#bencode #serialization #deserialization #bittorent

bendy

A rust library for encoding and decoding bencode with enforced cannonicalization rules

5 unstable releases

✓ Uses Rust 2018 edition

0.2.0 Feb 28, 2019
0.1.2 Aug 14, 2018
0.1.1 Aug 7, 2018
0.1.0 Jul 24, 2018
0.0.0 Jul 5, 2018

#27 in Encoding

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BSD-3-Clause

104KB
2K SLoC

Bendy

Build Status Current Version License: BSD-3-Clause

A Rust library for encoding and decoding bencode with enforced canonicalization rules. Bencode is a simple but very effective encoding scheme, originating with the BitTorrent peer-to-peer system.


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Known alternatives:

This is not the first library to implement Bencode. In fact there's several implementations already:

Why should I use it?

So why the extra work adding yet-another-version of a thing that already exists, you might ask?

Enforced correctness

Implementing a canonical encoding form is straight forward. It comes down to defining a proper way of handling unordered data. The next step is that bendy's sorting data before encoding it using the regular Bencode rules. If your data is already sorted bendy will of course skip the extra sorting step to gain efficiency. But bendy goes a step further to ensure correctness: If you hand the library data that you say is already sorted, bendy still does an in-place verification to ensure that your data actually is sorted and complains if it isn't. In the end, once bendy serialized your data, it's Bencode through and through. So it's perfectly compatible with every other Bencode library.

Just remember: At this point only bendy enforces the correctness of the canonical format if you read it back in.

Canonical representation

Bendy ensures that any de-serialize / serialize round trip produces the exact same and correct binary representation. This is relevant if you're dealing with unordered sets or map-structured data where theoretically the order is not relevant, but in practice it is, especially if you want to ensure that cryptographic signatures related to the data structure do not get invalidated accidentally.

Data Structure Default Impl Comment
Vec Defines own ordering
VecDeque Defines own ordering
LinkedList Defines own ordering
HashMap Ordering missing but content is ordered by key byte representation.
BTreeMap Defines own ordering
HashSet (Unordered) Set handling not yet defined
BTreeSet (Unordered) Set handling not yet defined
BinaryHeap Ordering missing
Iterator ~ emit_unchecked_list() allows to emit any iterable but user needs to ensure the ordering.

Attention:

  • Since most list types already define their inner ordering, data structures like Vec, VecDeque, and LinkedList will not get sorted during encoding!

  • There is no default implementation for handling generic iterators. This is by design. Bendy cannot tell from an iterator whether the underlying structure requires sorting or not and would have to take data as-is.

Usage

First you need to add bendy as a project dependency:

[dependencies]
bendy = "^0.2"

Encoding with ToBencode

To encode an object of a type which already implements the ToBencode trait it is enough to import the trait and call the to_bencode() function on the object.

use bendy::encoding::{ToBencode, Error};

fn main() -> Result<(), Error> {
    let my_data = vec!["hello", "world"];
    let encoded = my_data.to_bencode()?;

    assert_eq!(b"l5:hello5:worlde", encoded.as_slice());
    Ok(())
}

Implementing ToBencode

In most cases it should be enough to overwrite the associated encode function and keep the default implementation of to_bencode.

The function will provide you with a SingleItemEncoder which must be used to emit any relevant components of the current object. As long as these implement ToBencode themselves it is enough to pass them into the emit function of the encoder as this will serialize any type implementing the trait.

Next to emit the encoder also provides a list of functions to encode specific bencode primitives (i.e. emit_int and emit_str) and nested bencode elements (i.e. emit_dict and emit_list). These methods should be used if its necessary to output a specific non default data type.

