Cryptography Tutorials - Herong's Tutorial Examples - Version 5.36, by Dr. Herong Yang
Cleartext Message Padding and Revised Block Size
This section discusses how to pad the cleartext message so that it can divided into blocks of the same size. The padding idea is borrowed from the PKCS5Padding schema.
Once the block size is determined, we need to find a schema to pad the cleartext message if it is not multiples of the block size.
Instead of developing a new padding schema, we can borrow the idea from the PKCS5Padding schema, which is defined as:
But this padding schema has a limitation: The maximum number of bytes we can pad is 256, because that is the maximum value we can present in a single byte.
So we have to modify our block size logic and padding schema to make it work:
Example 1:
Given the RSA key size: x = 16 Given the number of bytes in the cleartext message: y = 12 The block size should be: min(floor((16-1)/8),256) = 1 The number of padding bytes should be: 1 - 12 mod 1 = 1 The padding byte value should be: 1 mod 1 = 0 The padding byte sequence should be: 0x00
Example 2:
Given the RSA key size: x = 64 Given the number of bytes in the cleartext message: y = 12 The block size should be: min(floor((64-1)/8),256) = 7 The number of padding bytes should be: 7 - 12 mod 7 = 2 The padding byte value should be: 2 mod 7 = 0x02 The padding byte sequence should be: 0x0202
Example 3:
Given the RSA key size: x = 64 Given the number of bytes in the cleartext message: y = 21 The block size should be: min(floor((64-1)/8),256) = 7 The number of padding bytes should be: 7 - 21 mod 7 = 7 The padding byte value should be: 7 mod 7 = 0x00 The padding byte sequence should be: 0x00000000000000
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)