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use alloc::vec::Vec;
use core::fmt::Debug;
use crate::error::InvalidMessage;
/// Wrapper over a slice of bytes that allows reading chunks from
/// with the current position state held using a cursor.
///
/// A new reader for a sub section of the buffer can be created
/// using the `sub` function or a section of a certain length can
/// be obtained using the `take` function
pub struct Reader<'a> {
/// The underlying buffer storing the readers content
buffer: &'a [u8],
/// Stores the current reading position for the buffer
cursor: usize,
}
impl<'a> Reader<'a> {
/// Creates a new Reader of the provided `bytes` slice with
/// the initial cursor position of zero.
pub fn init(bytes: &'a [u8]) -> Self {
Reader {
buffer: bytes,
cursor: 0,
}
}
/// Attempts to create a new Reader on a sub section of this
/// readers bytes by taking a slice of the provided `length`
/// will return None if there is not enough bytes
pub fn sub(&mut self, length: usize) -> Result<Self, InvalidMessage> {
match self.take(length) {
Some(bytes) => Ok(Reader::init(bytes)),
None => Err(InvalidMessage::MessageTooShort),
}
}
/// Borrows a slice of all the remaining bytes
/// that appear after the cursor position.
///
/// Moves the cursor to the end of the buffer length.
pub fn rest(&mut self) -> &'a [u8] {
let rest = &self.buffer[self.cursor..];
self.cursor = self.buffer.len();
rest
}
/// Attempts to borrow a slice of bytes from the current
/// cursor position of `length` if there is not enough
/// bytes remaining after the cursor to take the length
/// then None is returned instead.
pub fn take(&mut self, length: usize) -> Option<&'a [u8]> {
if self.left() < length {
return None;
}
let current = self.cursor;
self.cursor += length;
Some(&self.buffer[current..current + length])
}
/// Used to check whether the reader has any content left
/// after the cursor (cursor has not reached end of buffer)
pub fn any_left(&self) -> bool {
self.cursor < self.buffer.len()
}
pub fn expect_empty(&self, name: &'static str) -> Result<(), InvalidMessage> {
match self.any_left() {
true => Err(InvalidMessage::TrailingData(name)),
false => Ok(()),
}
}
/// Returns the cursor position which is also the number
/// of bytes that have been read from the buffer.
pub fn used(&self) -> usize {
self.cursor
}
/// Returns the number of bytes that are still able to be
/// read (The number of remaining takes)
pub fn left(&self) -> usize {
self.buffer.len() - self.cursor
}
}
/// Trait for implementing encoding and decoding functionality
/// on something.
pub trait Codec<'a>: Debug + Sized {
/// Function for encoding itself by appending itself to
/// the provided vec of bytes.
fn encode(&self, bytes: &mut Vec<u8>);
/// Function for decoding itself from the provided reader
/// will return Some if the decoding was successful or
/// None if it was not.
fn read(_: &mut Reader<'a>) -> Result<Self, InvalidMessage>;
/// Convenience function for encoding the implementation
/// into a vec and returning it
fn get_encoding(&self) -> Vec<u8> {
let mut bytes = Vec::new();
self.encode(&mut bytes);
bytes
}
/// Function for wrapping a call to the read function in
/// a Reader for the slice of bytes provided
///
/// Returns `Err(InvalidMessage::ExcessData(_))` if
/// `Self::read` does not read the entirety of `bytes`.
fn read_bytes(bytes: &'a [u8]) -> Result<Self, InvalidMessage> {
let mut reader = Reader::init(bytes);
Self::read(&mut reader).and_then(|r| {
reader.expect_empty("read_bytes")?;
Ok(r)
})
}
}
impl Codec<'_> for u8 {
fn encode(&self, bytes: &mut Vec<u8>) {
bytes.push(*self);
}
fn read(r: &mut Reader<'_>) -> Result<Self, InvalidMessage> {
match r.take(1) {
Some(&[byte]) => Ok(byte),
_ => Err(InvalidMessage::MissingData("u8")),
}
}
}
pub(crate) fn put_u16(v: u16, out: &mut [u8]) {
let out: &mut [u8; 2] = (&mut out[..2]).try_into().unwrap();
*out = u16::to_be_bytes(v);
}
impl Codec<'_> for u16 {
fn encode(&self, bytes: &mut Vec<u8>) {
let mut b16 = [0u8; 2];
put_u16(*self, &mut b16);
bytes.extend_from_slice(&b16);
}
fn read(r: &mut Reader<'_>) -> Result<Self, InvalidMessage> {
match r.take(2) {
Some(&[b1, b2]) => Ok(Self::from_be_bytes([b1, b2])),
_ => Err(InvalidMessage::MissingData("u16")),
}
}
}
// Make a distinct type for u24, even though it's a u32 underneath
#[allow(non_camel_case_types)]
#[derive(Debug, Copy, Clone)]
pub struct u24(pub u32);
#[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
impl From<u24> for usize {
#[inline]
fn from(v: u24) -> Self {
v.0 as Self
}
}
impl Codec<'_> for u24 {
fn encode(&self, bytes: &mut Vec<u8>) {
let be_bytes = u32::to_be_bytes(self.0);
bytes.extend_from_slice(&be_bytes[1..]);
}
fn read(r: &mut Reader<'_>) -> Result<Self, InvalidMessage> {
match r.take(3) {
Some(&[a, b, c]) => Ok(Self(u32::from_be_bytes([0, a, b, c]))),
_ => Err(InvalidMessage::MissingData("u24")),
}
}
}
impl Codec<'_> for u32 {
fn encode(&self, bytes: &mut Vec<u8>) {
bytes.extend(Self::to_be_bytes(*self));
}
fn read(r: &mut Reader<'_>) -> Result<Self, InvalidMessage> {
match r.take(4) {
Some(&[a, b, c, d]) => Ok(Self::from_be_bytes([a, b, c, d])),
_ => Err(InvalidMessage::MissingData("u32")),
}
}
}
pub(crate) fn put_u64(v: u64, bytes: &mut [u8]) {
let bytes: &mut [u8; 8] = (&mut bytes[..8]).try_into().unwrap();
*bytes = u64::to_be_bytes(v);
}
impl Codec<'_> for u64 {
fn encode(&self, bytes: &mut Vec<u8>) {
let mut b64 = [0u8; 8];
put_u64(*self, &mut b64);
bytes.extend_from_slice(&b64);
}
fn read(r: &mut Reader<'_>) -> Result<Self, InvalidMessage> {
match r.take(8) {
Some(&[a, b, c, d, e, f, g, h]) => Ok(Self::from_be_bytes([a, b, c, d, e, f, g, h])),
_ => Err(InvalidMessage::MissingData("u64")),
}
}
}
/// Implement `Codec` for lists of elements that implement `TlsListElement`.
