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//! Padding for OpenPGP messages.
//!
//! To reduce the amount of information leaked via the message length,
//! encrypted OpenPGP messages (see [Section 11.3 of RFC 4880]) should
//! be padded.
//!
//! [Section 11.3 of RFC 4880]: https://tools.ietf.org/html/rfc4880#section-11.3
//!
//! To pad a message using the streaming serialization interface, the
//! [`Padder`] needs to be inserted into the writing stack between the
//! [`Encryptor`] and [`Signer`]. This is illustrated in this
//! [example].
//!
//! [`Encryptor`]: super::Encryptor
//! [`Signer`]: super::Signer
//! [example]: Padder#examples
//!
//! # Padding in OpenPGP
//!
//! There are a number of ways to pad messages within the boundaries
//! of the OpenPGP protocol, keeping an eye on backwards-compatibility
//! with common implementations:
//!
//! - Add a decoy notation to a signature packet (up to about 60k)
//!
//! - Add a signature with a private algorithm and store the decoy
//! traffic in the MPIs (up to 4 GB)
//!
//! - Use a compression container and store the decoy traffic in a
//! chunk that decompresses to the empty string (unlimited)
//!
//! - Use a bunch of marker packets, which are ignored (each packet:
//! 3 bytes for the body, 5 bytes for the header)
//!
//! - Apparently, GnuPG understands a comment packet (tag: 61),
//! which is not standardized (up to 64k)
//!
//! We believe that padding the compressed data stream is the best
//! option, because as far as OpenPGP is concerned, it is completely
//! transparent for the recipient (for example, no weird packets are
//! inserted).
//!
//! Unfortunately, [testing] discovered problems when the resulting
//! messages are consumed by (at the time of this writing) OpenPGP.js,
//! RNP, and GnuPG. If compatibility with these implementations is a
//! concern, using this padding method is not advisable.
//!
//! [testing]: https://tests.sequoia-pgp.org/#Packet_excess_consumption
//!
//! To be effective, the padding layer must be placed inside the
//! encryption container. To increase compatibility, the padding
//! layer must not be signed. That is to say, the message structure
//! should be `(encryption (padding ops literal signature))`, the
//! exact structure GnuPG emits by default.
use std::fmt;
use std::io::{self, Write};
use crate::{
Result,
packet::prelude::*,
};
use crate::packet::header::CTB;
use crate::serialize::{
Marshal,
stream::{
writer,
Cookie,
Message,
PartialBodyFilter,
},
};
use crate::types::{
CompressionAlgorithm,
CompressionLevel,
};
/// Pads a packet stream.
///
/// Writes a compressed data packet containing all packets written to
/// this writer, and pads it according to the given policy.
///
/// The policy is a `Fn(u64) -> u64`, that given the number of bytes
/// written to this writer `N`, computes the size the compression
/// container should be padded up to. It is an error to return a
/// number that is smaller than `N`.
///
/// # Compatibility
///
/// This implementation uses the [DEFLATE] compression format. The
/// packet structure contains a flag signaling the end of the stream
/// (see [Section 3.2.3 of RFC 1951]), and any data appended after
/// that is not part of the stream.
///
/// [DEFLATE]: https://tools.ietf.org/html/rfc1951
/// [Section 3.2.3 of RFC 1951]: https://tools.ietf.org/html/rfc1951#page-9
///
/// [Section 9.3 of RFC 4880] recommends that this algorithm should be
/// implemented, therefore support across various implementations
/// should be good.
///
/// [Section 9.3 of RFC 4880]: https://tools.ietf.org/html/rfc4880#section-9.3
///
/// # Examples
///
/// This example illustrates the use of `Padder` with the [Padmé]
/// policy. Note that for brevity, the encryption and signature
/// filters are omitted.
///
/// [Padmé]: padme()
///
/// ```
/// use std::io::Write;
/// use sequoia_openpgp as openpgp;
/// use openpgp::serialize::stream::{Message, LiteralWriter};
/// use openpgp::serialize::stream::padding::Padder;
/// use openpgp::types::CompressionAlgorithm;
/// # fn main() -> sequoia_openpgp::Result<()> {
///
/// let mut unpadded = vec![];
/// {
/// let message = Message::new(&mut unpadded);
/// // XXX: Insert Encryptor here.
