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