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|
//! A simple order-preserving encryption function.
//!
//! This function is used to generate revision counters for onion service
//! descriptors. It is not suitable for other purposes.
//!
//! The scheme here is the one described in the specifications
//! as "Encrypted Time In Period".
//!
//! It is loosely based on the scheme first described in
//! G. Bebek. "Anti-tamper database research: Inference control
//! techniques."" Technical Report EECS 433 Final Report, Case
//! Western Reserve University, November 2002.
// NOTE:
//
// We use the same algorithm here as in C tor, not because it is a great
// algorithm, but because there has been a community of onion service operators
// who try to achieve load balancing by running multiple onion services with the
// same keys, and letting them "race" to publish at the HsDirs. This only
// works if all the onion service instances produce the same revision counters.
use cipher::{KeyIvInit as _, StreamCipher as _};
use digest::Digest as _;
use tor_llcrypto::{cipher::aes::Aes256Ctr, d::Sha3_256};
use zeroize::Zeroizing;
/// Key for a simple order-preserving encryption on the offset from the start of an SRV protocol
/// run.
///
/// The algorithm here is chosen to be the same as used in the C tor
/// implementation.
#[derive(Clone, Debug)]
pub struct AesOpeKey {
/// The key for our counter-mode cipher.
key: Zeroizing<[u8; 32]>,
}
/// A prefix used when deriving an AES key for this purpose.
const DERIVATION_PREFIX: &[u8] = b"rev-counter-generation";
impl AesOpeKey {
/// Construct a new [`AesOpeKey`] from a given secret.
///
/// The secret should be unpredictable by an adversary.
pub fn from_secret(secret: &[u8]) -> Self {
let mut h = Sha3_256::new();
h.update(DERIVATION_PREFIX);
h.update(secret);
let key = Zeroizing::new(h.finalize().into());
AesOpeKey { key }
}
/// Encrypt `offset` to a 64-bit number.
///
/// (We do not implement a decryption operation.)
///
/// # Limitations
///
/// Like all order-preserving encryption, this scheme leaks information by
/// its nature. It also leaks more information than necessary: (the
/// adversary can get a rough estimate for our input by dividing the output
/// by 0x8001). The only security property that this algorithm tries to
/// provide is that it prevents an adversary from inferring our clock skew.
///
/// This algorithm is also not very efficient in its implementation.
/// We expect that the result will become unacceptable if the time period is
/// ever larger than a few days.
pub fn encrypt(&self, offset: SrvPeriodOffset) -> u64 {
// We add "1" here per the spec, since the encryption of 0 is 0.
self.encrypt_inner(offset.0.saturating_add(1))
}
/// Implementation for the order-preserving encryption algorithm:
///
/// For security, requires that `n` is at least 1.
fn encrypt_inner(&self, n: u32) -> u64 {
let iv = [0; 16].into();
let mut ctr = Aes256Ctr::new((&*self.key).into(), &iv);
/// Number of u16s to create at once.
const BUF_LEN: usize = 8 * 1024;
/// Number of bytes in a u16
const STEP: usize = 2;
// We start our accumulator at `n` because we want every increase in the
// input to increase our output by at least 1, but it is otherwise
// possible for one of our randomly generated u16s to be 0x0000.
let mut result = u64::from(n);
let mut n = n as usize;
let mut buf = [0_u8; BUF_LEN * STEP];
while n >= BUF_LEN {
buf.fill(0);
ctr.apply_keystream(&mut buf[..]);
result += add_slice_as_le_u16(&buf[..]);
n -= BUF_LEN;
}
if n > 0 {
buf.fill(0);
ctr.apply_keystream(&mut buf[..n * STEP]);
result += add_slice_as_le_u16(&buf[..n * STEP]);
}
result
}
}
/// An opaque offset within an SRV period.
///
/// Used by onion services to compute a HsDesc revision counter.
#[derive(Copy, Clone, Debug, PartialEq, derive_more::From)]
pub struct SrvPeriodOffset(
// An offset, in seconds.
pub(crate) u32,
);
/// Treating `slice` as a sequence of little-endian 2-byte words,
/// add them into a u64.
///
/// # Panics
///
/// Panics if slice is not even in size.
fn add_slice_as_le_u16(slice: &[u8]) -> u64 {
assert_eq!(slice.len() % 2, 0);
let s = slice
.chunks_exact(2)
.map(|bytepair| {
let a: [u8; 2] = bytepair.try_into().expect("chunk was not of size 2!");
u64::from(u16::from_le_bytes(a))
})
.sum();
s
}
#[cfg(test)]
mod test {
use super::*;
use hex_literal::hex;
#[test]
fn add_slice() {
assert_eq!(6, add_slice_as_le_u16(&[1, 0, 2, 0, 3, 0]));
assert_eq!(0x600, add_slice_as_le_u16(&[0, 1, 0, 2, 0, 3]));
assert_eq!(
419477,
add_slice_as_le_u16(b"This is a string of moderate length!")
);
}
#[test]
fn test_vec() {
let key = hex!("19e05891d55232c08c2cad91d612fdb9cbd6691949a0742434a76c80bc6992fe");
let key = AesOpeKey { key: key.into() };
// Test vectors taken from C tor.
for (inp, expected) in [
(82283, 2695743564_u64),
(72661, 2381548866_u64),
(72941, 2390408421_u64),
(123122, 4036781069_u64),
(12154, 402067100_u64),
(121574, 3986197593_u64),
(11391, 376696838_u64),
(65845, 2161801517_u64),
(86301, 2828270975_u64),
(61284, 2013616892_u64),
(70505, 2313368870_u64),
(30438, 1001394664_u64),
(60150, 1977329668_u64),
(114800, 3764946628_u64),
(109403, 3585352477_u64),
(21893, 721388468_u64),
(123569, 4051780471_u64),
(95617, 3134921876_u64),
(48561, 1597596985_u64),
(53334, 1753691710_u64),
(92746, 3040874493_u64),
(7110, 234966492_u64),
(9612, 318326551_u64),
(106958, 3506124249_u64),
(46889, 1542219146_u64),
(87790, 2877361609_u64),
(68878, 2260369112_u64),
(47917, 1576681737_u64),
(121128, 3971553290_u64),
(108602, 3559176081_u64),
(28217, 929692460_u64),
(69498, 2280554161_u64),
(63870, 2098322675_u64),
(57542, 1891698992_u64),
(122148, 4004515805_u64),
(46254, 1521227949_u64),
(42850, 1408996941_u64),
(92661, 3037901517_u64),
(57720, 1897369989_u64),
] {
assert_eq!(key.encrypt_inner(inp), expected);
}
}
}
|
