rustls/msgs/
codec.rs

1use crate::error::InvalidMessage;
2
3use alloc::vec::Vec;
4use core::fmt::Debug;
5
6/// Wrapper over a slice of bytes that allows reading chunks from
7/// with the current position state held using a cursor.
8///
9/// A new reader for a sub section of the the buffer can be created
10/// using the `sub` function or a section of a certain length can
11/// be obtained using the `take` function
12pub struct Reader<'a> {
13    /// The underlying buffer storing the readers content
14    buffer: &'a [u8],
15    /// Stores the current reading position for the buffer
16    cursor: usize,
17}
18
19impl<'a> Reader<'a> {
20    /// Creates a new Reader of the provided `bytes` slice with
21    /// the initial cursor position of zero.
22    pub fn init(bytes: &[u8]) -> Reader {
23        Reader {
24            buffer: bytes,
25            cursor: 0,
26        }
27    }
28
29    /// Attempts to create a new Reader on a sub section of this
30    /// readers bytes by taking a slice of the provided `length`
31    /// will return None if there is not enough bytes
32    pub fn sub(&mut self, length: usize) -> Result<Reader, InvalidMessage> {
33        match self.take(length) {
34            Some(bytes) => Ok(Reader::init(bytes)),
35            None => Err(InvalidMessage::MessageTooShort),
36        }
37    }
38
39    /// Borrows a slice of all the remaining bytes
40    /// that appear after the cursor position.
41    ///
42    /// Moves the cursor to the end of the buffer length.
43    pub fn rest(&mut self) -> &[u8] {
44        let rest = &self.buffer[self.cursor..];
45        self.cursor = self.buffer.len();
46        rest
47    }
48
49    /// Attempts to borrow a slice of bytes from the current
50    /// cursor position of `length` if there is not enough
51    /// bytes remaining after the cursor to take the length
52    /// then None is returned instead.
53    pub fn take(&mut self, length: usize) -> Option<&[u8]> {
54        if self.left() < length {
55            return None;
56        }
57        let current = self.cursor;
58        self.cursor += length;
59        Some(&self.buffer[current..current + length])
60    }
61
62    /// Used to check whether the reader has any content left
63    /// after the cursor (cursor has not reached end of buffer)
64    pub fn any_left(&self) -> bool {
65        self.cursor < self.buffer.len()
66    }
67
68    pub fn expect_empty(&self, name: &'static str) -> Result<(), InvalidMessage> {
69        match self.any_left() {
70            true => Err(InvalidMessage::TrailingData(name)),
71            false => Ok(()),
72        }
73    }
74
75    /// Returns the cursor position which is also the number
76    /// of bytes that have been read from the buffer.
77    pub fn used(&self) -> usize {
78        self.cursor
79    }
80
81    /// Returns the number of bytes that are still able to be
82    /// read (The number of remaining takes)
83    pub fn left(&self) -> usize {
84        self.buffer.len() - self.cursor
85    }
86}
87
88/// Trait for implementing encoding and decoding functionality
89/// on something.
90pub trait Codec: Debug + Sized {
91    /// Function for encoding itself by appending itself to
92    /// the provided vec of bytes.
93    fn encode(&self, bytes: &mut Vec<u8>);
94
95    /// Function for decoding itself from the provided reader
96    /// will return Some if the decoding was successful or
97    /// None if it was not.
