Crate der_parser

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.

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

DER parser design

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 macros can be used: parse_der_sequence_defined and similar functions, parse_der_struct, etc.

For example, to read a sequence containing two integers:

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

fn localparse_seq(i:&[u8]) -> BerResult {
    parse_der_sequence_defined!(i,
        parse_ber_integer >>
        parse_ber_integer
    )
}

let bytes = [ 0x30, 0x0a,
              0x02, 0x03, 0x01, 0x00, 0x01,
              0x02, 0x03, 0x01, 0x00, 0x00,
];
let parsed = localparse_seq(&bytes);

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.

Notes

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

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.

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.

Serialization

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

References

• [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).

Modules

ber	Basic Encoding Rules (BER) objects and parser
der	Basic Encoding Rules (BER) objects and parser
error	Error type for BER/DER parsers
oid	Object ID (OID) representations.

Macros

oid	Procedural macro to get encoded oids, see the oid module.
parse_der_application	Parse an application DER element
parse_der_optional	Parse an optional DER element
parse_der_sequence_defined	Parse a defined sequence of DER elements
parse_der_sequence_defined_m	Deprecated Parse a defined sequence of DER elements (macro version)
parse_der_sequence_of	Parse a sequence of identical DER elements
parse_der_set_defined	Parse a defined set of DER elements
parse_der_set_defined_m	Deprecated Parse a defined set of DER elements
parse_der_set_of	Parse a set of identical DER elements
parse_der_struct	Parse a constructed DER element
parse_der_tagged	Parse a tagged DER element

Functions

parse_ber	Parse BER object recursively
parse_der	Parse DER object recursively