem);
$Crypted = openssl_private_decrypt($ciphertext, $decryptedtext, $privateKey);
if (!$Crypted) {
echo 'Cannot Decrypt ' . openssl_error_string() . '<br>';
} else {
openssl_free_key($privateKey);
//echo "decryptedtext: " . $decryptedtext . "<br>";
}
return $decryptedtext;
}
echo 'php version: ' . PHP_VERSION . ' openssl version: ' . OPENSSL_VERSION_TEXT . '<br>';
$plaintext = "this is the message to encrypt";
echo "plaintext: " . $plaintext . "<br>";
// RSA 512 keys from Java GenerateKeysSo.java
echo 'rsa encryption with original keys' . '<br>';
$priBase64 = "MIIBVgIBADANBgkqhkiG9w0BAQEFAASCAUAwggE8AgEAAkEA2wFgcni89ijJ/uijQkzCGF4JiUB1+mEJ48u4Lk0vxB7ym3/FCvOEnN2H7FLUzsGvXRhFriLBiSJlg2tOhV5eiwIDAQABAkEAkDpf4gNRrms+W/mpSshyKsoDTbh9+d5ePP601QlQI79lrsjdy2GLgk4RV1XmwYinM9Sk8G+ssyXTYHdby6A2wQIhAPcRtl6tub6PFiIE1jcuIkib/HzAdRYHZx3ZdzRTYDetAiEA4uv43xpGl5N8yG27Kv0DkRoOlr4Ch6oM24hLVw7ClhcCIFgdRAo+MQlqJH2bdf6WAHoez4x6YwepOjhmD2Jk/eK9AiEAtHgI6J5EEB56+gfS+CBa6tZ3Tcl1x6ElMp8Vk/ooJScCIQDUa3LUkcc58yjJYq8ZNQC/86+HIzd5MldTwg5buR1lpw==";
$pubBase64 = "MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBANsBYHJ4vPYoyf7oo0JMwhheCYlAdfphCePLuC5NL8Qe8pt/xQrzhJzdh+xS1M7Br10YRa4iwYkiZYNrToVeXosCAwEAAQ==";
echo 'priBase64:' . $priBase64 . '<br>';
echo 'pubBase64:' . $pubBase64 . '<br>';
$ciphertextBase64 = encrypt($pubBase64, $plaintext);
echo 'ciphertext Base64:' . $ciphertextBase64 . '<br>';
$ciphertext = base64_decode($ciphertextBase64);
$decryptedtext = decrypt($priBase64, $ciphertext);
echo "decryptedtext: " . $decryptedtext . "<br><br>";
// keys created via modulus & exponent
$priBase64 = "MIGzAgEAMA0GCSqGSIb3DQEBAQUABIGeMIGbAgEAAkEA2wFgcni89ijJ/uijQkzCGF4JiUB1+mEJ48u4Lk0vxB7ym3/FCvOEnN2H7FLUzsGvXRhFriLBiSJlg2tOhV5eiwIBAAJBAJA6X+IDUa5rPlv5qUrIcirKA024ffneXjz+tNUJUCO/Za7I3cthi4JOEVdV5sGIpzPUpPBvrLMl02B3W8ugNsECAQACAQACAQACAQACAQA=";
$pubBase64 = "MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBANsBYHJ4vPYoyf7oo0JMwhheCYlAdfphCePLuC5NL8Qe8pt/xQrzhJzdh+xS1M7Br10YRa4iwYkiZYNrToVeXosCAwEAAQ==";
echo 'rsa encryption with keys created via modulus & exponents' . '<br>';
echo 'priBase64:' . $priBase64 . '<br>';
echo 'pubBase64:' . $pubBase64 . '<br>';
$ciphertextBase64 = encrypt($pubBase64, $plaintext);
echo 'ciphertext Base64:' . $ciphertextBase64 . '<br>';
$ciphertext = base64_decode($ciphertextBase64);
$decryptedtext = decrypt($priBase64, $ciphertext);
echo "decryptedtext: " . $decryptedtext . "<br><br>";
echo 'decrypt error: error:0909006C:PEM routines:get_name:no start line' . '<br>';
?>
Final Edit solution & conclusion
If we want to use a RSA private-public keypair for encryption (and signing as well ?) in other systems than Java it is of importance that the private key is saved immeditaly. If we are trying to rebuild the private key from the encoded form (via PKCS8EncodedKeySpec) some data are definitely missing. Those rebuild private keys will fail to work (here in PHP/openssl).
If we need to rebuild a private key from the encoded-form (byte[]) the keys need to get enhanced via a method called "createCrtKey" - this method was written by President James K. Polk and all credits go to him. As links sometimes date out I marked my own answer below as accepted one because the createCrtKey-method is documented there.
Thanks to #President James K. Polk, #Topaco and #michalk for guiding me into the right direction.

