Can PKCS5Padding be in AES/GCM mode?

java algorithm aes padding aes-gcm
C.c picture C.c · Jul 6, 2015 · Viewed 10.2k times · Source

What's the padding mode for AES/GCM? I understood it can be NoPadding, as in ECB mode it can be PKCS5Padding, how about in GCM mode? in JCE interface, we need provide "algorithm/mode/padding" (Reference).

So I used the following code to get the instance and it works in JDK but failed in IBM SDK which says

cannot find provider for supporting AES/GCM/PKCS5Padding

 Cipher.getInstance("AES/GCM/PKCS5Padding");

What's real use case for padding?

Answer

Artjom B. picture Artjom B. · Jul 6, 2015

GCM is a streaming mode which means that the ciphertext is only as long as the plaintext (not including authentication tag). GCM doesn't require a padding. This means that the PKCS5Padding version is actually only a synonym for NoPadding for convenience during programming. Some providers don't have this strange mode.

The are cases where padding the plaintext makes sense. For example, you can hide the length of the actual plaintext by appending a random length PKCS5Padding.

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