What Is PKCS5Padding?

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∟DES Algorithm - Java Implementation in JDK JCE

∟What Is PKCS5Padding?

This section describes what is PKCS5Padding - a schema to pad cleartext to be multiples of 8-byte blocks.

As you can see from previous tutorials, DES algorithm requires that the input data to be 8-byte blocks. If you want to encrypt a text message that is not multiples of 8-byte blocks, the text message must be padded with additional bytes to make the text message to be multiples of 8-byte blocks.

PKCS5Padding is a padding scheme described in: RSA Laboratories, "PKCS #5: Password-Based Encryption Standard," version 1.5, November 1993.

PKCS5Padding schema is actually very simple. It follows the following rules:

The number of bytes to be padded equals to "8 - numberOfBytes(clearText) mod 8". So 1 to 8 bytes will be padded to the clear text data depending on the length of the clear text data.
All padded bytes have the same value - the number of bytes padded.

PKCS5Padding schema can also be explained with the diagram below, if M is the original clear text and PM is the padded clear text:

If numberOfBytes(clearText) mod 8 == 7, PM = M + 0x01
If numberOfBytes(clearText) mod 8 == 6, PM = M + 0x0202
If numberOfBytes(clearText) mod 8 == 5, PM = M + 0x030303
...
If numberOfBytes(clearText) mod 8 == 0, PM = M + 0x0808080808080808