AES Remove Block Padding - go

Question

I have a program that decrypts a ciphertext (which I don't control) using this example from the go docs: https://pkg.go.dev/crypto/cipher#NewCBCDecrypter
The problem is this part:
// If the original plaintext lengths are not a multiple of the block
// size, padding would have to be added when encrypting, which would be
// removed at this point. For an example, see
// https://tools.ietf.org/html/rfc5246#section-6.2.3.2
My original plaintext length is indeed not a multiple of the block size, so I need to remove the padding. How can I do this? If I do not remove the padding, I can not decompress the plaintext because of https://github.com/golang/go/issues/47809 which go maintainers made clear is very strict about gzip compliance on purpose. Currently my workaround is to invoke gunzip as a shell command which can successfully decompress the plaintext (ignoring the trailing padding).
I checked the ietf link but I found no examples (or at least no go examples)

Answer 1

It turns out the encrypter and decrypter have to agree on a padding scheme in advance. In my case, the encrypter was using the padding scheme described here: https://www.rfc-editor.org/rfc/rfc5652#section-6.3
In other words, the plaintext is padded with 0x01 if there is one byte of padding, 0x02 0x02 if there are 2 bytes of padding, 0x03 0x03 0x03, etc.
So for a toy example, assume block size is 4 bytes. Here are some example plaintexts:
plaintext ... plaintext with padding
01 01 03 03 03
01 02 01 02 02 02
01 02 03 01 02 03 01
01 02 03 04 01 02 03 04 04 04 04 04
Therefore, I was able to remove plaintext padding using:
func removePadding(pt []byte) []byte {
padLength := int(pt[len(pt)-1])
return pt[:len(pt)-padLength]
}
See also: https://crypto.stackexchange.com/a/2805