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/* * Copyright (C) 2016 Leo Gaspard * * This program is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program. If not, see <http://www.gnu.org/licenses/>. */ /*! * hotboot * ======= * * `hotboot` allows to secure private data with a weak secret, using as a * protection access control of the system and defense especially designed against * cold-boot attacks. * * * Usage * ----- * * ``` * # fn get_secret_from_user() -> Vec<u8> { vec![1, 5, 32, 46, 152] } * # fn very_private_data() -> Vec<u8> { vec![9, 12, 42, 10, 43, 19, 140, 158] } * * let data: Vec<u8> = very_private_data(); * let secret: Vec<u8> = get_secret_from_user(); * let hidden = hotboot::hide(data, secret, 100000).unwrap(); * * // `data` and `secret` no longer exist in memory * // A cold boot attack is highly unlikely to succeed in retrieving `data`, even * // if `secret` is known * * let secret: Vec<u8> = get_secret_from_user(); * let recovered = hotboot::recover(hidden, secret).unwrap(); * // `recovered` is the same as `data` was * * # assert_eq!(recovered, very_private_data()); * ``` * * Threat model * ------------ * * The threat model is that of a physical attacker who attacks a * weak-secret-protected data using a cold boot attack. * * The special use case this was designed for is screenlockers: the screen * unlocking password may not be strong, but there are timing delays that make * brute-force impractical. The use of hotboot to protect the secure data in memory * with the unlocking password allows to also be secure against a cold boot attack. * * * Design * ------ * * During a cold boot attack, some bits get corrupted. The aim of hotboot is to * minimize the ratio of bits that have to get corrupted to make it impossible to * recover the private data. * * In order to do so, it encrypts the data with a random key, then encrypts the * random key with another random key, iterates a number of times, and then * encrypts the last random key with a key derived from the password. * * If any bit in this chain is corrupted, garbage will be found at the end, without * being able to know which bit caused the issue. * * The choice of cryptographic primitives is AES256-CTR for the encryption (with a * random 128-bits IV and a random 256-bits key, making for 384 bits that have not * to be corrupted to decrypt one step), and PBKDF2-SHA256 with 10000 iterations * for initial key derivation. * * * Troubleshooting * --------------- * * Be it a support request, a bug, a lack in documentation or anything else that * doesn't just work as expected, please report it as a [GitHub * issue](https://github.com/Ekleog/hotboot/issues/new). * * * History * ------- * * * 2020-04-12: 0.1.1 released, uses a newer openssl * * 2017-03-05: 0.1.0 released * * 2017-03-04: Project launch * * * License * ------- * * `hotboot` is licensed under GPLv3, please see the file called `LICENSE.md`. */ #![warn(missing_docs)] extern crate openssl; use openssl::error::ErrorStack; use openssl::hash::MessageDigest; use openssl::symm::Cipher; const SALT_SIZE: usize = 32; const PBKDF_ITERS: usize = 10000; // const HASH: MessageDigest = MessageDigest::sha256(); // const CIPHER: Cipher = Cipher::aes_256_ctr(); const KEY_SIZE: usize = 256 / 8; const IV_SIZE: usize = 16; /** * Opaque struct encapsulating data encrypted with a secret. * * See `hide` and `recover` for information on how to hide or recover data */ pub struct HiddenData { salt: Vec<u8>, blocks: Vec<EncryptedBlock>, } #[derive(Debug)] struct EncryptedBlock { iv: [u8; IV_SIZE], data: Vec<u8>, } /** * Error that can be raised by a failed `hide` or `recover` call. */ #[derive(Debug)] pub enum Error { /// Error occuring during encryption EncryptionError(ErrorStack), /// Error occuring during decryption DecryptionError(ErrorStack), /// Error occuring while trying to gather random bytes RandomBytesError(ErrorStack), /// Error occuring while trying to derive a key from the secret KeyDerivationError(ErrorStack), } /** * Cleans up an array */ fn cleanup(mut data: