What is the difference between PKCS#5 padding and PKCS#7 padding

The difference between the PKCS#5 and PKCS#7 padding mechanisms is the block size; PKCS#5 padding is defined for 8-byte block sizes, PKCS#7 padding would work for any block size from 1 to 255 bytes.

This is the definition of PKCS#5 padding (6.2) as defined in the RFC:

The padding string PS shall consist of 8 - (||M|| mod 8) octets all having value 8 - (||M|| mod 8).

The RFC that contains the PKCS#7 standard is the same except that it allows block sizes up to 255 bytes in size (10.3 note 2):

For such algorithms, the method shall be to pad the input at the trailing end with k - (l mod k) octets all having value k - (l mod k), where l is the length of the input.

So fundamentally PKCS#5 padding is a subset of PKCS#7 padding for 8 byte block sizes. Hence, PKCS#5 padding can not be used for AES. PKCS#5 padding was only defined with (triple) DES operation in mind.

Many cryptographic libraries use an identifier indicating PKCS#5 or PKCS#7 to define the same padding mechanism. The identifier should indicate PKCS#7 if block sizes other than 8 are used within the calculation. Some cryptographic libraries such as the SUN provider in Java indicate PKCS#5 where PKCS#7 should be used - "PKCS5Padding" should have been "PKCS7Padding". This is a legacy from the time that only 8 byte block ciphers such as (triple) DES symmetric cipher were available.

Note that neither PKCS#5 nor PKCS#7 is a standard created to describe a padding mechanism. The padding part is only a small subset of the defined functionality. PKCS#5 is a standard for Password Based Encryption or PBE, and PKCS#7 defines the Cryptographic Message Syntax or CMS.

在PKCS5Padding中，明确定义Block的大小是8位，而在PKCS7Padding定义中，对于块的大小是不确定的，可以在1-255之间（块长度超出255的尚待研究），填充值的算法都是一样的：

value=k - (l mod k)  ,K=块大小，l=数据长度，如果l=8, 则需要填充额外的8个byte的8

在.net中，例如TripleDESCryptoServiceProvider ，默认BlockSize=64bits=8bytes，所以在这种情况下在PKCS5Padding=PKCS7Padding。

如果在C#中自己定义了一个不是64bits的加密块大小，同时使用PKCS7Padding，那么在java中使用JDK标准的PKCS5Padding就不能解密了。

https://crypto.stackexchange.com/questions/9043/what-is-the-difference-between-pkcs5-padding-and-pkcs7-padding