The minimal information needed to perform the RSA decrypt operation is the modulus n and the decrypt exponent d. There's an optimization that can be applied to RSA decryption involving the Chinese Remainder Theorem whereby exponentiations are done mod the RSA primes separately and then combined to produce a final value, and thus there are some extra fields fields for this purpose in the RSA Private Key syntax and the Java RSAPrivateCrtKey interface modeled after it.
Now the question being raised here is: When are two RSAPrivateCrtKey instances equal? I would argue that they are equal when they function identically in the RSA algorithm. You are asking for a more narrow definition, namely they are equal when their encoded forms are equal. The problem with this definition is that it is too implementation-specific. Currently, when the "Sun" provider generates a key pair it always orders the primes p and q such that p > q. But I like it the other way, where p < q. The RSAPrivateCrtKey interface does not care either way as it does no checking. The Javadocs for the interface do not specify an order. You can change my code to produce what should be the same encoded form as the current "Sun" implementation simply by reversing the comparison in p.compareTo(q) > 0. However, the default implementation can change to match my preference in the future, and it will if my plan to take over the world succeeds. The Javadocs are the specification, and the implementation may change as long as it complies with the Javadocs.
Below I have provided an implementation of an equality function in which I have tried to incorporate the widest possible notion of equality consistent with the specification. That is, any two RSAPrivateCRTKey instances for which keyEquals returns true should produce identical results when used in the RSA algorithm, and if false is returned then there should be at least one value for which they produce different results.
public static boolean keyEquals(RSAPrivateCrtKey k1, RSAPrivateCrtKey k2) {
final BigInteger ZERO = BigInteger.ZERO;
boolean result = true;
result = result && isConsistent(k1) && isConsistent(k2);
result = result && k1.getModulus().equals(k2.getModulus());
BigInteger lambda = computeCarmichaelLambda(k1.getPrimeP(), k1.getPrimeQ());
result = result && k1.getPublicExponent().subtract(k2.getPublicExponent()).mod(lambda).equals(ZERO);
result = result && k1.getPrivateExponent().subtract(k2.getPrivateExponent()).mod(lambda).equals(ZERO);
return result;
}
private static boolean isConsistent(RSAPrivateCrtKey k1) {
final BigInteger ZERO = BigInteger.ZERO;
final BigInteger ONE = BigInteger.ONE;
BigInteger n = k1.getModulus();
BigInteger p = k1.getPrimeP();
BigInteger q = k1.getPrimeQ();
BigInteger e = k1.getPublicExponent();
BigInteger d = k1.getPrivateExponent();
boolean result = true;
result = p.multiply(q).equals(n);
BigInteger lambda = computeCarmichaelLambda(p, q);
result = result && e.multiply(d).mod(lambda).equals(ONE);
result = result && d.subtract(key.getPrimeExponentP()).mod(p.subtract(ONE)).equals(ZERO);
result = result && d.subtract(key.getPrimeExponentQ()).mod(q.subtract(ONE)).equals(ZERO);
result = result && q.multiply(k1.getCrtCoefficient()).mod(p).equals(ONE);
return result;
}
private static BigInteger computeCarmichaelLambda(BigInteger p, BigInteger q) {
return lcm(p.subtract(BigInteger.ONE), q.subtract(BigInteger.ONE));
}
private static BigInteger lcm(BigInteger x, BigInteger y) {
return x.multiply(y).divide(x.gcd(y));
}

