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#8 in Parser implementations

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BER/DER Parser

A parser for Basic Encoding Rules (BER [X.690]) and Distinguished Encoding Rules(DER [X.690]), implemented with the nom parser combinator framework.

It is written in pure Rust, fast, and makes extensive use of zero-copy. A lot of care is taken to ensure security and safety of this crate, including design (recursion limit, defensive programming), tests, and fuzzing. It also aims to be panic-free.

Historically, this parser was intended for DER only, and BER support was added later. This may still reflect on some naming schemes, but has no other consequence: the BerObject and DerObject used in this crate are type aliases, so all functions are compatible.

DER parsing functions have additional constraints verification, however.

Serialization has also been added (see Serialization )

The code is available on Github and is part of the Rusticata project.

BER/DER parsers

BER stands for Basic Encoding Rules, and is defined in X.690. It defines a set of rules to encode and decode ASN.1 objects in binary.

X.690 also defines Distinguished Encoding Rules (DER), which is BER with added rules to ensure canonical and unequivocal binary representation of objects.

The choice of which one to use is usually guided by the speficication of the data format based on BER or DER: for example, X.509 uses DER as encoding representation.

See the related modules for object definitions, functions, and example:

  • [ber]: Basic Encoding Rules
  • [der]: Distinguished Encoding Rules


Parse two BER integers:

use der_parser::ber::parse_ber_integer;

let bytes = [ 0x02, 0x03, 0x01, 0x00, 0x01,
              0x02, 0x03, 0x01, 0x00, 0x00,

let (rem, obj1) = parse_ber_integer(&bytes).expect("parsing failed");
let (rem, obj2) = parse_ber_integer(&bytes).expect("parsing failed");

Parse a DER sequence of integers:

use der_parser::der::{parse_der_integer, parse_der_sequence_of};

let bytes = [ 0x30, 0x0a,
              0x02, 0x03, 0x01, 0x00, 0x01,
              0x02, 0x03, 0x01, 0x00, 0x00,

let (rem, seq) = parse_der_sequence_of(parse_der_integer)(&bytes)
                    .expect("parsing failed");

Note: all parsing functions return the remaining (unparsed) bytes and the parsed object, or an error.

DER parser design

Parsing functions are inspired from nom, and follow the same interface. The most common return type is BerResult, that stores the remaining bytes and parsed BerObject, or an error. Reading the nom documentation may help understanding how to write parsers and use the output.

There are two different approaches for parsing DER objects: reading the objects recursively as long as the tags are known, or specifying a description of the expected objects (generally from the ASN.1 description).

The first parsing method can be done using the parse_ber and parse_der methods. It is useful when decoding an arbitrary DER object. However, it cannot fully parse all objects, especially those containing IMPLICIT, OPTIONAL, or DEFINED BY items.

use der_parser::parse_der;

let bytes = [ 0x30, 0x0a,
              0x02, 0x03, 0x01, 0x00, 0x01,
              0x02, 0x03, 0x01, 0x00, 0x00,

let parsed = parse_der(&bytes);

The second (and preferred) parsing method is to specify the expected objects recursively. The following functions can be used:

For example, to read a BER sequence containing two integers:

use der_parser::ber::*;
use der_parser::error::BerResult;

fn localparse_seq(i:&[u8]) -> BerResult {
    parse_ber_sequence_defined(|data| {
        let (rem, a) = parse_ber_integer(data)?;
        let (rem, b) = parse_ber_integer(rem)?;
        Ok((rem, vec![a, b]))

let bytes = [ 0x30, 0x0a,
              0x02, 0x03, 0x01, 0x00, 0x01,
              0x02, 0x03, 0x01, 0x00, 0x00,

let (_, parsed) = localparse_seq(&bytes).expect("parsing failed");

assert_eq!(parsed[0].as_u64(), Ok(65537));
assert_eq!(parsed[1].as_u64(), Ok(65536));

All functions return a BerResult object: the parsed BerObject, an Incomplete value, or an error.

Note that this type is also a Result, so usual functions (map, unwrap etc.) are available.


BER/DER Integers

DER integers can be of any size, so it is not possible to store them as simple integers (they are stored as raw bytes).

To get a simple value, use BerObject::as_u32 (knowning that this method will return an error if the integer is too large), BerObject::as_u64, or use the bigint feature of this crate and use BerObject::as_bigint.

use der_parser::ber::*;

let data = &[0x02, 0x03, 0x01, 0x00, 0x01];

let (_, object) = parse_ber_integer(data).expect("parsing failed");
assert_eq!(object.as_u64(), Ok(65537));

Access to the raw value is possible using the as_slice method.

Parsers, combinators, macros

Some parsing tools (for ex for tagged objects) are available in different forms:

  • parsers: (regular) functions that takes input and create an object
  • combinators: functions that takes parsers (or combinators) as input, and return a function (usually, the parser). They are used (combined) as building blocks to create more complex parsers.
  • macros: these are generally previous (historic) versions of parsers, kept for compatibility. They can sometime reduce the amount of code to write, but are hard to debug. Parsers should be preferred when possible.

Misc Notes

  • The DER constraints are verified if using parse_der.
  • BerObject and DerObject are the same objects (type alias). The only difference is the verification of constraints during parsing.

Rust version requirements

The 5.0 series of der-parser requires Rustc version 1.44 or greater, based on nom 6 dependencies.


Support for encoding BER/DER objects is currently being tested and can be used by activating the serialize feature. Note that current status is experimental.

See the ber_encode_* functions in the ber module, and BerObject::to_vec


  • [X.680] Abstract Syntax Notation One (ASN.1): Specification of basic notation.
  • [X.690] ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER).


See CHANGELOG.md, and UPGRADING.md for instructions for upgrading major versions.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


~40K SLoC