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package s3crypto
// Copyright 2017 Amazon.com, Inc. or its affiliates. All Rights Reserved.
//
// Portions Licensed under the MIT License. Copyright (c) 2016 Carl Jackson
import (
"bytes"
"crypto/subtle"
"github.com/aws/aws-sdk-go/aws/awserr"
)
const (
pkcs7MaxPaddingSize = 255
)
type pkcs7Padder struct {
blockSize int
}
// NewPKCS7Padder follows the RFC 2315: https://www.ietf.org/rfc/rfc2315.txt
// PKCS7 padding is subject to side-channel attacks and timing attacks. For
// the most secure data, use an authenticated crypto algorithm.
func NewPKCS7Padder(blockSize int) Padder {
return pkcs7Padder{blockSize}
}
var errPKCS7Padding = awserr.New("InvalidPadding", "invalid padding", nil)
// Pad will pad the data relative to how many bytes have been read.
// Pad follows the PKCS7 standard.
func (padder pkcs7Padder) Pad(buf []byte, n int) ([]byte, error) {
if padder.blockSize < 1 || padder.blockSize > pkcs7MaxPaddingSize {
return nil, awserr.New("InvalidBlockSize", "block size must be between 1 and 255", nil)
}
size := padder.blockSize - (n % padder.blockSize)
pad := bytes.Repeat([]byte{byte(size)}, size)
buf = append(buf, pad...)
return buf, nil
}
// Unpad will unpad the correct amount of bytes based off
// of the PKCS7 standard
func (padder pkcs7Padder) Unpad(buf []byte) ([]byte, error) {
if len(buf) == 0 {
return nil, errPKCS7Padding
}
// Here be dragons. We're attempting to check the padding in constant
// time. The only piece of information here which is public is len(buf).
// This code is modeled loosely after tls1_cbc_remove_padding from
// OpenSSL.
padLen := buf[len(buf)-1]
toCheck := pkcs7MaxPaddingSize
good := 1
if toCheck > len(buf) {
toCheck = len(buf)
}
for i := 0; i < toCheck; i++ {
b := buf[len(buf)-1-i]
outOfRange := subtle.ConstantTimeLessOrEq(int(padLen), i)
equal := subtle.ConstantTimeByteEq(padLen, b)
good &= subtle.ConstantTimeSelect(outOfRange, 1, equal)
}
good &= subtle.ConstantTimeLessOrEq(1, int(padLen))
good &= subtle.ConstantTimeLessOrEq(int(padLen), len(buf))
if good != 1 {
return nil, errPKCS7Padding
}
return buf[:len(buf)-int(padLen)], nil
}
func (padder pkcs7Padder) Name() string {
return "PKCS7Padding"
}
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