98    fn read(_: &mut Reader) -> Result<Self, InvalidMessage>;
99
100    /// Convenience function for encoding the implementation
101    /// into a vec and returning it
102    fn get_encoding(&self) -> Vec<u8> {
103        let mut bytes = Vec::new();
104        self.encode(&mut bytes);
105        bytes
106    }
107
108    /// Function for wrapping a call to the read function in
109    /// a Reader for the slice of bytes provided
110    fn read_bytes(bytes: &[u8]) -> Result<Self, InvalidMessage> {
111        let mut reader = Reader::init(bytes);
112        Self::read(&mut reader)
113    }
114}
115
116impl Codec for u8 {
117    fn encode(&self, bytes: &mut Vec<u8>) {
118        bytes.push(*self);
119    }
120
121    fn read(r: &mut Reader) -> Result<Self, InvalidMessage> {
122        match r.take(1) {
123            Some(&[byte]) => Ok(byte),
124            _ => Err(InvalidMessage::MissingData("u8")),
125        }
126    }
127}
128
129pub(crate) fn put_u16(v: u16, out: &mut [u8]) {
130    let out: &mut [u8; 2] = (&mut out[..2]).try_into().unwrap();
131    *out = u16::to_be_bytes(v);
132}
133
134impl Codec for u16 {
135    fn encode(&self, bytes: &mut Vec<u8>) {
136        let mut b16 = [0u8; 2];
137        put_u16(*self, &mut b16);
138        bytes.extend_from_slice(&b16);
139    }
140
141    fn read(r: &mut Reader) -> Result<Self, InvalidMessage> {
142        match r.take(2) {
143            Some(&[b1, b2]) => Ok(Self::from_be_bytes([b1, b2])),
144            _ => Err(InvalidMessage::MissingData("u8")),
145        }
146    }
147}
148
149// Make a distinct type for u24, even though it's a u32 underneath
150#[allow(non_camel_case_types)]
151#[derive(Debug, Copy, Clone)]
152pub struct u24(pub u32);
153
154#[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
155impl From<u24> for usize {
156    #[inline]
157    fn from(v: u24) -> Self {
158        v.0 as Self
159    }
160}
161
162impl Codec for u24 {
163    fn encode(&self, bytes: &mut Vec<u8>) {
164        let be_bytes = u32::to_be_bytes(self.0);
165        bytes.extend_from_slice(&be_bytes[1..]);
166    }
167
168    fn read(r: &mut Reader) -> Result<Self, InvalidMessage> {
169        match r.take(3) {
170            Some(&[a, b, c]) => Ok(Self(u32::from_be_bytes([0, a, b, c]))),
171            _ => Err(InvalidMessage::MissingData("u24")),
172        }
173    }
174}
175
176impl Codec for u32 {
177    fn encode(&self, bytes: &mut Vec<u8>) {
178        bytes.extend(Self::to_be_bytes(*self));
179    }
180
181    fn read(r: &mut Reader) -> Result<Self, InvalidMessage> {
182        match r.take(4) {
183            Some(&[a, b, c, d]) => Ok(Self::from_be_bytes([a, b, c, d])),
184            _ => Err(InvalidMessage::MissingData("u32")),
185        }
186    }
187}
188
189pub(crate) fn put_u64(v: u64, bytes: &mut [u8]) {
190    let bytes: &mut [u8; 8] = (&mut bytes[..8]).try_into().unwrap();
191    *bytes = u64::to_be_bytes(v);
192}
193
194impl Codec for u64 {
195    fn encode(&self, bytes: &mut Vec<u8>) {
196        let mut b64 = [0u8; 8];
197        put_u64(*self, &mut b64);
198        bytes.extend_from_slice(&b64);
199    }
200
201    fn read(r: &mut Reader) -> Result<Self, InvalidMessage> {
202        match r.take(8) {
203            Some(&[a, b, c, d, e, f, g, h]) => Ok(Self::from_be_bytes([a, b, c, d, e, f, g, h])),
204            _ => Err(InvalidMessage::MissingData("u64")),
205        }
206    }
207}
208
209/// Implement `Codec` for lists of elements that implement `TlsListElement`.
210///
211/// `TlsListElement` provides the size of the length prefix for the list.
212impl<T: Codec + TlsListElement + Debug> Codec for Vec<T> {
213    fn encode(&self, bytes: &mut Vec<u8>) {
214        let nest = LengthPrefixedBuffer::new(T::SIZE_LEN, bytes);
215
216        for i in self {
217            i.encode(nest.buf);
218        }
219    }
220
221    fn read(r: &mut Reader) -> Result<Self, InvalidMessage> {
222        let len = match T::SIZE_LEN {
223            ListLength::U8 => usize::from(u8::read(r)?),
224            ListLength::U16 => usize::from(u16::read(r)?),
225            ListLength::U24 { max } => Ord::min(usize::from(u24::read(r)?), max),
226        };
227
228        let mut sub = r.sub(len)?;
229        let mut ret = Self::new();
230        while sub.any_left() {
231            ret.push(T::read(&mut sub)?);
232        }
233
234        Ok(ret)
235    }
236}
237
238/// A trait for types that can be encoded and decoded in a list.