This is the modified version of my program that has the additional code from #President James K. Polk (see link of Topaco above). Even if the rebuild CRT-private key is now longer than the rebuild Private Key it does not match the original (encoded)
private key. As I'm using the encoded private and public keys for a PHP RSA encryption/decryption there is the funny fact that the original keys run successfully but the rebuild ones not...
This version uses a 512 bit keylength that is insecure is for demonstration only (to keep the keys shorter).
result:
Rebuilding of a RSA PrivateKey from modulus & exponent
privateKey equals rebuild: false
publicKey equals rebuild: true
privateKey original encoded: 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
privateKey rebuild encoded: 3081b2020100300d06092a864886f70d010101050004819d30819a020100024100a45477b9f00f51c8e1d5cb961a485c74ee123aa6da5c5bfd43f62acee9b684a8f140bb7a68996a77d04bdaabc5f259cb38a7bef909f4d85c6a597519a09aec9b020100024066ea4fa12f6b28b93a567f0e1e9fbae7b041d261b4d7aaf4ce9f58e8050ebdbd5e2a6261f06de2d72c4fdc6a62465f9cad9e8f5860bb2f8395cd903a214fb441020100020100020100020100020100
privateKey rebuild CRT encoded: 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
code:
import java.math.BigInteger;
import java.security.*;
import java.security.interfaces.RSAPrivateCrtKey;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.RSAPrivateCrtKeySpec;
import java.security.spec.RSAPrivateKeySpec;
import java.security.spec.RSAPublicKeySpec;
import java.util.Arrays;
public class RebuildRSAPrivateKey2 {
public static void main(String[] args) throws NoSuchAlgorithmException, InvalidKeySpecException {
System.out.println("Rebuilding of a RSA PrivateKey from modulus & exponent");
// rsa key generation
KeyPairGenerator kpGen = KeyPairGenerator.getInstance("RSA");
//kpGen.initialize(2048, new SecureRandom());
kpGen.initialize(512, new SecureRandom()); // don't use 512 bit keys as they are insecure !!
KeyPair keyPair = kpGen.generateKeyPair();
// private key
PrivateKey privateKey = keyPair.getPrivate();
// get modulus & exponent
RSAPrivateKey rsaPrivateKey = (RSAPrivateKey) privateKey;
BigInteger modulus = rsaPrivateKey.getModulus();
BigInteger privateExponent = rsaPrivateKey.getPrivateExponent();
// rebuild the private key
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
RSAPrivateKeySpec rsaPrivateKeySpec = new RSAPrivateKeySpec(modulus, privateExponent);
PrivateKey privateKeyRebuild = keyFactory.generatePrivate(rsaPrivateKeySpec);
System.out.println("privateKey equals rebuild: " + Arrays.equals(privateKey.getEncoded(), privateKeyRebuild.getEncoded()));
// public key
PublicKey publicKey = keyPair.getPublic();
// get modulus & exponent
RSAPublicKey rsaPublicKey = (RSAPublicKey) publicKey;
BigInteger modulusPub = rsaPublicKey.getModulus();
BigInteger publicExponent = rsaPublicKey.getPublicExponent();
// rebuild the public key
KeyFactory keyFactoryPub = KeyFactory.getInstance("RSA");
RSAPublicKeySpec rsaPublicKeySpec = new RSAPublicKeySpec(modulusPub, publicExponent);
PublicKey publicKeyRebuild = keyFactory.generatePublic(rsaPublicKeySpec);
System.out.println("publicKey equals rebuild: " + Arrays.equals(publicKey.getEncoded(), publicKeyRebuild.getEncoded()));
System.out.println("\nprivateKey original encoded: " + bytesToHex(privateKey.getEncoded()));
System.out.println("privateKey rebuild encoded: " + bytesToHex(privateKeyRebuild.getEncoded()));
RSAPrivateKey rsaPrivateKeyRebuild = (RSAPrivateKey) privateKeyRebuild;
RSAPublicKey rsaPublicKeyRebuild = (RSAPublicKey) publicKeyRebuild;
RSAPrivateCrtKey rsaPrivateCrtKey = createCrtKey(rsaPublicKeyRebuild, rsaPrivateKeyRebuild);
System.out.println("privateKey rebuild CRT encoded: " + bytesToHex(rsaPrivateCrtKey.getEncoded()));
}
/**
* https://stackoverflow.com/questions/43136036/how-to-get-a-rsaprivatecrtkey-from-a-rsaprivatekey
* answered Mar 31 '17 at 18:16 President James K. Polk
* Find a factor of n by following the algorithm outlined in Handbook of Applied Cryptography, section
* 8.2.2(i). See http://cacr.uwaterloo.ca/hac/about/chap8.pdf.
*
*/
private static BigInteger findFactor(BigInteger e, BigInteger d, BigInteger n) {
BigInteger edMinus1 = e.multiply(d).subtract(BigInteger.ONE);
int s = edMinus1.getLowestSetBit();
BigInteger t = edMinus1.shiftRight(s);
for (int aInt = 2; true; aInt++) {
BigInteger aPow = BigInteger.valueOf(aInt).modPow(t, n);
for (int i = 1; i <= s; i++) {
if (aPow.equals(BigInteger.ONE)) {
break;
}
if (aPow.equals(n.subtract(BigInteger.ONE))) {
break;
}
BigInteger aPowSquared = aPow.multiply(aPow).mod(n);
if (aPowSquared.equals(BigInteger.ONE)) {
return aPow.subtract(BigInteger.ONE).gcd(n);
}
aPow = aPowSquared;
}
}
}
public static RSAPrivateCrtKey createCrtKey(RSAPublicKey rsaPub, RSAPrivateKey rsaPriv) throws NoSuchAlgorithmException, InvalidKeySpecException {
BigInteger e = rsaPub.getPublicExponent();
BigInteger d = rsaPriv.getPrivateExponent();
BigInteger n = rsaPub.getModulus();
BigInteger p = findFactor(e, d, n);
BigInteger q = n.divide(p);
if (p.compareTo(q) > 0) {
BigInteger t = p;
p = q;
q = t;
}
BigInteger exp1 = d.mod(p.subtract(BigInteger.ONE));
BigInteger exp2 = d.mod(q.subtract(BigInteger.ONE));
BigInteger coeff = q.modInverse(p);
RSAPrivateCrtKeySpec keySpec = new RSAPrivateCrtKeySpec(n, e, d, p, q, exp1, exp2, coeff);
KeyFactory kf = KeyFactory.getInstance("RSA");
return (RSAPrivateCrtKey) kf.generatePrivate(keySpec);
}
private static String bytesToHex(byte[] bytes) {
StringBuffer result = new StringBuffer();
for (byte b : bytes) result.append(Integer.toString((b & 0xff) + 0x100, 16).substring(1));
return result.toString();
}
}

