1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428
//! 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]. //! //! [`Padder`]: struct.Padder.html //! [`Encryptor`]: ../stream/struct.Encryptor.html //! [`Signer`]: ../stream/struct.Signer.html //! [example]: struct.Padder.html#example //! //! # 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] (RNP, DKGPG, PGPy, OpenKeychain, GnuPG classic //! and modern) revealed no problems. //! //! [testing]: https://tests.sequoia-pgp.org/#Encrypt-Decrypt_roundtrip_with_key__Bob___AES256 //! //! 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 /// /// # 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 /// /// ``` /// use std::io::Write; /// use sequoia_openpgp as openpgp; /// use openpgp::serialize::stream::{Message, LiteralWriter}; /// use openpgp::serialize::stream::padding::{Padder, padme}; /// use openpgp::types::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 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, padme)?; /// // 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, 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. /// /// # Example /// /// This example illustrates the use of `Padder` with the [Padmé] /// policy. /// /// [Padmé]: fn.padme.html /// /// 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`]: ../struct.Signer.html /// [`LiteralWriter`]: ../struct.LiteralWriter.html /// /// ``` /// # f().unwrap(); fn f() -> 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, padme)?; /// // Optionally add a `Signer` here. /// // Add a `LiteralWriter` here. /// # let _ = message; /// # Ok(()) } /// ``` pub fn new(inner: Message<'a>, p: P) -> Result<Message<'a>> { 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: 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. let inner: Message<'a> = writer::ZIP::new(inner, Cookie::new(level), CompressionLevel::none()); Ok(Message::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") } fn inner_ref(&self) -> Option<&dyn writer::Stackable<'a, Cookie>> { Some(self.inner.as_ref()) } fn inner_mut(&mut self) -> Option<&mut dyn writer::Stackable<'a, Cookie>> { 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]. /// /// [`Padder`]: struct.Padder.html /// [example]: struct.Padder.html#example 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 } #[test] fn padme_max_overhead() { assert!(0.111 < padme_multiplicative_overhead(9)); assert!(padme_multiplicative_overhead(9) < 0.112); } 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 < 0.112); assert!(o <= max_overhead, "padme({}): overhead {} exceeds maximum overhead {}", l, 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, padme).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, padme).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"); } }