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