Padding is in whole bytes. The value of each added byte is the number of bytes that are added, i.e. N bytes, each of value N are added. The number of bytes added will depend on the block boundary to which the message needs to be extended.
The padding will be one of:
01 02 02 03 03 03 04 04 04 04 05 05 05 05 05 etc.
This padding method (as well as the previous two) is well-defined if and only if N is less than 256.
Example: In the following example the block size is 8 bytes and padding is required for 4 bytes
... | DD DD DD DD DD DD DD DD | DD DD DD DD 04 04 04 04 |
If the original data is a multiple of N bytes, then an extra block of bytes with value N is added.
PKCS#5 padding is identical to PKCS#7 padding, except that it has only been defined for block ciphers that use a 64-bit (8 byte) block size. In practice the two can be used interchangeably.
@老三的古代: 最简单的情况，如果加密操作的块大小为 64 位（8字节）时，PKCS#5 同 PKCS#7 填充结果一致。