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# PKCS#7

The rules for PKCS padding are very simple:

* Padding bytes are always added to the clear text before it is encrypted.
* Each padding byte has a value equal to the total number of padding bytes that are added. For example, if 6 padding bytes must be added, each of those bytes will have the value 0x06.
* The total number of padding bytes is at least one, and is the number that is required in order to bring the data length up to a multiple of the cipher algorithm block size.

AES, for example, has a cipher block size of 16 bytes. The total number of padding bytes added to the clear text will always be between 1 and 16.

| Value of clear text length | Number of bytes added | Value of each padding byte |
| -------------------------- | --------------------- | -------------------------- |
| 0                          | 16                    | 0x10                       |
| 1                          | 15                    | 0x0F                       |
| 2                          | 14                    | 0x0E                       |
| 3                          | 13                    | 0x0D                       |
| 4                          | 12                    | 0x0C                       |
| 5                          | 11                    | 0x0B                       |
| 6                          | 10                    | 0x0A                       |
| 7                          | 9                     | 0x09                       |
| 8                          | 8                     | 0x08                       |
| 9                          | 7                     | 0x07                       |
| 10                         | 6                     | 0x06                       |
| 11                         | 5                     | 0x05                       |
| 12                         | 4                     | 0x04                       |
| 13                         | 3                     | 0x03                       |
| 14                         | 2                     | 0x02                       |
| 15                         | 1                     | 0x01                       |