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|
//! Padding for OpenPGP messages.
//!
//! To reduce the amount of information leaked via the message length,
//! encrypted OpenPGP messages should be padded.
//!
//! # 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).
//!
//! Cursory testing (OpenKeychain, GnuPG) revealed no problems.
//!
//! 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::ctb::CTB;
use super::{
PartialBodyFilter,
Serialize,
writer,
stream::Cookie,
};
use crate::constants::{
CompressionAlgorithm,
};
/// 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
///
/// # Example
///
/// This example illustrates the use of `Padder` with the [Padmé]
/// policy. Note that for brevity, the encryption and signature
/// filters are omitted.
///
/// [Padmé]: fn.padme.html
///
/// ```
/// extern crate sequoia_openpgp as openpgp;
/// use std::io::Write;
/// use openpgp::constants::DataFormat;
/// use openpgp::serialize::stream::{Message, LiteralWriter};
/// use openpgp::serialize::padding::{Padder, padme};
/// use openpgp::constants::CompressionAlgorithm;
/// # use openpgp::Result;
/// # f().unwrap();
/// # fn f() -> Result<()> {
///
/// let mut unpadded = vec![];
/// {
/// let message = Message::new(&mut unpadded);
/// // XXX: Insert Encryptor here.
/// // XXX: Insert Signer here.
/// let mut w = LiteralWriter::new(message, DataFormat::Text, None, None)?;
/// w.write_all(b"Hello world.")?;
/// w.finalize()?;
/// }
///
/// let mut padded = vec![];
/// {
/// let message = Message::new(&mut padded);
/// // XXX: Insert Encryptor here.
/// let padder = Padder::new(message, padme)?;
/// // XXX: Insert Signer here.
/// let mut w = LiteralWriter::new(padder, DataFormat::Text, None, None)?;
/// w.write_all(b"Hello world.")?;
/// w.finalize()?;
/// }
/// assert!(unpadded.len() < padded.len());
/// # Ok(())
/// # }
pub struct Padder<'a, P: Fn(u64) -> u64 + 'a> {
inner: writer::BoxStack<'a, Cookie>,
policy: P,
}
impl<'a, P: Fn(u64) -> u64 + 'a> Padder<'a, P> {
/// Creates a new padder with the given policy.
pub fn new(inner: writer::Stack<'a, Cookie>, p: P)
-> Result<writer::Stack<'a, Cookie>> {
let mut inner = writer::BoxStack::from(inner);
let level = inner.cookie_ref().level + 1;
// Packet header.
CTB::new(Tag::CompressedData).serialize(&mut inner)?;
let mut inner: writer::Stack<'a, Cookie>
= PartialBodyFilter::new(writer::Stack::from(inner),
Cookie::new(level));
// Compressed data header.
inner.as_mut().write_u8(CompressionAlgorithm::Zip.into())?;
// Create an appropriate filter.
let inner: writer::Stack<'a, Cookie> =
writer::ZIP::new(inner, Cookie::new(level),
writer::CompressionLevel::none());
Ok(writer::Stack::from(Box::new(Self {
inner: inner.into(),
policy: p,
})))
}
}
impl<'a, P: Fn(u64) -> u64 + 'a> fmt::Debug for Padder<'a, P> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("Padder")
.field("inner", &self.inner)
.finish()
}
}
impl<'a, P: Fn(u64) -> u64 + 'a> io::Write for Padder<'a, P> {
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
self.inner.write(buf)
}
fn flush(&mut self) -> io::Result<()> {
self.inner.flush()
}
}
impl<'a, P: Fn(u64) -> u64 + 'a> writer::Stackable<'a, Cookie> for Padder<'a, P>
{
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")
}
|