239///
240/// This trait is used to implement `Codec` for `Vec<T>`. Lists in the TLS wire format are
241/// prefixed with a length, the size of which depends on the type of the list elements.
242/// As such, the `Codec` implementation for `Vec<T>` requires an implementation of this trait
243/// for its element type `T`.
244pub(crate) trait TlsListElement {
245    const SIZE_LEN: ListLength;
246}
247
248/// The length of the length prefix for a list.
249///
250/// The types that appear in lists are limited to three kinds of length prefixes:
251/// 1, 2, and 3 bytes. For the latter kind, we require a `TlsListElement` implementer
252/// to specify a maximum length.
253pub(crate) enum ListLength {
254    U8,
255    U16,
256    U24 { max: usize },
257}
258
259/// Tracks encoding a length-delimited structure in a single pass.
260pub(crate) struct LengthPrefixedBuffer<'a> {
261    pub(crate) buf: &'a mut Vec<u8>,
262    len_offset: usize,
263    size_len: ListLength,
264}
265
266impl<'a> LengthPrefixedBuffer<'a> {
267    /// Inserts a dummy length into `buf`, and remembers where it went.
268    ///
269    /// After this, the body of the length-delimited structure should be appended to `LengthPrefixedBuffer::buf`.
270    /// The length header is corrected in `LengthPrefixedBuffer::drop`.
271    pub(crate) fn new(size_len: ListLength, buf: &'a mut Vec<u8>) -> Self {
272        let len_offset = buf.len();
273        buf.extend(match size_len {
274            ListLength::U8 => &[0xff][..],
275            ListLength::U16 => &[0xff, 0xff],
276            ListLength::U24 { .. } => &[0xff, 0xff, 0xff],
277        });
278
279        Self {
280            buf,
281            len_offset,
282            size_len,
283        }
284    }
285}
286
287impl<'a> Drop for LengthPrefixedBuffer<'a> {
288    /// Goes back and corrects the length previously inserted at the start of the structure.
289    fn drop(&mut self) {
290        match self.size_len {
291            ListLength::U8 => {
292                let len = self.buf.len() - self.len_offset - 1;
293                debug_assert!(len <= 0xff);
294                self.buf[self.len_offset] = len as u8;
295            }
296            ListLength::U16 => {
297                let len = self.buf.len() - self.len_offset - 2;
298                debug_assert!(len <= 0xffff);
299                let out: &mut [u8; 2] = (&mut self.buf[self.len_offset..self.len_offset + 2])
300                    .try_into()
301                    .unwrap();
302                *out = u16::to_be_bytes(len as u16);
303            }
304            ListLength::U24 { .. } => {
305                let len = self.buf.len() - self.len_offset - 3;
306                debug_assert!(len <= 0xff_ffff);
307                let len_bytes = u32::to_be_bytes(len as u32);
308                let out: &mut [u8; 3] = (&mut self.buf[self.len_offset..self.len_offset + 3])
309                    .try_into()
310                    .unwrap();
311                out.copy_from_slice(&len_bytes[1..]);
312            }
313        }
314    }
315}
316
317#[cfg(test)]
318mod tests {
319    use super::*;
320    use std::prelude::v1::*;
321    use std::vec;
322
323    #[test]
324    fn interrupted_length_prefixed_buffer_leaves_maximum_length() {
325        let mut buf = Vec::new();
326        let nested = LengthPrefixedBuffer::new(ListLength::U16, &mut buf);
327        nested.buf.push(0xaa);
328        assert_eq!(nested.buf, &vec![0xff, 0xff, 0xaa]);
329        // <- if the buffer is accidentally read here, there is no possiblity
330        //    that the contents of the length-prefixed buffer are interpretted
331        //    as a subsequent encoding (perhaps allowing injection of a different
332        //    extension)
333        drop(nested);
334        assert_eq!(buf, vec![0x00, 0x01, 0xaa]);
335    }
336}
rustls/msgs/codec.rs

rustls/msgs/
codec.rs