Implementing Integer Encoding

As bencode has native integer support bendy provides default implementations for all of rusts native integer types. This allows to call to_bencode on any integer object and to pass these objects into the encoder's emit_int function.

use bendy::encoding::{ToBencode, SingleItemEncoder, Error};

struct IntegerWrapper(i64);

impl ToBencode for IntegerWrapper {
    const MAX_DEPTH: usize = 0;

    fn encode(&self, encoder: SingleItemEncoder) -> Result<(), Error> {
        encoder.emit_int(self.0)
    } 
}

fn main() -> Result<(), Error> {
    let example = IntegerWrapper(21);
    
    let encoded = example.to_bencode()?;
    assert_eq!(b"i21e", encoded.as_slice());
    
    let encoded = 21.to_bencode()?;
    assert_eq!(b"i21e", encoded.as_slice());
    
    Ok(())
}

Encode a byte string

Another data type bencode natively supports are byte strings. Therefore bendy provides default implementations for String and &str.

use bendy::encoding::{ToBencode, SingleItemEncoder, Error};

struct StringWrapper(String);

impl ToBencode for StringWrapper {
    const MAX_DEPTH: usize = 0;

    fn encode(&self, encoder: SingleItemEncoder) -> Result<(), Error> {
        encoder.emit_str(&self.0)
    } 
}

fn main() -> Result<(), Error> {
    let example = StringWrapper("content".to_string());

    let encoded = example.to_bencode()?;
    assert_eq!(b"7:content", encoded.as_slice());
    
    let encoded = "content".to_bencode()?;
    assert_eq!(b"7:content", encoded.as_slice());
    
    Ok(())
}

As its a very common pattern to represent a byte string as Vec<u8> bendy exposes the AsString wrapper. This can be used to encapsulate any element implementing AsRef<[u8]> to output itself as a bencode string instead of a list.

use bendy::encoding::{ToBencode, SingleItemEncoder, Error, AsString};

struct ByteStringWrapper(Vec<u8>);

impl ToBencode for ByteStringWrapper {
    const MAX_DEPTH: usize = 0;

    fn encode(&self, encoder: SingleItemEncoder) -> Result<(), Error> {
        let content = AsString(&self.0);
        encoder.emit(&content)
    } 
}

fn main() -> Result<(), Error> {
    let example = ByteStringWrapper(b"content".to_vec());
    
    let encoded = example.to_bencode()?;
    assert_eq!(b"7:content", encoded.as_slice());
    
    let encoded = AsString(b"content").to_bencode()?;
    assert_eq!(b"7:content", encoded.as_slice());
    
    Ok(())
}

Encode a dictionary

If a data structure contains key-value pairs its most likely a good idea to encode it as a bencode dictionary. This is also true for most structs with more then one member as it might be helpful to represent their names to ensure the existence of specific (optional) member.

Attention: To ensure a canonical representation bendy requires that the keys of a dictionary emitted via emit_dict are sorted in ascending order or the encoding will fail with an error of kind UnsortedKeys. In case of an unsorted dictionary it might be useful to use emit_and_sort_dict instead.

use bendy::encoding::{ToBencode, SingleItemEncoder, Error};

struct Example {
    label: String,
    counter: u64,
}

impl ToBencode for Example {
    const MAX_DEPTH: usize = 1;

    fn encode(&self, encoder: SingleItemEncoder) -> Result<(), Error> {
        encoder.emit_dict(|mut e| {
            e.emit_pair(b"counter", &self.counter)?;
            e.emit_pair(b"label", &self.label)?;
            
            Ok(())
        })
    }
}

fn main() -> Result<(), Error> {
    let example = Example { label: "Example".to_string(), counter: 0 };
    
    let encoded = example.to_bencode()?;
    assert_eq!(b"d7:counteri0e5:label7:Examplee", encoded.as_slice());
    
    Ok(())
}

Encode a list

While encoding a list bendy assumes the elements inside this list are inherently sorted through their position inside the list. The implementation is therefore free to choose its own sorting.

use bendy::encoding::{ToBencode, SingleItemEncoder, Error};

struct Location(i64, i64);

impl ToBencode for Location {
    const MAX_DEPTH: usize = 1;

    fn encode(&self, encoder: SingleItemEncoder) -> Result<(), Error> {
        encoder.emit_list(|e| {
            e.emit_int(self.0)?;
            e.emit_int(self.1)
        })
    }
}

fn main() -> Result<(), Error> {
    let example = Location(2, 3);

    let encoded = example.to_bencode()?;
    assert_eq!(b"li2ei3ee", encoded.as_slice());
    
    Ok(())
}

Decoding with FromBencode

To decode an object of a type which already implements the FromBencode trait it is enough to import the trait and call the from_bencode() function on the object.

use bendy::decoding::{FromBencode, Error};

fn main() -> Result<(), Error> {
    let encoded = b"l5:hello5:worlde".to_vec();
    let decoded = Vec::<String>::from_bencode(&encoded)?;
    
    assert_eq!(vec!["hello", "world"], decoded);
    Ok(())
}

Implementing FromBencode

In most cases it should be enough to overwrite the associated decode_bencode_object function and keep the default implementation of from_bencode.

