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//! DER/BER decoder implementation
#[cfg(not(feature = "std"))]
use alloc::{boxed::Box, vec::Vec};
use crate::config::DecoderConfig;
use crate::error::{Error, Result};
use crate::{Encoding, Length, Tag};
/// Decoder for ASN.1 DER/BER encoded data
///
/// Uses a cursor-based design for efficient zero-copy parsing.
/// For BER indefinite-length encoding, assembled content is stored inside
/// the decoder and freed when the decoder is dropped.
pub struct Decoder<'a> {
/// Original input for position calculation
original: &'a [u8],
/// Remaining input to decode
cursor: &'a [u8],
/// Encoding type (DER, BER, or CER)
encoding: Encoding,
/// Configuration limits (borrowed to avoid cloning on every nested construct)
config: &'a DecoderConfig,
/// Current nesting depth
current_depth: usize,
/// Cached position in original input (for O(1) access)
position: usize,
/// Buffers assembled from BER/CER indefinite-length content.
/// `cursor` (or sub-slices of it) may point into these allocations.
/// `Box<[u8]>` heap data is stable across moves, so the raw pointers
/// embedded in `cursor` remain valid after the decoder is moved.
indefinite_buffers: Vec<Box<[u8]>>,
}
/// Default decoder configuration (shared by all decoders using default config)
static DEFAULT_CONFIG: DecoderConfig = DecoderConfig {
max_depth: 32,
max_sequence_elements: 10_000,
max_length: 16 * 1024 * 1024, // 16 MB
};
impl<'a> Decoder<'a> {
/// Create a new decoder for the given input
pub fn new(input: &'a [u8], encoding: Encoding) -> Self {
Self {
original: input,
cursor: input,
encoding,
config: &DEFAULT_CONFIG,
current_depth: 0,
position: 0,
indefinite_buffers: Vec::new(),
}
}
/// Create a new decoder with custom configuration
pub fn new_with_config(input: &'a [u8], encoding: Encoding, config: &'a DecoderConfig) -> Self {
Self {
original: input,
cursor: input,
encoding,
config,
current_depth: 0,
position: 0,
indefinite_buffers: Vec::new(),
}
}
/// Get current position in original input
#[inline]
pub fn position(&self) -> usize {
self.position
}
/// Get remaining bytes in current scope
pub fn remaining(&self) -> &'a [u8] {
self.cursor
}
/// Check if at end of current scope
pub fn is_empty(&self) -> bool {
self.cursor.is_empty()
}
/// Advance the cursor past all remaining bytes in the current scope.
///
/// After this call `is_empty()` returns `true`. Used by sequence
/// iterators to terminate cleanly after a decode error: without this the
/// iterator would keep retrying the same failing position forever.
#[inline]
pub(crate) fn exhaust(&mut self) {
self.position += self.cursor.len();
self.cursor = &self.cursor[self.cursor.len()..];
}
/// Get the encoding type
pub fn encoding(&self) -> Encoding {
self.encoding
}
/// Read exactly `n` bytes from the current cursor position, advancing it.
///
/// Returns a slice borrowed from the original input with lifetime `'a`.
///
/// # Errors
///
/// Returns [`Error::UnexpectedEof`] if fewer than `n` bytes remain in
/// the current scope.
#[inline]
pub fn read_bytes(&mut self, n: usize) -> Result<&'a [u8]> {
if self.cursor.len() < n {
return Err(Error::UnexpectedEof {
position: self.position,
});
}
let (chunk, rest) = self.cursor.split_at(n);
self.cursor = rest;
self.position += n;
Ok(chunk)
}
/// Peek at the next tag without consuming any bytes.
///
/// Returns the [`Tag`] that would be read by the next call to
/// [`read_tag`](Self::read_tag). The cursor position is unchanged.
///
/// Useful for dispatching on the tag type before committing to a specific
/// `Decode` implementation.
///
/// # Errors
///
/// Returns an error if the remaining input is empty or if the tag encoding
/// is invalid.
pub fn peek_tag(&self) -> Result<Tag> {
let (tag, _) = Tag::decode(self.cursor, self.position)?;
Ok(tag)
}
/// Read and consume the next ASN.1 tag from the input.
///
/// Advances the cursor past the tag bytes (1 byte for low-tag-number form;
/// more for high-tag-number form).
///
/// # Errors
///
/// Returns [`Error::UnexpectedEof`] if the input is empty, or an error if
/// the tag encoding is malformed (e.g. an overlong high-tag-number form
/// that would overflow a `u32`).
pub fn read_tag(&mut self) -> Result<Tag> {
let (tag, consumed) = Tag::decode(self.cursor, self.position)?;
self.cursor = &self.cursor[consumed..];
self.position += consumed;
Ok(tag)
}
/// Assemble indefinite-length content into an owned buffer.