How to set padding for signed key encryption in java? [duplicate]

I am using rsa key to encrypt a long string which I will send to my server(will encrypt it with server's public key and my private key) But it throws an exception like javax.crypto.IllegalBlockSizeException: Data must not be longer than 256 bytes
I feel that I have not understood the working of rsa properly till now(using the inbuilt libraries are the cause for this). Can some one please explain why this exception is being thrown. Is it not at all possible to send long string encrypted?

The RSA algorithm can only encrypt data that has a maximum byte length
of the RSA key length in bits divided with eight minus eleven padding
bytes, i.e. number of maximum bytes = key length in bits / 8 - 11.
So basicly you divide the key length with 8 -11(if you have padding). For example if you have a 2048bit key you can encrypt 2048/8 = 256 bytes (- 11 bytes if you have padding). So, either use a larger key or you encrypt the data with a symmetric key, and encrypt that key with rsa (which is the recommended approach).
That will require you to:
generate a symmetric key
Encrypt the data with the symmetric key
Encrypt the symmetric key with rsa
send the encrypted key and the data
Decrypt the encrypted symmetric key with rsa
decrypt the data with the symmetric key
done :)

Based on #John Snow answer, I did an example
Generate Symmetric Key (AES with 128 bits)
KeyGenerator generator = KeyGenerator.getInstance("AES");
generator.init(128); // The AES key size in number of bits
SecretKey secKey = generator.generateKey();
Encrypt plain text using AES
String plainText = "Please encrypt me urgently..."
Cipher aesCipher = Cipher.getInstance("AES");
aesCipher.init(Cipher.ENCRYPT_MODE, secKey);
byte[] byteCipherText = aesCipher.doFinal(plainText.getBytes());
Encrypt the key using RSA public key
KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
kpg.initialize(2048);
KeyPair keyPair = kpg.generateKeyPair();
PublicKey puKey = keyPair.getPublic();
PrivateKey prKey = keyPair.getPrivate();
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.PUBLIC_KEY, puKey);
byte[] encryptedKey = cipher.doFinal(secKey.getEncoded()/*Seceret Key From Step 1*/);
Send encrypted data (byteCipherText) + encrypted AES Key (encryptedKey)
On the client side, decrypt symmetric key using RSA private key
cipher.init(Cipher.PRIVATE_KEY, prKey);
byte[] decryptedKey = cipher.doFinal(encryptedKey);
Decrypt the cipher text using decrypted symmetric key
//Convert bytes to AES SecertKey
SecretKey originalKey = new SecretKeySpec(decryptedKey , 0, decryptedKey .length, "AES");
Cipher aesCipher = Cipher.getInstance("AES");
aesCipher.init(Cipher.DECRYPT_MODE, originalKey);
byte[] bytePlainText = aesCipher.doFinal(byteCipherText);
String plainText = new String(bytePlainText);`

You should not use RSA on your secret data directly. You should only ever use RSA on pseudo-random or completely random data, such as session keys or message authentication codes.
You've gotten the problem at 256 bytes -- that is because you're probably working with 2048 bit keys. The keys are able to encrypt any integer in the range 0 to 2^2048 - 1 into the same range, and that means your data must be 256 bytes or smaller.
If you intend to encrypt more than this, please use one RSA encryption to encrypt a session key for a symmetric algorithm, and use that to encrypt your data.

To follow on from John Snow's answer above I created a simple random-symmetric-crypt library that you can use to simply encrypt any length data using a private key.
You can find the library at GitHub - random-symmetric-crypto
final RandomSymmetricCipher cipher = new RandomSymmetricCipher();
// Encrypt the data and the random symmetric key.
final CryptoPacket cryptoPacket = cipher.encrypt(inputData, PRIVATE_KEY_BASE64);
