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// Copyright 2020 Google LLC
//
// 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 subtle
import (
"encoding/binary"
"errors"
"fmt"
"github.com/tink-crypto/tink-go/v2/tink"
)
// EncryptThenAuthenticate performs an encrypt-then-MAC operation on plaintext
// and associated data (ad). The MAC is computed over (ad ||
// ciphertext || size of ad). This implementation is based on
// http://tools.ietf.org/html/draft-mcgrew-aead-aes-cbc-hmac-sha2-05.
type EncryptThenAuthenticate struct {
indCPACipher INDCPACipher
mac tink.MAC
tagSize int
}
const (
minTagSizeInBytes = 10
)
// Assert that EncryptThenAuthenticate implements the AEAD interface.
var _ tink.AEAD = (*EncryptThenAuthenticate)(nil)
// uint64ToByte stores a uint64 to a slice of bytes in big endian format.
func uint64ToByte(n uint64) []byte {
buf := make([]byte, 8)
binary.BigEndian.PutUint64(buf, n)
return buf
}
// NewEncryptThenAuthenticate returns a new instance of EncryptThenAuthenticate.
func NewEncryptThenAuthenticate(indCPACipher INDCPACipher, mac tink.MAC, tagSize int) (*EncryptThenAuthenticate, error) {
if tagSize < minTagSizeInBytes {
return nil, fmt.Errorf("encrypt_then_authenticate: tag size too small")
}
return &EncryptThenAuthenticate{indCPACipher, mac, tagSize}, nil
}
// Encrypt encrypts plaintext with associatedData.
// The resulting ciphertext allows for checking authenticity and
// integrity of associatedData, but does not guarantee its secrecy.
//
// The plaintext is encrypted with an INDCPACipher, then MAC is computed over
// (associatedData || ciphertext || n) where n is associatedData's length
// in bits represented as a 64-bit bigendian unsigned integer. The final
// ciphertext format is (IND-CPA ciphertext || mac).
func (e *EncryptThenAuthenticate) Encrypt(plaintext, associatedData []byte) ([]byte, error) {
ciphertext, err := e.indCPACipher.Encrypt(plaintext)
if err != nil {
return nil, fmt.Errorf("encrypt_then_authenticate: %v", err)
}
adSizeInBits := uint64(len(associatedData)) * 8
adSizeInBitsEncoded := uint64ToByte(adSizeInBits)
toAuthData := make([]byte, 0, len(associatedData)+len(ciphertext)+len(adSizeInBitsEncoded))
toAuthData = append(toAuthData, associatedData...)
toAuthData = append(toAuthData, ciphertext...)
toAuthData = append(toAuthData, adSizeInBitsEncoded...)
tag, err := e.mac.ComputeMAC(toAuthData)
if err != nil {
return nil, fmt.Errorf("encrypt_then_authenticate: %v", err)
}
if len(tag) != e.tagSize {
return nil, errors.New("encrypt_then_authenticate: invalid tag size")
}
ciphertext = append(ciphertext, tag...)
return ciphertext, nil
}
// Decrypt decrypts ciphertext with associatedData.
func (e *EncryptThenAuthenticate) Decrypt(ciphertext, associatedData []byte) ([]byte, error) {
if len(ciphertext) < e.tagSize {
return nil, errors.New("ciphertext too short")
}
// payload contains everything except the tag.
payload := ciphertext[:len(ciphertext)-e.tagSize]
// Authenticate the following data:
// associatedData || payload || adSizeInBits
adSizeInBits := uint64(len(associatedData)) * 8
adSizeInBitsEncoded := uint64ToByte(adSizeInBits)
toAuthData := make([]byte, 0, len(associatedData)+len(payload)+len(adSizeInBitsEncoded))
toAuthData = append(toAuthData, associatedData...)
toAuthData = append(toAuthData, payload...)
toAuthData = append(toAuthData, adSizeInBitsEncoded...)
err := e.mac.VerifyMAC(ciphertext[len(ciphertext)-e.tagSize:], toAuthData)
if err != nil {
return nil, fmt.Errorf("encrypt_then_authenticate: %v", err)
}
plaintext, err := e.indCPACipher.Decrypt(payload)
if err != nil {
return nil, fmt.Errorf("encrypt_then_authenticate: %v", err)
}
return plaintext, nil
}
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