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|
//! Memory protection and encryption.
//!
//! Sequoia makes an effort to protect secrets stored in memory. Even
//! though a process's memory should be protected from being read by an
//! adversary, there may be bugs in the program or the architecture
//! the program is running on that allow (partial) recovery of data.
//! Or, the process may be serialized to persistent storage, and its
//! memory may be inspected while it is not running.
//!
//! To reduce the window for these kind of exfiltrations, we use
//! [`Protected`] to clear the memory once it is no longer in use, and
//! [`Encrypted`] to protect long-term secrets like passwords and
//! secret keys.
//!
//!
//! Furthermore, operations involving secrets must be carried out in a
//! way that avoids leaking information. For example, comparison
//! must be done in constant time with [`secure_cmp`].
//!
//! [`secure_cmp`]: secure_cmp()
use std::cmp::{min, Ordering};
use std::fmt;
use std::hash::{Hash, Hasher};
use std::ops::{Deref, DerefMut};
/// Protected memory.
///
/// The memory is guaranteed not to be copied around, and is cleared
/// when the object is dropped.
///
/// # Examples
///
/// ```rust
/// use sequoia_openpgp::crypto::mem::Protected;
///
/// {
/// let p: Protected = vec![0, 1, 2].into();
/// assert_eq!(p.as_ref(), &[0, 1, 2]);
/// }
///
/// // p is cleared once it goes out of scope.
/// ```
// # Note on the implementation
//
// We use a boxed slice, then Box::leak the Box. This takes the
// knowledge about the shape of the heap allocation away from Rust,
// preventing any optimization based on that.
//
// For example, Rust could conceivably compact the heap: The borrow
// checker knows when no references exist, and this is an excellent
// opportunity to move the object on the heap because only one pointer
// needs to be updated.
pub struct Protected(*mut [u8]);
// Safety: Box<[u8]> is Send and Sync, we do not expose any
// functionality that was not possible before, hence Protected may
// still be Send and Sync.
unsafe impl Send for Protected {}
unsafe impl Sync for Protected {}
impl Clone for Protected {
fn clone(&self) -> Self {
// Make a vector with the correct size to avoid potential
// reallocations when turning it into a `Protected`.
let mut p = Vec::with_capacity(self.len());
p.extend_from_slice(self);
p.into_boxed_slice().into()
}
}
impl PartialEq for Protected {
fn eq(&self, other: &Self) -> bool {
secure_cmp(self, other) == Ordering::Equal
}
}
impl Eq for Protected {}
impl Hash for Protected {
fn hash<H: Hasher>(&self, state: &mut H) {
self.as_ref().hash(state);
}
}
impl Protected {
/// Allocates a chunk of protected memory.
///
/// Effective protection of sensitive values requires avoiding any
/// copying and reallocations. Therefore, it is required to
/// provide the size upfront at allocation time, then copying the
/// secrets into this protected memory region.
pub fn new(size: usize) -> Protected {
vec![0; size].into_boxed_slice().into()
}
/// Converts to a buffer for modification.
///
/// Don't expose `Protected` values unless you know what you're doing.
pub(crate) fn expose_into_unprotected_vec(self) -> Vec<u8> {
let mut p = Vec::with_capacity(self.len());
p.extend_from_slice(&self);
p
}
}
impl Deref for Protected {
type Target = [u8];
fn deref(&self) -> &Self::Target {
self.as_ref()
}
}
impl AsRef<[u8]> for Protected {
fn as_ref(&self) -> &[u8] {
unsafe { &*self.0 }
}
}
impl AsMut<[u8]> for Protected {
fn as_mut(&mut self) -> &mut [u8] {
unsafe { &mut *self.0 }
}
}
impl DerefMut for Protected {
fn deref_mut(&mut self) -> &mut [u8] {
self.as_mut()
}
}
impl From<Vec<u8>> for Protected {
fn from(mut v: Vec<u8>) -> Self {
// Make a careful copy of the data. We do this instead of
// reusing v's allocation so that our allocation has the exact
// size.
let p = Protected::from(&v[..]);
// Now clear the previous allocation. Just to be safe, we
// clear the whole allocation.
let capacity = v.capacity();
unsafe {
// Safety: New size is equal to the capacity, and we
// initialize all elements.
v.set_len(capacity);
memsec::memzero(v.as_mut_ptr(), capacity);
}
p
}
}
impl From<Box<[u8]>> for Protected {
fn from(v: Box<[u8]>) -> Self {
Protected(Box::leak(v))
}
}
impl From<&[u8]> for Protected {
fn from(v: &[u8]) -> Self {
let mut p = Protected::new(v.len());
// Very carefully copy the slice. The obvious
// `p.copy_from_slice(v);` indeed leaks secrets.
v.iter().zip(p.iter_mut()).for_each(|(f, t)| *t = *f);
p
}
}
impl Drop for Protected {
fn drop(&mut self) {
unsafe {
let len = self.len();
memsec::memzero(self.as_mut().as_mut_ptr(), len);
drop(Box::from_raw(self.0));
}
}
}
impl fmt::Debug for Protected {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
if cfg!(debug_assertions) {
write!(f, "{:?}", self.0)
} else {
f.write_str("[<Redacted>]")
}
}
}
/// Encrypted memory.
///
/// This type encrypts sensitive data, such as secret keys, in memory
/// while they are unused, and decrypts them on demand. This protects
/// against cross-protection-boundary readout via microarchitectural
/// flaws like Spectre or Meltdown, via attacks on physical layout
/// like Rowbleed, and even via coldboot attacks.
///
/// The key insight is that these kinds of attacks are imperfect,
/// i.e. the recovered data contains bitflips, or the attack only
/// provides a probability for any given bit. Applied to
/// cryptographic keys, these kind of imperfect attacks are enough to
/// recover the actual key.
///
/// This implementation on the other hand, derives a sealing key from
/// a large area of memory, the "pre-key", using a key derivation
/// function. Now, any single bitflip in the readout of the pre-key
/// will avalanche through all the bits in the sealing key, rendering
/// it unusable with no indication of where the error occurred.
///
/// This kind of protection was pioneered by OpenSSH. The commit
/// adding it can be found
/// [here](https://marc.info/?l=openbsd-cvs&m=156109087822676).
///
/// # Examples
///
/// ```rust
/// use sequoia_openpgp::crypto::mem::Encrypted;
///
/// let e = Encrypted::new(vec![0, 1, 2].into());
/// e.map(|p| {
/// // e is temporarily decrypted and made available to the closure.
/// assert_eq!(p.as_ref(), &[0, 1, 2]);
/// // p is cleared once the function returns.
/// });
/// ```
#[derive(Clone, Debug)]
pub struct Encrypted {
ciphertext: Protected,
salt: [u8; 32],
plaintext_len: usize,
}
assert_send_and_sync!(Encrypted);
impl PartialEq for Encrypted {
fn eq(&
|