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// Copyright 2024 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 aead
import (
"crypto/aes"
"crypto/cipher"
"fmt"
"github.com/tink-crypto/tink-go/v2/subtle/random"
)
const (
aesCTRMinIVSize = 12
)
// AESCTR is an implementation of IndCpa Interface.
type AESCTR struct {
key []byte
ivSize int
}
// NewAESCTR returns an instance of [AESCTR] unauthenticated encryption.
func NewAESCTR(key []byte, ivSize int) (*AESCTR, error) {
keySize := uint32(len(key))
if err := ValidateAESKeySize(keySize); err != nil {
return nil, fmt.Errorf("aes_ctr: %s", err)
}
if ivSize < aesCTRMinIVSize || ivSize > aes.BlockSize {
return nil, fmt.Errorf("aes_ctr: invalid IV size: %d", ivSize)
}
return &AESCTR{key: key, ivSize: ivSize}, nil
}
// Encrypt encrypts plaintext using AES in CTR mode.
func (a *AESCTR) Encrypt(dst, plaintext []byte) ([]byte, error) {
if len(plaintext) > maxInt-a.ivSize {
return nil, fmt.Errorf("aes_ctr: plaintext too long")
}
ctSize := len(plaintext) + a.ivSize
if len(dst) == 0 {
dst = make([]byte, ctSize)
}
if len(dst) < ctSize {
return nil, fmt.Errorf("aes_ctr: destination buffer too small (%d vs %d)", len(dst), ctSize)
}
iv := random.GetRandomBytes(uint32(a.ivSize))
stream, err := newCipher(a.key, iv)
if err != nil {
return nil, err
}
if n := copy(dst, iv); n != a.ivSize {
return nil, fmt.Errorf("aes_ctr: failed to copy IV (copied %d/%d bytes)", n, a.ivSize)
}
stream.XORKeyStream(dst[a.ivSize:], plaintext)
return dst, nil
}
// Decrypt decrypts ciphertext in the format (prefix || iv || ciphertext).
func (a *AESCTR) Decrypt(dst, ciphertext []byte) ([]byte, error) {
if len(ciphertext) < a.ivSize {
return nil, fmt.Errorf("aes_ctr: ciphertext too short")
}
ptSize := len(ciphertext) - a.ivSize
if len(dst) == 0 {
dst = make([]byte, ptSize)
}
if len(dst) < ptSize {
return nil, fmt.Errorf("aes_ctr: destination buffer too small (%d vs %d)", len(dst), ptSize)
}
stream, err := newCipher(a.key, ciphertext[:a.ivSize])
if err != nil {
return nil, err
}
stream.XORKeyStream(dst, ciphertext[a.ivSize:])
return dst, nil
}
func newCipher(key, iv []byte) (cipher.Stream, error) {
block, err := aes.NewCipher(key)
if err != nil {
return nil, fmt.Errorf("aes_ctr: failed to create block cipher, error: %v", err)
}
// If the IV is less than BlockSize bytes we need to pad it with zeros
// otherwise NewCTR will panic.
if len(iv) < aes.BlockSize {
paddedIV := make([]byte, aes.BlockSize)
if n := copy(paddedIV, iv); n != len(iv) {
return nil, fmt.Errorf("aes_ctr: failed to pad IV")
}
iv = paddedIV
}
return cipher.NewCTR(block, iv), nil
}
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