// Convert the CryptoPacket into a Base64 String that can be readily reconstituted at the other end.
final CryptoPacketConverter cryptoPacketConverter = new CryptoPacketConverter();
final String base64EncryptedData = cryptoPacketConverter.convert(cryptoPacket);
System.out.println("Base64EncryptedData=" + base64EncryptedData);
// Decrypt the Base64 encoded (and encrypted) String.
final byte[] outputData = cipher.decrypt(base64EncryptedData, PUBLIC_KEY_BASE64);

I went through the same problem, this is how I solved it.
AES can encrypt data as a standalone algorithm and can also do it with the help of RSA algorithm. Using AES standalone algorithm combined with RSA algorithm in the same block code(function) will cause increase in Data size affecting the AES key Size. You shouldn't do as shown with the code:
//encryption without using RSA KEY both can't run at the same time.
byte[] bytes = Files.readAllBytes(Paths.get(pvtKeyFile));
ks = new PKCS8EncodedKeySpec(bytes);
kf = KeyFactory.getInstance("RSA");
pvt = kf.generatePrivate(ks);
cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pvt);
processFiles(cipher, localFile, localFile + ".enc");
System.out.println("The encrypted files have been created successfully.");
//encryption using RSA.
byte[] bytes = Files.readAllBytes(Paths.get(pvtKeyFile));
ks = new PKCS8EncodedKeySpec(bytes);
kf = KeyFactory.getInstance("RSA");
pvt = kf.generatePrivate(ks);
kgen = KeyGenerator.getInstance("AES");
kgen.init(128);
skey = kgen.generateKey();
byte[] iv = new byte[128/8];
srandom.nextBytes(iv);
ivspec = new IvParameterSpec(iv);
try (FileOutputStream out = new FileOutputStream(localFile + ".enc")) {
{
cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pvt);
byte[] b = cipher.doFinal(skey.getEncoded());
out.write(b);
System.err.println("AES Key Length: " + b.length);
}
out.write(iv);
System.err.println("IV Length: " + iv.length);
ciphers = Cipher.getInstance("AES/CBC/PKCS5Padding");
ciphers.init(Cipher.ENCRYPT_MODE, skey, ivspec);
System.out.println("The encrypted files have been created successfully.");
try (FileInputStream in = new FileInputStream(localFile)) {
processFile(ciphers, in, out);
}
}
You can't do as shown above, it will cause error during the decryption process. if you are to use AES standalone algorithm use it in one block of code without including the RSA algorithm and the vice versa is true, as show below.
//encryption without using RSA KEY both can't run at the same time.
byte[] bytes = Files.readAllBytes(Paths.get(pvtKeyFile));
ks = new PKCS8EncodedKeySpec(bytes);
kf = KeyFactory.getInstance("RSA");
pvt = kf.generatePrivate(ks);
cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pvt);
processFiles(cipher, localFile, localFile + ".enc");
System.out.println("The encrypted files have been created successfully.");
OR
You can only use the RSA Algorithm as shown:
byte[] bytes = Files.readAllBytes(Paths.get(pvtKeyFile));
ks = new PKCS8EncodedKeySpec(bytes);
kf = KeyFactory.getInstance("RSA");
pvt = kf.generatePrivate(ks);
kgen = KeyGenerator.getInstance("AES");
kgen.init(128);
skey = kgen.generateKey();
byte[] iv = new byte[128/8];
srandom.nextBytes(iv);
ivspec = new IvParameterSpec(iv);
try (FileOutputStream out = new FileOutputStream(localFile + ".enc")) {
{
cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pvt);
byte[] b = cipher.doFinal(skey.getEncoded());
out.write(b);
System.err.println("AES Key Length: " + b.length);
}
out.write(iv);
System.err.println("IV Length: " + iv.length);
ciphers = Cipher.getInstance("AES/CBC/PKCS5Padding");
ciphers.init(Cipher.ENCRYPT_MODE, skey, ivspec);
System.out.println("The encrypted files have been created successfully.");
try (FileInputStream in = new FileInputStream(localFile)) {
processFile(ciphers, in, out);
}
}
Thanks I hope it will help someone regards.