The function will provide you with an representation of a bencode Object which must be processed to receive any relevant components of the expected data type. As long as these implement FromBencode themselves it is enough to call decode_bencode_object on the expected data type of the element as this will deserialize any type implementing the trait.

Next to from_bencode the bencode Object representation also provides a list of helper functions to itself into specific bencode primitives and container (i.e. bytes_or, integer_or_else or try_into_list). Which than can be used to restore the actual element.

Decode an integer

As bencode has native integer support bendy provides default implementations for all of rusts native integer types. This allows to call from_bencode on any type of integer.

Attention: If it's necessary to handle a big integer which has no representation through one of the default data types it's always possible to access the string version of the number during decoding.

use bendy::decoding::{FromBencode, Object, Error};

#[derive(Debug, Eq, PartialEq)]
struct IntegerWrapper(i64);

impl FromBencode for IntegerWrapper {
    const EXPECTED_RECURSION_DEPTH: usize = 0;

    fn decode_bencode_object(object: Object) -> Result<Self, Error> {
        // This is an example for content handling. It would also be possible
        // to call  `i64::decode_bencode_object(object)` directly.
        let content = object.try_into_integer()?;
        let number = content.parse::<i64>()?;

        Ok(IntegerWrapper(number))
    }
}

fn main() -> Result<(), Error> {
    let encoded = b"i21e".to_vec();
    
    let example = IntegerWrapper::from_bencode(&encoded)?;
    assert_eq!(IntegerWrapper(21), example);
    
    let example = i64::from_bencode(&encoded)?;
    assert_eq!(21, example);
    
    Ok(())
}

Decode a byte string

In most cases it is possible to restore a string from its bencode representation as a byte sequence via the String::from_utf8 and str::from_utf8.

use bendy::decoding::{FromBencode, Object, Error};

#[derive(Debug, Eq, PartialEq)]
struct StringWrapper(String);

impl FromBencode for StringWrapper {
    const EXPECTED_RECURSION_DEPTH: usize = 0;

    fn decode_bencode_object(object: Object) -> Result<Self, Error> {
        // This is an example for content handling. It would also be possible
        // to call  `String::decode_bencode_object(object)` directly.
        let content = object.try_into_bytes()?;
        let content = String::from_utf8(content.to_vec())?;
        
        Ok(StringWrapper(content))
    }
}

fn main() -> Result<(), Error> {
    let encoded = b"7:content".to_vec();
    
    let example = StringWrapper::from_bencode(&encoded)?;
    assert_eq!(StringWrapper("content".to_string()), example);
    
    let example = String::from_bencode(&encoded)?;
    assert_eq!("content".to_string(), example);
    
    Ok(())
}

If the content is a non utf8 encoded string or an actual byte sequence the AsString wrapper might be useful to restore the bencode string object as a sequence of bytes through an object of type Vec<u8>.

use bendy::{
    decoding::{FromBencode, Object, Error},
    encoding::AsString,
};

#[derive(Debug, Eq, PartialEq)]
struct ByteStringWrapper(Vec<u8>);

impl FromBencode for ByteStringWrapper {
    const EXPECTED_RECURSION_DEPTH: usize = 0;

    fn decode_bencode_object(object: Object) -> Result<Self, Error> {
        let content = AsString::decode_bencode_object(object)?;
        Ok(ByteStringWrapper(content.0))
    }
}

fn main() -> Result<(), Error> {
    let encoded = b"7:content".to_vec();
    
    let example = ByteStringWrapper::from_bencode(&encoded)?;
    assert_eq!(ByteStringWrapper(b"content".to_vec()), example);
    
    let example = AsString::from_bencode(&encoded)?;
    assert_eq!(b"content".to_vec(), example.0);
    
    Ok(())
}

Decode a dictionary

Unwrapping the bencode object into a dictionary will provide a dictionary decoder which can be used to access the included key-value pairs.