/// // XXX: Insert Signer here.
/// let mut message = LiteralWriter::new(message).build()?;
/// message.write_all(b"Hello world.")?;
/// message.finalize()?;
/// }
///
/// let mut padded = vec![];
/// {
/// let message = Message::new(&mut padded);
/// // XXX: Insert Encryptor here.
/// let message = Padder::new(message).build()?;
/// // XXX: Insert Signer here.
/// let mut message = LiteralWriter::new(message).build()?;
/// message.write_all(b"Hello world.")?;
/// message.finalize()?;
/// }
/// assert!(unpadded.len() < padded.len());
/// # Ok(())
/// # }
pub struct Padder<'a> {
inner: writer::BoxStack<'a, Cookie>,
policy: fn(u64) -> u64,
}
assert_send_and_sync!(Padder<'_>);
impl<'a> Padder<'a> {
/// Creates a new padder with the given policy.
///
/// # Examples
///
/// This example illustrates the use of `Padder` with the [Padmé]
/// policy.
///
/// [Padmé]: padme()
///
/// The most useful filter to push to the writer stack next is the
/// [`Signer`] or the [`LiteralWriter`]. Finally, literal data
/// *must* be wrapped using the [`LiteralWriter`].
///
/// [`Signer`]: super::Signer
/// [`LiteralWriter`]: super::LiteralWriter
///
/// ```
/// # fn main() -> sequoia_openpgp::Result<()> {
/// use sequoia_openpgp as openpgp;
/// use openpgp::serialize::stream::padding::Padder;
///
/// # let message = openpgp::serialize::stream::Message::new(vec![]);
/// let message = Padder::new(message).build()?;
/// // Optionally add a `Signer` here.
/// // Add a `LiteralWriter` here.
/// # let _ = message;
/// # Ok(()) }
/// ```
pub fn new(inner: Message<'a>) -> Self {
Self {
inner: writer::BoxStack::from(inner),
policy: padme,
}
}
/// Sets padding policy, returning the padder.
///
/// # Examples
///
/// This example illustrates the use of `Padder` with an explicit policy.
///
/// ```
/// # fn main() -> sequoia_openpgp::Result<()> {
/// use sequoia_openpgp as openpgp;
/// use openpgp::serialize::stream::padding::{Padder, padme};
///
/// # let message = openpgp::serialize::stream::Message::new(vec![]);
/// let message = Padder::new(message).with_policy(padme).build()?;
/// // Optionally add a `Signer` here.
/// // Add a `LiteralWriter` here.
/// # let _ = message;
/// # Ok(()) }
/// ```
pub fn with_policy(mut self, p: fn(u64) -> u64) -> Self {
self.policy = p;
self
}
/// Builds the padder, returning the writer stack.
///
/// # Examples
///
/// This example illustrates the use of `Padder` with the [Padmé]
/// policy.
///
/// [Padmé]: padme()
///
/// The most useful filter to push to the writer stack next is the
/// [`Signer`] or the [`LiteralWriter`]. Finally, literal data
/// *must* be wrapped using the [`LiteralWriter`].
///
/// [`Signer`]: super::Signer
/// [`LiteralWriter`]: super::LiteralWriter
///
/// ```
/// # fn main() -> sequoia_openpgp::Result<()> {
/// use sequoia_openpgp as openpgp;
/// use openpgp::serialize::stream::padding::Padder;
///
/// # let message = openpgp::serialize::stream::Message::new(vec![]);
/// let message = Padder::new(message).build()?;
/// // Optionally add a `Signer` here.
/// // Add a `LiteralWriter` here.
/// # let _ = message;
/// # Ok(()) }
/// ```
pub fn build(mut self) -> Result<Message<'a>> {
let mut inner = self.inner;
let level = inner.cookie_ref().level + 1;
// Packet header.