///
/// Reads TLV elements from the cursor until the outermost end-of-contents
/// octets (`0x00 0x00`) are reached, advancing the cursor past them.
/// Nested indefinite-length encodings are handled transparently: inner
/// end-of-contents pairs are copied into the returned buffer and the depth
/// counter is decremented accordingly.
///
/// The returned `Box<[u8]>` contains all assembled bytes, excluding the
/// outermost terminating `0x00 0x00` pair. Callers must store the box
/// (e.g. in `indefinite_buffers`) before constructing any slice into it.
fn assemble_indefinite_content(&mut self) -> Result<Box<[u8]>> {
// Pre-allocate to reduce reallocations (typical ASN.1 indefinite content is < 4KB)
let mut content = Vec::with_capacity(4096);
let mut depth = 0usize;
let start_position = self.position;
loop {
// Need at least 2 bytes to check for end-of-contents
if self.cursor.len() < 2 {
return Err(Error::UnexpectedEof {
position: self.position,
});
}
// Check for end-of-contents octets (0x00 0x00)
if self.cursor[0] == 0x00 && self.cursor[1] == 0x00 {
if depth == 0 {
// Found the terminator at our level
self.cursor = &self.cursor[2..]; // Skip the 0x00 0x00
self.position += 2;
break;
} else {
// This terminates a nested indefinite length
depth -= 1;
content.extend_from_slice(&self.cursor[..2]);
self.cursor = &self.cursor[2..];
self.position += 2;
continue;
}
}
// Read tag
let (_tag, tag_len) = Tag::decode(self.cursor, self.position)?;
content.extend_from_slice(&self.cursor[..tag_len]);
self.cursor = &self.cursor[tag_len..];
self.position += tag_len;
// Read length
let (length, length_len) = Length::decode(self.cursor, self.position)?;
content.extend_from_slice(&self.cursor[..length_len]);
self.cursor = &self.cursor[length_len..];
self.position += length_len;
// Handle length
match length {
Length::Definite(len) => {
// Read the definite length content
if len > self.config.max_length {
return Err(Error::LengthExceedsMaximum {
position: self.position,
length: len,
max_length: self.config.max_length,
});
}
let bytes = self.read_bytes(len)?;
content.extend_from_slice(bytes);
}
Length::Indefinite => {
// Nested indefinite length - increase depth
depth += 1;
}
}
// Safety check: prevent infinite loops with huge buffers
if content.len() > self.config.max_length {
return Err(Error::LengthExceedsMaximum {
position: start_position,
length: content.len(),
max_length: self.config.max_length,
});
}
}
Ok(content.into_boxed_slice())
}
/// Read indefinite-length content until end-of-contents octets.
///
/// Assembles the bytes, stores the buffer in `self.indefinite_buffers`, and
/// returns a `&'a [u8]` into it.
///
/// # Safety invariant
///
/// The returned reference is valid for as long as `self` is alive. All
/// current call sites satisfy this: the result is either discarded or used
/// to construct a local decoder that does not outlive `self`. Do **not**
/// pass the returned slice to a decoder that may outlive `self`; use
/// [`enter_constructed`](Self::enter_constructed) instead, which places the
/// buffer in the child decoder.
pub(crate) fn read_indefinite_content(&mut self) -> Result<&'a [u8]> {
let boxed = self.assemble_indefinite_content()?;
let ptr = boxed.as_ptr();
let len = boxed.len();
self.indefinite_buffers.push(boxed);
// SAFETY: `boxed` was just pushed into `self.indefinite_buffers`; the
// heap allocation is stable across moves of `Box<[u8]>`, so `ptr`
// remains valid until `self` is dropped. All callers use the returned
// slice only within the lifetime of `self`.
Ok(unsafe { core::slice::from_raw_parts(ptr, len) })
}
/// Read length
pub fn read_length(&mut self) -> Result<Length> {
let position_before = self.position;
let (length, consumed) = Length::decode(self.cursor, self.position)?;
// DER doesn't allow indefinite length
if self.encoding == Encoding::Der && length.is_indefinite() {
return Err(Error::IndefiniteLengthInDer {
position: self.position,
});
}
// DER requires shortest form for length encoding
if self.encoding == Encoding::Der {
if let Length::Definite(len) = length {
let first_byte = self.cursor[0];
// Check if long form was used when short form should have been used
if first_byte > 0x80 {
// Long form: check if length < 128 (should have used short form)
if len < 128 {
return Err(Error::DerViolation {
position: position_before,
#[cfg(feature = "std")]
reason: "DER requires short form for lengths < 128".to_string(),
#[cfg(not(feature = "std"))]
reason: "DER requires shortest length form",
});
}
// Check for unnecessary leading zeros in long form
if consumed > 2 {
let first_length_byte = self.cursor[1];
if first_length_byte == 0 {
return Err(Error::DerViolation {
position: position_before,
#[cfg(feature = "std")]
reason: "DER requires minimal length encoding (no leading zeros)"
.to_string(),
#[cfg(not(feature = "std"))]
reason: "DER requires minimal length encoding",
});
}
}
}
}
}
self.cursor = &self.cursor[consumed..];
self.position += consumed;
Ok(length)
}
/// Enter a constructed ASN.1 element and return a child decoder scoped to its content.