Vec<u8>) { for x in data.iter_mut() { let y = x as *mut u8; unsafe { std::ptr::write_volatile(y, 0) } } } /** * Encrypts data with key returns a block that can be decrypted with the key * * data and key will be erased at the end of the function */ fn encrypt_and_destroy_key(data: Vec<u8>, key: Vec<u8>) -> Result<EncryptedBlock, Error> { let mut iv = [0; IV_SIZE]; openssl::rand::rand_bytes(&mut iv).map_err(Error::RandomBytesError)?; // Encrypt let enc = openssl::symm::encrypt(/* CIPHER */ Cipher::aes_256_ctr(), &key, Some(&iv), &data) .map_err(Error::EncryptionError)?; // Clean up cleanup(data); cleanup(key); // Return Ok(EncryptedBlock { iv: iv, data: enc }) } /** * Encrypts data and returns both the key used and a block that can be decrypted with the key * * data will be erased at the end of the function */ fn encrypt_and_destroy(data: Vec<u8>) -> Result<(Vec<u8>, EncryptedBlock), Error> { // Generate the parameters let mut key = vec![0; KEY_SIZE]; openssl::rand::rand_bytes(&mut key).map_err(Error::RandomBytesError)?; let keyret = key.clone(); // Return Ok((keyret, encrypt_and_destroy_key(data, key)?)) } /** * Decrypts encrypted block data with the key key * * The key will be erased at the end of the function */ fn decrypt(data: EncryptedBlock, key: Vec<u8>) -> Result<Vec<u8>, Error> { let res = openssl::symm::decrypt(/* CIPHER */ Cipher::aes_256_ctr(), &key, Some(&data.iv), &data.data) .map_err(Error::DecryptionError)?; cleanup(key); cleanup(data.data); Ok(res) } /** * Derives a key from a secret and a salt * * The secret will be erased at the end of the function */ fn derive_key_salt(secret: Vec<u8>, salt: Vec<u8>) -> Result<Vec<u8>, Error> { // Generate key let mut key = vec![0; KEY_SIZE]; openssl::pkcs5::pbkdf2_hmac(&secret, &salt, PBKDF_ITERS, /* HASH */ MessageDigest::sha256(), &mut key) .map_err(Error::KeyDerivationError)?; // Clean up cleanup(secret); // Return Ok(key) } /** * Derives a key from a secret, and returns a couple (salt, key) * * The secret will be erased at the end of the function */ fn derive_key(secret: Vec<u8>) -> Result<(Vec<u8>, Vec<u8>), Error> { // Generate parameters let mut salt = vec![0; SALT_SIZE]; openssl::rand::rand_bytes(&mut salt).map_err(Error::RandomBytesError)?; let saltret = salt.clone(); // Return Ok((saltret, derive_key_salt(secret, salt)?)) } /** * Hides data so that it cannot be recovered by a cold boot attack without the secret secret. * * Please note both data and secret will be erased at the end of this function, so that it is hard * to forget cleaning them up. * * `iters` is the number of iterations to run, the number of bits that will be required uncorrupted * to recover the data with the secret is then approximately `384 * iters` */ pub fn hide(data: Vec<u8>, secret: Vec<u8>, iters: usize) -> Result<HiddenData, Error> { let mut blocks = Vec::new(); // Encrypt data with random key let (key, block) = encrypt_and_destroy(data)?; blocks.push(block); // Encrypt key with random key a number of times let mut oldkey = key; for _ in 0..iters { let (key, block) = encrypt_and_destroy(oldkey)?; blocks.push(block); oldkey = key; } // Encrypt last random key with the secret let (salt, key) = derive_key(secret)?; blocks.push(encrypt_and_destroy_key(oldkey, key)?); // Return Ok(HiddenData { salt: salt, blocks: blocks, }) } /** * Recovers data hidden with secret secret. * * Please note secret will be erased at the end of this function, so that it is hard to forget * cleaning it up. */ pub fn recover(mut data: HiddenData, secret: Vec<u8>) -> Result<Vec<u8>, Error> { // Decrypt random keys one by one let mut key = derive_key_salt(secret, data.salt)?; while let Some(data) = data.blocks.pop() { key = decrypt(data, key)?; } // Here, key is the last "key", ie. the stored data // Return Ok(key) } #[cfg(test)] mod tests { use ::*; #[test] fn it_works() { let secret1 = vec![0, 1, 2, 3]; let secret2 = secret1.clone(); let data1 = vec![4, 5, 6, 6]; let data2 = data1.clone(); assert_eq!(*recover(hide(data1, secret1, 100000).unwrap(), secret2).unwrap(), *data2); } }