you need split your data by the publicKey
int keyLength = publicKey.getModulus().bitLength() / 16;
String[] datas = splitString(data, keyLength - 11);
String mi = ""//the data after encrypted;
for (String s : datas) {
mi += bcd2Str(cipher.doFinal(s.getBytes()));
}
return mi;
public static String bcd2Str(byte[] bytes) {
char temp[] = new char[bytes.length * 2], val;
for (int i = 0; i < bytes.length; i++) {
val = (char) (((bytes[i] & 0xf0) >> 4) & 0x0f);
temp[i * 2] = (char) (val > 9 ? val + 'A' - 10 : val + '0');
val = (char) (bytes[i] & 0x0f);
temp[i * 2 + 1] = (char) (val > 9 ? val + 'A' - 10 : val + '0');
}
return new String(temp);
}

Getting different RSA encryption result in different language?

I am trying to encypt some data using RSA public key and signing with SHA-512 algo. But response recevied is differnet in different plat
form.
In C#:
RSACryptoServiceProvider crypto = new RSACryptoServiceProvider();
crypto.ImportCspBlob(Convert.FromBase64String(publickey));
crypto.exportParameters(false); // and got the public key modulus and exp
byte[] response = crypto.SignData(data, "SHA512");
In Java:
// got modulus and exp for public key from c#
byte[] modulo = {.....};
byte[] exp = {1,0,1};
BigInteger modulus = new BigInteger(1, modulo);
BigInteger pubExp = new BigInteger(1, exp);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
RSAPublicKeySpec priKeySpec = new RSAPublicKeySpec(modulus, pubExp);
RSAPublicKey Key = (RSAPublicKey)keyFactory.generatePublic(priKeySpec);
// Calculate Hash
MessageDigest sha1 = MessageDigest.getInstance("SHA-512");
byte[] digest = sha1.digest(data);
// Encrypt digest
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher.ENCRYPT_MODE, Key);
byte[] response = cipher.doFinal(digest);
but both response are not matching.C# generate correct one but java not generating the same byte[]
Any missing part in java code.

private static final String algorithm = "AES/CBC/NOPadding";
if we convert type 中國傳統的 languages then we get null value when we encryt.To overcome this problem we use below thing.
private static final String algorithm = "AES/CBC/PKCS5Padding";
if we do diifferent type languages like 中國傳統的 converion can be encryted.

getting a IllegalBlockSizeException: Data must not be longer than 256 bytes when using rsa

I am using rsa key to encrypt a long string which I will send to my server(will encrypt it with server's public key and my private key) But it throws an exception like javax.crypto.IllegalBlockSizeException: Data must not be longer than 256 bytes
I feel that I have not understood the working of rsa properly till now(using the inbuilt libraries are the cause for this). Can some one please explain why this exception is being thrown. Is it not at all possible to send long string encrypted?