///
/// Reads the leading tag and verifies it equals `tag`, then reads the
/// length and returns a new `Decoder` whose input is limited to the
/// content bytes of this element. The parent decoder's cursor advances
/// past the entire TLV.
///
/// The child decoder's nesting depth is the parent's depth plus one.
/// If that would exceed [`DecoderConfig::max_depth`], the method returns
/// [`Error::MaxDepthExceeded`].
///
/// # Errors
///
/// Returns [`Error::UnexpectedTag`] if the tag does not match `tag`,
/// [`Error::MaxDepthExceeded`] if the depth limit is reached, or any
/// encoding-rule violation error (e.g. indefinite length in DER mode).
pub fn enter_constructed(&mut self, tag: Tag) -> Result<Decoder<'a>> {
if self.current_depth >= self.config.max_depth {
return Err(Error::MaxDepthExceeded {
position: self.position,
max_depth: self.config.max_depth,
});
}
let content_tag = self.read_tag()?;
if content_tag != tag {
return Err(Error::UnexpectedTag {
position: self.position,
expected: tag,
actual: content_tag,
});
}
let length = self.read_length()?;
// CER validation: constructed types >1000 bytes must use indefinite length
if self.encoding == Encoding::Cer {
match length {
Length::Definite(len) if len > 1000 => {
#[cfg(feature = "std")]
return Err(Error::CerViolation {
position: self.position,
reason: "CER requires indefinite length for constructed types >1000 bytes"
.into(),
});
#[cfg(not(feature = "std"))]
return Err(Error::CerViolation {
position: self.position,
reason: "CER requires indefinite length for constructed types >1000 bytes",
});
}
_ => {}
}
}
match length {
Length::Definite(len) => {
if len > self.config.max_length {
return Err(Error::LengthExceedsMaximum {
position: self.position,
length: len,
max_length: self.config.max_length,
});
}
let content_bytes = self.read_bytes(len)?;
Ok(Decoder {
original: self.original,
cursor: content_bytes,
encoding: self.encoding,
config: self.config,
current_depth: self.current_depth + 1,
position: 0,
indefinite_buffers: Vec::new(),
})
}
Length::Indefinite => {
// Indefinite length is only allowed in BER/CER
if self.encoding == Encoding::Der {
return Err(Error::IndefiniteLengthInDer {
position: self.position,
});
}
// Store the assembled buffer in the *child* decoder so it is
// freed when the child is dropped, regardless of when `self`
// (the parent) is dropped.
let boxed = self.assemble_indefinite_content()?;
let ptr = boxed.as_ptr();
let len = boxed.len();
// SAFETY: `boxed` is moved into the child's `indefinite_buffers`.
// `Box<[u8]>` heap data does not move when the `Box` is moved,
// so `ptr` stays valid for the child's entire lifetime.
let cursor: &'a [u8] = unsafe { core::slice::from_raw_parts(ptr, len) };
Ok(Decoder {
original: self.original,
cursor,
encoding: self.encoding,
config: self.config,
current_depth: self.current_depth + 1,
position: 0,
indefinite_buffers: vec![boxed],
})
}
}
}
/// Decode the next ASN.1 value using the [`Decode`](crate::traits::Decode) trait.
///
/// Convenience wrapper equivalent to `T::decode(self)`.
///
/// # Errors
///
/// Propagates any error returned by `T::decode`.
pub fn decode<T: crate::traits::Decode<'a>>(&mut self) -> Result<T> {
T::decode(self)
}
/// Decode the next ASN.1 value after verifying its tag matches `expected_tag`.
///
/// Peeks at the next tag and returns [`Error::UnexpectedTag`] without
/// consuming any bytes if it does not match. On a tag match, delegates
/// to `T::decode`.
///
/// # Errors
///
/// Returns [`Error::UnexpectedTag`] if the leading tag does not match, or
/// any error from `T::decode`.
pub fn decode_with_tag<T: crate::traits::Decode<'a>>(
&mut self,
expected_tag: Tag,
) -> Result<T> {
let tag = self.peek_tag()?;
if tag != expected_tag {
return Err(Error::UnexpectedTag {
position: self.position,
expected: expected_tag,
actual: tag,
});
}
self.decode()
}
}