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//! Implementation of the AES decryption for zip files.
//! This was implemented according to the [WinZip specification](
//! Note that using CRC with AES depends on the used encryption specification, AE-1 or AE-2.
//! If the file is marked as encrypted with AE-2 the CRC field is ignored, even if it isn't set to 0.
use crate::aes_ctr;
use crate::types::AesMode;
use constant_time_eq::constant_time_eq;
use hmac::{Hmac, Mac};
use sha1::Sha1;
use std::io::{self, Read};
/// The length of the password verifcation value in bytes
const PWD_VERIFY_LENGTH: usize = 2;
/// The length of the authentication code in bytes
const AUTH_CODE_LENGTH: usize = 10;
/// The number of iterations used with PBKDF2
const ITERATION_COUNT: u32 = 1000;
/// Create a AesCipher depending on the used `AesMode` and the given `key`.
/// # Panics
/// This panics if `key` doesn't have the correct size for the chosen aes mode.
fn cipher_from_mode(aes_mode: AesMode, key: &[u8]) -> Box<dyn aes_ctr::AesCipher> {
match aes_mode {
AesMode::Aes128 => Box::new(aes_ctr::AesCtrZipKeyStream::<aes_ctr::Aes128>::new(key))
as Box<dyn aes_ctr::AesCipher>,
AesMode::Aes192 => Box::new(aes_ctr::AesCtrZipKeyStream::<aes_ctr::Aes192>::new(key))
as Box<dyn aes_ctr::AesCipher>,
AesMode::Aes256 => Box::new(aes_ctr::AesCtrZipKeyStream::<aes_ctr::Aes256>::new(key))
as Box<dyn aes_ctr::AesCipher>,
// An aes encrypted file starts with a salt, whose length depends on the used aes mode
// followed by a 2 byte password verification value
// then the variable length encrypted data
// and lastly a 10 byte authentication code
pub struct AesReader<R> {
reader: R,
aes_mode: AesMode,
data_length: u64,
impl<R: Read> AesReader<R> {
pub fn new(reader: R, aes_mode: AesMode, compressed_size: u64) -> AesReader<R> {
let data_length = compressed_size
- (PWD_VERIFY_LENGTH + AUTH_CODE_LENGTH + aes_mode.salt_length()) as u64;
Self {
/// Read the AES header bytes and validate the password.
/// Even if the validation succeeds, there is still a 1 in 65536 chance that an incorrect
/// password was provided.
/// It isn't possible to check the authentication code in this step. This will be done after
/// reading and decrypting the file.
/// # Returns
/// If the password verification failed `Ok(None)` will be returned to match the validate
/// method of ZipCryptoReader.
pub fn validate(mut self, password: &[u8]) -> io::Result<Option<AesReaderValid<R>>> {
let salt_length = self.aes_mode.salt_length();
let key_length = self.aes_mode.key_length();
let mut salt = vec![0; salt_length];
self.reader.read_exact(&mut salt)?;
// next are 2 bytes used for password verification
let mut pwd_verification_value = vec![0; PWD_VERIFY_LENGTH];
self.reader.read_exact(&mut pwd_verification_value)?;
// derive a key from the password and salt
// the length depends on the aes key length
let derived_key_len = 2 * key_length + PWD_VERIFY_LENGTH;
let mut derived_key: Vec<u8> = vec![0; derived_key_len];
// use PBKDF2 with HMAC-Sha1 to derive the key
pbkdf2::pbkdf2::<Hmac<Sha1>>(password, &salt, ITERATION_COUNT, &mut derived_key);
let decrypt_key = &derived_key[0..key_length];
let hmac_key = &derived_key[key_length..key_length * 2];
let pwd_verify = &derived_key[derived_key_len - 2..];
// the last 2 bytes should equal the password verification value
if pwd_verification_value != pwd_verify {
// wrong password
return Ok(None);
let cipher = cipher_from_mode(self.aes_mode, decrypt_key);
let hmac = Hmac::<Sha1>::new_from_slice(hmac_key).unwrap();
Ok(Some(AesReaderValid {
reader: self.reader,
data_remaining: self.data_length,
finalized: false,
/// A reader for aes encrypted files, which has already passed the first password check.
/// There is a 1 in 65536 chance that an invalid password passes that check.
/// After the data has been read and decrypted an HMAC will be checked and provide a final means
/// to check if either the password is invalid or if the data has been changed.
pub struct AesReaderValid<R: Read> {
reader: R,
data_remaining: u64,
cipher: Box<dyn aes_ctr::AesCipher>,
hmac: Hmac<Sha1>,
finalized: bool,
impl<R: Read> Read for AesReaderValid<R> {
/// This implementation does not fulfill all requirements set in the trait documentation.
/// ```txt
/// "If an error is returned then it must be guaranteed that no bytes were read."
/// ```
/// Whether this applies to errors that occur while reading the encrypted data depends on the
/// underlying reader. If the error occurs while verifying the HMAC, the reader might become
/// practically unusable, since its position after the error is not known.
fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
if self.data_remaining == 0 {
return Ok(0);
// get the number of bytes to read, compare as u64 to make sure we can read more than
// 2^32 bytes even on 32 bit systems.
let bytes_to_read = self.data_remaining.min(buf.len() as u64) as usize;
let read = buf[0..bytes_to_read])?;
self.data_remaining -= read as u64;
// Update the hmac with the encrypted data
// decrypt the data
self.cipher.crypt_in_place(&mut buf[]);
// if there is no data left to read, check the integrity of the data
if self.data_remaining == 0 {
"Tried to use an already finalized HMAC. This is a bug!"
self.finalized = true;
// Zip uses HMAC-Sha1-80, which only uses the first half of the hash
// see
let mut read_auth_code = [0; AUTH_CODE_LENGTH];
self.reader.read_exact(&mut read_auth_code)?;
let computed_auth_code = &self.hmac.finalize_reset().into_bytes()[0..AUTH_CODE_LENGTH];
// use constant time comparison to mitigate timing attacks
if !constant_time_eq(computed_auth_code, &read_auth_code) {
return Err(
"Invalid authentication code, this could be due to an invalid password or errors in the data"
impl<R: Read> AesReaderValid<R> {
/// Consumes this decoder, returning the underlying reader.
pub fn into_inner(self) -> R {