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| // pkcs7strip remove pkcs7 padding | |
| func pkcs7strip(data []byte, blockSize int) ([]byte, error) { | |
| length := len(data) | |
| if length == 0 { | |
| return nil, errors.New("pkcs7: Data is empty") | |
| } | |
| if length%blockSize != 0 { | |
| return nil, errors.New("pkcs7: Data is not block-aligned") | |
| } | |
| padLen := int(data[length-1]) | |
| ref := bytes.Repeat([]byte{byte(padLen)}, padLen) | |
| if padLen > blockSize || padLen == 0 || !bytes.HasSuffix(data, ref) { | |
| return nil, errors.New("pkcs7: Invalid padding") | |
| } | |
| return data[:length-padLen], nil | |
| } | |
| // pkcs7pad add pkcs7 padding | |
| func pkcs7pad(data []byte, blockSize int) ([]byte, error) { | |
| if blockSize <= 1 || blockSize >= 256 { | |
| return nil, fmt.Errorf("pkcs7: Invalid block size %d", blockSize) | |
| } else { | |
| padLen := blockSize - len(data) % blockSize | |
| padding := bytes.Repeat([]byte{byte(padLen)}, padLen) | |
| return append(data, padding...), nil | |
| } | |
| } |
What's the meaning of the
16on line 23? Shouldn't it be equal toblockSize?
Opps, I was using it only for AES.
You are right, thanks for notification. I have altered it to blockSize.
Cool. It helped me.
Nice, thanks
This has a subtle off by one bug in line 20. The block size must fit in a byte; the condition should be >= 256 (also, zero is invalid)
There is a bug at line 23.
It should be
padLen := (blockSize - len(data)) % blockSize
There is a bug at line 23.
It should be
padLen := (blockSize - len(data)) % blockSize
I failed to see the reason. Why?
There is a bug at line 23.
It should be
padLen := (blockSize - len(data)) % blockSize
The original code is correct. If the data is an integral number of blocks there should be a full block of padding. It’s necessary in order for the last byte to make sense.
So current implementation is correct, yes?
padLen := blockSize - len(data) % blockSizeIt is effectively the same as:
padLen := blockSize - (len(data) % blockSize)if we use
padLen := (blockSize - len(data)) % blockSizelen(data) could be greater than blockSize and produces minus result.
https://www.rfc-editor.org/rfc/rfc2315
There is an error in line 20 of the code, as described on page 21 of this link.
Kaliski Informational [Page 21]
[RFC 2315](https://www.rfc-editor.org/rfc/rfc2315) PKCS #7: Crytographic Message Syntax March 1998
2. Some content-encryption algorithms assume the
input length is a multiple of k octets, where k > 1, and
let the application define a method for handling inputs
whose lengths are not a multiple of k octets. 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. In other
words, the input is padded at the trailing end with one of
the following strings:
01 -- if l mod k = k-1
02 02 -- if l mod k = k-2
.
.
.
k k ... k k -- if l mod k = 0
The padding can be removed unambiguously since all input is
padded and no padding string is a suffix of another. This
padding method is well-defined if and only if k < 256;
methods for larger k are an open issue for further study.
https://www.rfc-editor.org/rfc/rfc2315
There is an error in line 20 of the code, as described on page 21 of this link.
Hi @aimuz , thanks for your feedback. What change should be made? Is it this?
if blockSize <= 1 || blockSize >= 256 {
Yes, I think that should be the case
Yes, I think that should be the case
Thanks, I have updated the gist per your suggestion.
What's the meaning of the
16on line 23? Shouldn't it be equal toblockSize?