CTB::new(Tag::CompressedData).serialize(&mut inner)?;
let mut inner: Message<'a>
= PartialBodyFilter::new(Message::from(inner),
Cookie::new(level));
// Compressed data header.
inner.as_mut().write_u8(CompressionAlgorithm::Zip.into())?;
// Create an appropriate filter.
self.inner =
writer::ZIP::new(inner, Cookie::new(level),
CompressionLevel::none()).into();
Ok(Message::from(Box::new(self)))
}
}
impl<'a> fmt::Debug for Padder<'a> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("Padder")
.field("inner", &self.inner)
.finish()
}
}
impl<'a> io::Write for Padder<'a> {
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
self.inner.write(buf)
}
fn flush(&mut self) -> io::Result<()> {
self.inner.flush()
}
}
impl<'a> writer::Stackable<'a, Cookie> for Padder<'a>
{
fn into_inner(self: Box<Self>)
-> Result<Option<writer::BoxStack<'a, Cookie>>> {
// Make a note of the amount of data written to this filter.
let uncompressed_size = self.position();
// Pop-off us and the compression filter, leaving only our
// partial body encoder on the stack. This finalizes the
// compression.
let mut pb_writer = Box::new(self.inner).into_inner()?.unwrap();
// Compressed size is what we've actually written out, modulo
// partial body encoding.
let compressed_size = pb_writer.position();
// Sometimes, the compression step expands the data. Handle
// this by padding the maximum of both sizes.
let size = std::cmp::max(uncompressed_size, compressed_size);
// Compute the amount of padding required according to the
// given policy.
let padded_size = (self.policy)(size);
if padded_size < size {
return Err(crate::Error::InvalidOperation(
format!("Padding policy({}) returned {}: smaller than argument",
size, padded_size)).into());
}
let mut amount = padded_size - compressed_size;
if false {
eprintln!("u: {}, c: {}, amount: {}",
uncompressed_size, compressed_size, amount);
}
// Write 'amount' of padding.
const BUFFER_SIZE: usize = 4096;
let mut padding = vec![0; BUFFER_SIZE];
while amount > 0 {
let n = std::cmp::min(BUFFER_SIZE as u64, amount) as usize;
crate::crypto::random(&mut padding[..n]);
pb_writer.write_all(&padding[..n])?;
amount -= n as u64;
}
pb_writer.into_inner()
}
fn pop(&mut self) -> Result<Option<writer::BoxStack<'a, Cookie>>> {
unreachable!("Only implemented by Signer")
}
/// Sets the inner stackable.
fn mount(&mut self, _new: writer::BoxStack<'a, Cookie>) {
unreachable!("Only implemented by Signer")
}
fn inner_ref(&self) -> Option<&(dyn writer::Stackable<'a, Cookie> + Send + Sync)> {
Some(self.inner.as_ref())
}
fn inner_mut(&mut self) -> Option<&mut (dyn writer::Stackable<'a, Cookie> + Send + Sync)> {
Some(self.inner.as_mut())
}
fn cookie_set(&mut self, cookie: Cookie) -> Cookie {
self.inner.cookie_set(cookie)
}
fn cookie_ref(&self) -> &Cookie {
self.inner.cookie_ref()
}
fn cookie_mut(&mut self) -> &mut Cookie {
self.inner.cookie_mut()
}
fn position(&self) -> u64 {
self.inner.position()
}
}
/// Padmé padding scheme.
///
/// Padmé leaks at most O(log log M) bits of information (with M being
/// the maximum length of all messages) with an overhead of at most
/// 12%, decreasing with message size.
///
/// This scheme leaks the same order of information as padding to the
/// next power of two, while avoiding an overhead of up to 100%.
///
/// See Section 4 of [Reducing Metadata Leakage from Encrypted Files
/// and Communication with
/// PURBs](https://bford.info/pub/sec/purb.pdf).
///
/// This function is meant to be used with [`Padder`], see this
/// [example].
