Why PKS5Padding alters 128 bytes string in Java

Question

I have implemented an AES (CBC) algorithm in Java crypto with PKS5Padding. I noticed that for a input string size of 128 bytes; size of the cyphered text is 144 bytes. Is this normal? For my understanding AES algo should not pad this string since it is a multiplication of 16.

Answer 1

PKCS#7/PKCS#5 padding is always applied. Check the Wikipedia page . If your 16 byte aligned data would end with a byte 01 and you would remove the padding, then you would have one less byte of plaintext. So for AES 1 to 16 bytes are added, and having a multiple of the block size is therefore the worst situation instead of the best.