The RSA algorithm can only encrypt data that has a maximum byte length
of the RSA key length in bits divided with eight minus eleven padding
bytes, i.e. number of maximum bytes = key length in bits / 8 - 11.
So basicly you divide the key length with 8 -11(if you have padding). For example if you have a 2048bit key you can encrypt 2048/8 = 256 bytes (- 11 bytes if you have padding). So, either use a larger key or you encrypt the data with a symmetric key, and encrypt that key with rsa (which is the recommended approach).
That will require you to:
generate a symmetric key
Encrypt the data with the symmetric key
Encrypt the symmetric key with rsa
send the encrypted key and the data
Decrypt the encrypted symmetric key with rsa
decrypt the data with the symmetric key
done :)

Based on #John Snow answer, I did an example
Generate Symmetric Key (AES with 128 bits)
KeyGenerator generator = KeyGenerator.getInstance("AES");
generator.init(128); // The AES key size in number of bits
SecretKey secKey = generator.generateKey();
Encrypt plain text using AES
String plainText = "Please encrypt me urgently..."
Cipher aesCipher = Cipher.getInstance("AES");
aesCipher.init(Cipher.ENCRYPT_MODE, secKey);
byte[] byteCipherText = aesCipher.doFinal(plainText.getBytes());
Encrypt the key using RSA public key
KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
kpg.initialize(2048);
KeyPair keyPair = kpg.generateKeyPair();
PublicKey puKey = keyPair.getPublic();
PrivateKey prKey = keyPair.getPrivate();
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.PUBLIC_KEY, puKey);
byte[] encryptedKey = cipher.doFinal(secKey.getEncoded()/*Seceret Key From Step 1*/);
Send encrypted data (byteCipherText) + encrypted AES Key (encryptedKey)
On the client side, decrypt symmetric key using RSA private key
cipher.init(Cipher.PRIVATE_KEY, prKey);
byte[] decryptedKey = cipher.doFinal(encryptedKey);
Decrypt the cipher text using decrypted symmetric key
//Convert bytes to AES SecertKey
SecretKey originalKey = new SecretKeySpec(decryptedKey , 0, decryptedKey .length, "AES");
Cipher aesCipher = Cipher.getInstance("AES");
aesCipher.init(Cipher.DECRYPT_MODE, originalKey);
byte[] bytePlainText = aesCipher.doFinal(byteCipherText);
String plainText = new String(bytePlainText);`

You should not use RSA on your secret data directly. You should only ever use RSA on pseudo-random or completely random data, such as session keys or message authentication codes.
You've gotten the problem at 256 bytes -- that is because you're probably working with 2048 bit keys. The keys are able to encrypt any integer in the range 0 to 2^2048 - 1 into the same range, and that means your data must be 256 bytes or smaller.
If you intend to encrypt more than this, please use one RSA encryption to encrypt a session key for a symmetric algorithm, and use that to encrypt your data.

To follow on from John Snow's answer above I created a simple random-symmetric-crypt library that you can use to simply encrypt any length data using a private key.
You can find the library at GitHub - random-symmetric-crypto
final RandomSymmetricCipher cipher = new RandomSymmetricCipher();
// Encrypt the data and the random symmetric key.
final CryptoPacket cryptoPacket = cipher.encrypt(inputData, PRIVATE_KEY_BASE64);
// Convert the CryptoPacket into a Base64 String that can be readily reconstituted at the other end.
final CryptoPacketConverter cryptoPacketConverter = new CryptoPacketConverter();
final String base64EncryptedData = cryptoPacketConverter.convert(cryptoPacket);
System.out.println("Base64EncryptedData=" + base64EncryptedData);
// Decrypt the Base64 encoded (and encrypted) String.
final byte[] outputData = cipher.decrypt(base64EncryptedData, PUBLIC_KEY_BASE64);