///
/// [example]: Padder#examples
#[allow(clippy::many_single_char_names)]
pub fn padme(l: u64) -> u64 {
if l < 2 {
return 1; // Avoid cornercase.
}
let e = log2(l); // l's floating-point exponent
let s = log2(e as u64) + 1; // # of bits to represent e
let z = e - s; // # of low bits to set to 0
let m = (1 << z) - 1; // mask of z 1's in LSB
(l + (m as u64)) & !(m as u64) // round up using mask m to clear last z bits
}
/// Compute the log2 of an integer. (This is simply the most
/// significant bit.) Note: log2(0) = -Inf, but this function returns
/// log2(0) as 0 (which is the closest number that we can represent).
fn log2(x: u64) -> usize {
if x == 0 {
0
} else {
63 - x.leading_zeros() as usize
}
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn log2_test() {
for i in 0..64 {
assert_eq!(log2(1u64 << i), i);
if i > 0 {
assert_eq!(log2((1u64 << i) - 1), i - 1);
assert_eq!(log2((1u64 << i) + 1), i);
}
}
}
fn padme_multiplicative_overhead(p: u64) -> f32 {
let c = padme(p);
let (p, c) = (p as f32, c as f32);
(c - p) / p
}
/// Experimentally, we observe the maximum overhead to be ~11.63%
/// when padding 129 bytes to 144.
const MAX_OVERHEAD: f32 = 0.1163;
#[test]
fn padme_max_overhead() {
// The paper says the maximum multiplicative overhead is
// 11.(11)% when padding 9 bytes to 10.
assert!(0.111 < padme_multiplicative_overhead(9));
assert!(padme_multiplicative_overhead(9) < 0.112);
// Contrary to that, we observe the maximum overhead to be
// ~11.63% when padding 129 bytes to 144.
assert!(padme_multiplicative_overhead(129) < MAX_OVERHEAD);
}
quickcheck! {
fn padme_overhead(l: u32) -> bool {
if l < 2 {
return true; // Avoid cornercase.
}
let o = padme_multiplicative_overhead(l as u64);
let l_ = l as f32;
let e = l_.log2().floor(); // l's floating-point exponent
let s = e.log2().floor() + 1.; // # of bits to represent e
let max_overhead = (2.0_f32.powf(e-s) - 1.) / l_;
assert!(o < MAX_OVERHEAD,
"padme({}) => {}: overhead {} exceeds maximum overhead {}",
l, padme(l.into()), o, MAX_OVERHEAD);
assert!(o <= max_overhead,
"padme({}) => {}: overhead {} exceeds maximum overhead {}",
l, padme(l.into()), o, max_overhead);
true
}
}
/// Asserts that we can consume the padded messages.
#[test]
fn roundtrip() {
use std::io::Write;
use crate::parse::Parse;
use crate::serialize::stream::*;
let mut msg = vec![0; rand::random::<usize>() % 1024];
crate::crypto::random(&mut msg);
let mut padded = vec![];
{
let message = Message::new(&mut padded);
let padder = Padder::new(message).with_policy(padme).build().unwrap();
let mut w = LiteralWriter::new(padder).build().unwrap();
w.write_all(&msg).unwrap();
w.finalize().unwrap();
}
let m = crate::Message::from_bytes(&padded).unwrap();
assert_eq!(m.body().unwrap().body(), &msg[..]);
}
/// Asserts that no actual compression is done.
///
/// We want to avoid having the size of the data stream depend on
/// the data's compressibility, therefore it is best to disable
/// the compression.
#[test]
fn no_compression() {
use std::io::Write;
use crate::serialize::stream::*;
const MSG: &[u8] = b"@@@@@@@@@@@@@@";
let mut padded = vec![];
{
let message = Message::new(&mut padded);
let padder = Padder::new(message).build().unwrap();
let mut w = LiteralWriter::new(padder).build().unwrap();
w.write_all(MSG).unwrap();
w.finalize().unwrap();
}
assert!(padded.windows(MSG.len()).any(|ch| ch == MSG),
"Could not find uncompressed message");
}
}