To improve the error handling in case of huge or multiple nested dictionaries the decoding module provides a ResultExt trait which allows to add a context description in case of an error. If multiple context calls are nested they will concatenated in a dot notation like style.

use bendy::decoding::{FromBencode, Object, Error, ResultExt};

#[derive(Debug, Eq, PartialEq)]
struct Example {
    label: String,
    counter: u64,
}

impl FromBencode for Example {
    const EXPECTED_RECURSION_DEPTH: usize = 1;

    fn decode_bencode_object(object: Object) -> Result<Self, Error> {
        let mut counter = None;
        let mut label = None;
        
        let mut dict = object.try_into_dictionary()?;
        while let Some(pair) = dict.next_pair()? {
            match pair {
                (b"counter", value) => {
                    counter = u64::decode_bencode_object(value)
                        .context("counter")
                        .map(Some)?;
                },
                (b"label", value) => {
                    label = String::decode_bencode_object(value)
                        .context("label")
                        .map(Some)?;
                },
                (unknown_field, _) => {
                    return Err(Error::unexpected_field(String::from_utf8_lossy(
                        unknown_field,
                    )));
                },
            }
        }
        
        let counter = counter.ok_or_else(|| Error::missing_field("counter"))?;
        let label= label.ok_or_else(|| Error::missing_field("label"))?;

        Ok(Example { counter, label })
    }
}

fn main() -> Result<(), Error> {
    let encoded = b"d7:counteri0e5:label7:Examplee".to_vec();
    let expected = Example { label: "Example".to_string(), counter: 0 };
    
    let example = Example::from_bencode(&encoded)?;
    assert_eq!(expected, example);
    
    Ok(())
}

Decode a list

Unwrapping the bencode object into a list will provide a list decoder which can be used to access the contained elements.

use bendy::decoding::{FromBencode, Object, Error};

#[derive(Debug, PartialEq, Eq)]
struct Location(i64, i64);

impl FromBencode for Location {
    const EXPECTED_RECURSION_DEPTH: usize = 1;

    fn decode_bencode_object(object: Object) -> Result<Self, Error> {
        let mut list = object.try_into_list()?;
        
        let x = list.next_object()?.ok_or(Error::missing_field("x"))?;
        let x = i64::decode_bencode_object(x)?;
        
        let y = list.next_object()?.ok_or(Error::missing_field("y"))?;
        let y = i64::decode_bencode_object(y)?;

        Ok(Location(x, y))
    }
}

fn main() -> Result<(), Error> {
    let encoded = b"li2ei3ee".to_vec();
    let expected = Location(2, 3);

    let example = Location::from_bencode(&encoded)?;
    assert_eq!(expected, example);
    
    Ok(())
}

Optional: Limitation of recursive parsing

What?

The library allows to set an expected recursion depth limit for de- and encoding. If set, the parser will use this value as an upper limit for the validation of any nested data structure and abort with an error if an additional level of nesting is detected.

While the encoding limit itself is primarily there to increase the confidence of bendy users in their own validation code, the decoding limit should be used to avoid parsing of malformed or malicious external data.

  • The encoding limit can be set through the MAX_DEPTH constant in any implementation of the ToBencode trait.
  • The decoding limit can be set through the EXPECTED_RECURSION_DEPTH constant in any implementation of the FromBencode trait.

How?

The nesting level calculation always starts on level zero, is incremented by one when the parser enters a nested bencode element (i.e. list, dictionary) and decrement as soon as the related element ends. Therefore any values decoded as bencode strings or integers do not affect the nesting limit.

Usage of unsafe code

The parser would not require any unsafe code to work but it still contains a single unsafe call to str::from_utf8_unchecked. This call is used to avoid a duplicated UTF-8 check when the parser converts the bytes representing an incoming integer into a &str after its successful validation.

Disclaimer: Further unsafe code may be introduced through the dependency on the failure crate.

Contributing

We welcome everyone to ask questions, open issues or provide merge requests. Each merge request will be reviewed and either landed in the main tree or given feedback for changes that would be required.

All code in this repository is under the BSD-3-Clause license.

Dependencies

~361KB