I went through the same problem, this is how I solved it.
AES can encrypt data as a standalone algorithm and can also do it with the help of RSA algorithm. Using AES standalone algorithm combined with RSA algorithm in the same block code(function) will cause increase in Data size affecting the AES key Size. You shouldn't do as shown with the code:
//encryption without using RSA KEY both can't run at the same time.
byte[] bytes = Files.readAllBytes(Paths.get(pvtKeyFile));
ks = new PKCS8EncodedKeySpec(bytes);
kf = KeyFactory.getInstance("RSA");
pvt = kf.generatePrivate(ks);
cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pvt);
processFiles(cipher, localFile, localFile + ".enc");
System.out.println("The encrypted files have been created successfully.");
//encryption using RSA.
byte[] bytes = Files.readAllBytes(Paths.get(pvtKeyFile));
ks = new PKCS8EncodedKeySpec(bytes);
kf = KeyFactory.getInstance("RSA");
pvt = kf.generatePrivate(ks);
kgen = KeyGenerator.getInstance("AES");
kgen.init(128);
skey = kgen.generateKey();
byte[] iv = new byte[128/8];
srandom.nextBytes(iv);
ivspec = new IvParameterSpec(iv);
try (FileOutputStream out = new FileOutputStream(localFile + ".enc")) {
{
cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pvt);
byte[] b = cipher.doFinal(skey.getEncoded());
out.write(b);
System.err.println("AES Key Length: " + b.length);
}
out.write(iv);
System.err.println("IV Length: " + iv.length);
ciphers = Cipher.getInstance("AES/CBC/PKCS5Padding");
ciphers.init(Cipher.ENCRYPT_MODE, skey, ivspec);
System.out.println("The encrypted files have been created successfully.");
try (FileInputStream in = new FileInputStream(localFile)) {
processFile(ciphers, in, out);
}
}
You can't do as shown above, it will cause error during the decryption process. if you are to use AES standalone algorithm use it in one block of code without including the RSA algorithm and the vice versa is true, as show below.
//encryption without using RSA KEY both can't run at the same time.
byte[] bytes = Files.readAllBytes(Paths.get(pvtKeyFile));
ks = new PKCS8EncodedKeySpec(bytes);
kf = KeyFactory.getInstance("RSA");
pvt = kf.generatePrivate(ks);
cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pvt);
processFiles(cipher, localFile, localFile + ".enc");
System.out.println("The encrypted files have been created successfully.");
OR
You can only use the RSA Algorithm as shown:
byte[] bytes = Files.readAllBytes(Paths.get(pvtKeyFile));
ks = new PKCS8EncodedKeySpec(bytes);
kf = KeyFactory.getInstance("RSA");
pvt = kf.generatePrivate(ks);
kgen = KeyGenerator.getInstance("AES");
kgen.init(128);
skey = kgen.generateKey();
byte[] iv = new byte[128/8];
srandom.nextBytes(iv);
ivspec = new IvParameterSpec(iv);
try (FileOutputStream out = new FileOutputStream(localFile + ".enc")) {
{
cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
cipher.init(Cipher.ENCRYPT_MODE, pvt);
byte[] b = cipher.doFinal(skey.getEncoded());
out.write(b);
System.err.println("AES Key Length: " + b.length);
}
out.write(iv);
System.err.println("IV Length: " + iv.length);
ciphers = Cipher.getInstance("AES/CBC/PKCS5Padding");
ciphers.init(Cipher.ENCRYPT_MODE, skey, ivspec);
System.out.println("The encrypted files have been created successfully.");
try (FileInputStream in = new FileInputStream(localFile)) {
processFile(ciphers, in, out);
}
}
Thanks I hope it will help someone regards.

you need split your data by the publicKey
int keyLength = publicKey.getModulus().bitLength() / 16;
String[] datas = splitString(data, keyLength - 11);
String mi = ""//the data after encrypted;
for (String s : datas) {
mi += bcd2Str(cipher.doFinal(s.getBytes()));
}
return mi;
public static String bcd2Str(byte[] bytes) {
char temp[] = new char[bytes.length * 2], val;
for (int i = 0; i < bytes.length; i++) {
val = (char) (((bytes[i] & 0xf0) >> 4) & 0x0f);
temp[i * 2] = (char) (val > 9 ? val + 'A' - 10 : val + '0');
val = (char) (bytes[i] & 0x0f);
temp[i * 2 + 1] = (char) (val > 9 ? val + 'A' - 10 : val + '0');
}
return new String(temp);
}