Cryptography Tutorials - Herong's Tutorial Examples - Version 5.36, by Dr. Herong Yang
RsaKeyEncryption.java for RSA Encryption Operation
This section provides a tutorial example on how to implement RSA encryption operation using the java.math.BigInteger class. The important part of the implementation is to determine the cleartext block size, ciphertext block size, and the padding of the last block.
Finally, we are ready to implement the RSA public key encryption operation using the java.math.BigInteger class. Here is my initial draft:
/* RsaKeyEncryption.java
#- Copyright (c) 2013, HerongYang.com, All Rights Reserved.
*/
import java.math.BigInteger;
import java.io.*;
class RsaKeyEncryption {
private BigInteger n, e;
public static void main(String[] a) {
if (a.length<3) {
System.out.println("Usage:");
System.out.println("java RsaKeyEncryption key input output");
return;
}
String keyFile = a[0];
String input = a[1];
String output = a[2];
RsaKeyEncryption encryptor = new RsaKeyEncryption(keyFile);
encryptor.encrypt(input,output);
}
// Reading in RSA public key
RsaKeyEncryption(String input) {
try {
BufferedReader in = new BufferedReader(new FileReader(input));
String line = in.readLine();
while (line!=null) {
if (line.indexOf("Modulus: ")>=0) {
n = new BigInteger(line.substring(9));
}
if (line.indexOf("Public key: ")>=0) {
e = new BigInteger(line.substring(12));
}
line = in.readLine();
}
} catch (Exception ex) {
ex.printStackTrace();
}
System.out.println("--- Reading public key ---");
System.out.println("Modulus: "+n);
System.out.println("Key size: "+n.bitLength());
System.out.println("Public key: "+e);
}
// Encrypting original message
public void encrypt(String intput, String output) {
int keySize = n.bitLength(); // In bits
int clearTextSize = Math.min((keySize-1)/8,256); // In bytes
int cipherTextSize = 1 + (keySize-1)/8; // In bytes
System.out.println("Cleartext block size: "+clearTextSize);
System.out.println("Ciphertext block size: "+cipherTextSize);
try {
FileInputStream fis = new FileInputStream(intput);
FileOutputStream fos = new FileOutputStream(output);
byte[] clearTextBlock = new byte[clearTextSize];
byte[] cipherTextBlock = new byte[cipherTextSize];
long blocks = 0;
int dataSize = fis.read(clearTextBlock);
boolean isPadded = false;
// Reading input message
while (dataSize>0) {
blocks++;
if (dataSize<clearTextSize) {
padBytesBlock(clearTextBlock,dataSize);
isPadded = true;
}
BigInteger clearText = new BigInteger(1,clearTextBlock);
BigInteger cipherText = clearText.modPow(e,n);
byte[] cipherTextData = cipherText.toByteArray();
putBytesBlock(cipherTextBlock,cipherTextData);
fos.write(cipherTextBlock);
dataSize = fis.read(clearTextBlock);
}
// Adding a full padding block, if needed
if (!isPadded) {
blocks++;
padBytesBlock(clearTextBlock,0);
BigInteger clearText = new BigInteger(1,clearTextBlock);
BigInteger cipherText = clearText.modPow(e,n);
byte[] cipherTextData = cipherText.toByteArray();
putBytesBlock(cipherTextBlock,cipherTextData);
fos.write(cipherTextBlock);
}
fis.close();
fos.close();
System.out.println("Encryption block count: "+blocks);
} catch (Exception ex) {
ex.printStackTrace();
}
}
// Putting bytes data into a block
public static void putBytesBlock(byte[] block, byte[] data) {
int bSize = block.length;
int dSize = data.length;
int i = 0;
while (i<dSize && i<bSize) {
block[bSize-i-1] = data[dSize-i-1];
i++;
}
while (i<bSize) {
block[bSize-i-1] = (byte)0x00;
i++;
}
}
// Padding input message block
public static void padBytesBlock(byte[] block, int dataSize) {
int bSize = block.length;
int padSize = bSize - dataSize;
int padValue = padSize%bSize;
for (int i=0; i<padSize; i++) {
block[bSize-i-1] = (byte) padValue;
}
}
}
Some notes on RsaKeyEncryption.java:
Testing result is presented in the next tutorial.
Last update: 2013.
Table of Contents
Introduction to AES (Advanced Encryption Standard)
DES Algorithm - Illustrated with Java Programs
DES Algorithm Java Implementation
DES Algorithm - Java Implementation in JDK JCE
DES Encryption Operation Modes
PHP Implementation of DES - mcrypt
Blowfish - 8-Byte Block Cipher
Secret Key Generation and Management
Cipher - Secret Key Encryption and Decryption
►RSA Implementation using java.math.BigInteger Class
Generating Prime Number with BigInteger Class
Performance of Prime Number Generation
RSA Encryption Implementation using BigInteger Class
RsaKeyGenerator.java for RSA Key Generation
RSA Keys Generated by RsaKeyGenerator.java
RsaKeyValidator.java for RSA Key Validation
64-bit RSA Key Validated by RsaKeyValidator.java
Converting Byte Sequences to Positive Integers
Cleartext Block Size for RSA Encryption
Cleartext Message Padding and Revised Block Size
Ciphertext Block Size for RSA Encryption
►RsaKeyEncryption.java for RSA Encryption Operation
RsaKeyDecryption.java for RSA Decryption Operation
Testing RsaKeyEncryption.java with a 16-bit Key
Testing RsaKeyEncryption.java with a 64-bit Key
Testing RsaKeyEncryption.java with a 3072-bit Key
Introduction of DSA (Digital Signature Algorithm)
Java Default Implementation of DSA
Private key and Public Key Pair Generation
PKCS#8/X.509 Private/Public Encoding Standards
Cipher - Public Key Encryption and Decryption
OpenSSL Introduction and Installation
OpenSSL Generating and Managing RSA Keys
OpenSSL Generating and Signing CSR
OpenSSL Validating Certificate Path
"keytool" and "keystore" from JDK
"OpenSSL" Signing CSR Generated by "keytool"
Migrating Keys from "keystore" to "OpenSSL" Key Files
Certificate X.509 Standard and DER/PEM Formats
Migrating Keys from "OpenSSL" Key Files to "keystore"
Using Certificates in IE (Internet Explorer)