104 lines
2.9 KiB
Go
104 lines
2.9 KiB
Go
// Copyright 2012 Aaron Jacobs. All Rights Reserved.
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// Author: aaronjjacobs@gmail.com (Aaron Jacobs)
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package siv
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import (
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"crypto/aes"
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"crypto/cipher"
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"crypto/subtle"
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"fmt"
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)
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// *NotAuthenticError is returned by Decrypt if the input is otherwise
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// well-formed but the ciphertext doesn't check out as authentic. This could be
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// due to an incorrect key, corrupted ciphertext, or incorrect/corrupted
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// associated data.
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type NotAuthenticError struct {
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s string
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}
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func (e *NotAuthenticError) Error() string {
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return e.s
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}
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// Given ciphertext previously generated by Encrypt and the key and associated
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// data that were used when generating the ciphertext, return the original
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// plaintext given to Encrypt. If the input is well-formed but the key is
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// incorrect, return an instance of WrongKeyError.
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func Decrypt(key, ciphertext []byte, associated [][]byte) ([]byte, error) {
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keyLen := len(key)
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associatedLen := len(associated)
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// The first 16 bytes of the ciphertext are the SIV.
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if len(ciphertext) < aes.BlockSize {
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return nil, fmt.Errorf("Invalid ciphertext; length must be at least 16.")
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}
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v := ciphertext[0:aes.BlockSize]
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c := ciphertext[aes.BlockSize:]
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// Make sure the key length is legal.
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switch keyLen {
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case 32, 48, 64:
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default:
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return nil, fmt.Errorf("SIV requires a 32-, 48-, or 64-byte key.")
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}
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// Derive subkeys.
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k1 := key[:keyLen/2]
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k2 := key[keyLen/2:]
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// Make sure the number of associated data is legal, per RFC 5297 section 7.
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if associatedLen > 126 {
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return nil, fmt.Errorf("len(associated) may be no more than 126.")
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}
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// Create a CTR cipher using a version of v with the 31st and 63rd bits
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// zeroed out.
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q := dup(v)
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q[aes.BlockSize-4] &= 0x7f
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q[aes.BlockSize-8] &= 0x7f
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ciph, err := aes.NewCipher(k2)
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if err != nil {
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return nil, fmt.Errorf("aes.NewCipher: %v", err)
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}
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ctrCiph := cipher.NewCTR(ciph, q)
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// Decrypt the ciphertext.
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plaintext := make([]byte, len(c))
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ctrCiph.XORKeyStream(plaintext, c)
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// Verify the SIV.
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s2vStrings := make([][]byte, associatedLen+1)
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copy(s2vStrings, associated)
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s2vStrings[associatedLen] = plaintext
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t := s2v(k1, s2vStrings, nil)
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if len(t) != aes.BlockSize {
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panic(fmt.Sprintf("Unexpected output of S2V: %v", t))
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}
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if subtle.ConstantTimeCompare(t, v) != 1 {
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return nil, &NotAuthenticError{
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"Couldn't validate the authenticity of the ciphertext and " +
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"associated data."}
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}
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return plaintext, nil
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}
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