///
/// `TlsListElement` provides the size of the length prefix for the list.
impl<'a, T: Codec<'a> + TlsListElement + Debug> Codec<'a> for Vec<T> {
fn encode(&self, bytes: &mut Vec<u8>) {
let nest = LengthPrefixedBuffer::new(T::SIZE_LEN, bytes);
for i in self {
i.encode(nest.buf);
}
}
fn read(r: &mut Reader<'a>) -> Result<Self, InvalidMessage> {
let len = match T::SIZE_LEN {
ListLength::U8 => usize::from(u8::read(r)?),
ListLength::U16 => usize::from(u16::read(r)?),
ListLength::U24 { max, error } => match usize::from(u24::read(r)?) {
len if len > max => return Err(error),
len => len,
},
};
let mut sub = r.sub(len)?;
let mut ret = Self::new();
while sub.any_left() {
ret.push(T::read(&mut sub)?);
}
Ok(ret)
}
}
/// A trait for types that can be encoded and decoded in a list.
///
/// This trait is used to implement `Codec` for `Vec<T>`. Lists in the TLS wire format are
/// prefixed with a length, the size of which depends on the type of the list elements.
/// As such, the `Codec` implementation for `Vec<T>` requires an implementation of this trait
/// for its element type `T`.
pub(crate) trait TlsListElement {
const SIZE_LEN: ListLength;
}
/// The length of the length prefix for a list.
///
/// The types that appear in lists are limited to three kinds of length prefixes:
/// 1, 2, and 3 bytes. For the latter kind, we require a `TlsListElement` implementer
/// to specify a maximum length and error if the actual length is larger.
pub(crate) enum ListLength {
U8,
U16,
U24 { max: usize, error: InvalidMessage },
}
/// Tracks encoding a length-delimited structure in a single pass.
pub(crate) struct LengthPrefixedBuffer<'a> {
pub(crate) buf: &'a mut Vec<u8>,
len_offset: usize,
size_len: ListLength,
}
impl<'a> LengthPrefixedBuffer<'a> {
/// Inserts a dummy length into `buf`, and remembers where it went.
///
/// After this, the body of the length-delimited structure should be appended to `LengthPrefixedBuffer::buf`.
/// The length header is corrected in `LengthPrefixedBuffer::drop`.
pub(crate) fn new(size_len: ListLength, buf: &'a mut Vec<u8>) -> Self {
let len_offset = buf.len();
buf.extend(match size_len {
ListLength::U8 => &[0xff][..],
ListLength::U16 => &[0xff, 0xff],
ListLength::U24 { .. } => &[0xff, 0xff, 0xff],
});
Self {
buf,
len_offset,
size_len,
}
}
}
impl<'a> Drop for LengthPrefixedBuffer<'a> {
/// Goes back and corrects the length previously inserted at the start of the structure.
fn drop(&mut self) {
match self.size_len {
ListLength::U8 => {
let len = self.buf.len() - self.len_offset - 1;
debug_assert!(len <= 0xff);
self.buf[self.len_offset] = len as u8;
}
ListLength::U16 => {
let len = self.buf.len() - self.len_offset - 2;
debug_assert!(len <= 0xffff);
let out: &mut [u8; 2] = (&mut self.buf[self.len_offset..self.len_offset + 2])
.try_into()
.unwrap();
*out = u16::to_be_bytes(len as u16);
}
ListLength::U24 { .. } => {
let len = self.buf.len() - self.len_offset - 3;
debug_assert!(len <= 0xff_ffff);
let len_bytes = u32::to_be_bytes(len as u32);
let out: &mut [u8; 3] = (&mut self.buf[self.len_offset..self.len_offset + 3])
.try_into()
.unwrap();
out.copy_from_slice(&len_bytes[1..]);
}
}
}
}
#[cfg(test)]
mod tests {
use std::prelude::v1::*;
use std::vec;
use super::*;
#[test]
fn interrupted_length_prefixed_buffer_leaves_maximum_length() {
let mut buf = Vec::new();
let nested = LengthPrefixedBuffer::new(ListLength::U16, &mut buf);
nested.buf.push(0xaa);
assert_eq!(nested.buf, &vec![0xff, 0xff, 0xaa]);
// <- if the buffer is accidentally read here, there is no possibility
// that the contents of the length-prefixed buffer are interpreted
// as a subsequent encoding (perhaps allowing injection of a different
// extension)
drop(nested);
assert_eq!(buf, vec![0x00, 0x01, 0xaa]);
}
}