DER Parser

A DER (X.690) parser, implemented with the nom parser combinator framework.

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_der method. However, it cannot fully parse all objects, especially those containing IMPLICIT, OPTIONAL, or DEFINED BY items.

# #[macro_use] extern crate der_parser;
use der_parser::parse_der;

# fn main() {
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:

# #[macro_use] extern crate nom;
# #[macro_use] extern crate rusticata_macros;
# #[macro_use] extern crate der_parser;
use der_parser::*;
use nom::{IResult,Err,ErrorKind};

# fn main() {
fn localparse_seq(i:&[u8]) -> IResult<&[u8],DerObject> {
parse_der_sequence_defined!(i,
parse_der_integer,
parse_der_integer
)
}
let bytes = [ 0x30, 0x0a,
0x02, 0x03, 0x01, 0x00, 0x01,
0x02, 0x03, 0x01, 0x00, 0x00,
];
let parsed = localparse_seq(&bytes);
# }

All functions return an IResult object from nom: the parsed DerObject, an Incomplete value, or an error.

Notes

• The DER constraints are not enforced or verified. Because of that, this parser is mostly compatible with BER.
• 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 DerObject::as_u32 (knowning that this method will return an error if the integer is too large), or use the bigint feature of this crate and use DerObject::as_bigint.

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).