PKCS7-Padding

Implementation of PKCS7 padding in C. It can be used to pre-prepare data before block cipher using (for example, AES).

Algorithm description

PKCS7 is described in RFC 5652.

Some content-encryption algorithms assume the input length is a multiple of k octets, where k is greater than one. For such algorithms, the input shall be padded at the trailing end with k-(lth mod k) octets all having value k-(lth mod k), where lth is the length of the input. In other words, the input is padded at the trailing end with one of the following strings:

                 01 -- if lth mod k = k-1
              02 02 -- if lth mod k = k-2
                  .
                  .
                  .
        k k ... k k -- if lth mod k = 0

The padding can be removed unambiguously since all input is padded, including input values that are already a multiple of the block size, and no padding string is a suffix of another. This padding method is well defined if and only if k is less than 256.