asn2c is a project developed at Politecnico di Torino, created by extending and modifying the codebase of the open-source ASN.1 compiler asn1c. The goal of asn2c is to preserve the robustness and maturity of asn1c while introducing new features, improved constraint handling, and a more user-friendly approach to version management and maintainability. Among the enhancements introduced in asn2c are the support for additional ASN.1 constraints, better diagnostic messages, cleaner internal structures for constraint resolution, and several architectural improvements designed to ease future extensions of the compiler.
The original ASN.1 to C compiler (asn1c) takes ASN.1 module files and generates C/C++-compatible source code. This generated code can be used to serialize native C structures into compact and unambiguous BER/OER/PER/XER/JER-based encodings, and to deserialize the resulting data back into structured objects.
ASN.1-based formats are widely adopted across multiple industries. They are used to encode X.509 certificates in HTTPS handshakes, to exchange control messages between mobile phones and cellular networks, and to support car-to-car communication in intelligent transportation systems, among many other applications.
The ASN.1 family of standards is broad and highly complex, and no open-source compiler provides complete coverage of all its features. Nevertheless, asn1c is arguably the most advanced and widely used open-source ASN.1 compiler available today. The asn2c project builds upon this foundation, aiming to push the capabilities of the tool further and to provide a modern, extensible, and developer-friendly environment for ASN.1-to-C code generation.
The original author of asn1c is Lev Walkin vlm@lionet.info.
The author of asn2c, as an extension to asn1c, is Francesco Di Gregorio fra.digregorio.2002@gmail.com, who developed asn2c as part of his final master degree thesis at Politecnico di Torino.
ASN.1 encodings interoperability table
The ASN.1 family of standards define a number of ways to encode data, including byte-oriented (e.g., BER), bit-oriented (e.g., PER), and textual (e.g., XER). Some encoding variants (e.g., DER) are just stricter variants of the more general encodings (e.g., BER).
The interoperability table below specifies which API functions can be used to exchange data in a compatible manner. If you need to produce data conforming to the standard specified in the column 1, use the API function in the column 2. If you need to process data conforming to the standard(s) specified in the column 3, use the API function specified in column 4. See the doc/asn1c-usage.pdf for details.
| Encoding | API function | Understood by | API function |
|---|---|---|---|
| BER | der_encode() | BER | ber_decode() |
| DER | der_encode() | DER, BER | ber_decode() |
| CER | not supported | CER, BER | ber_decode() |
| BASIC-OER | oer_encode() | *-OER | oer_decode() |
| CANONICAL-OER | oer_encode() | *-OER | oer_decode() |
| BASIC-UPER | uper_encode() | *-UPER | uper_decode() |
| CANONICAL-UPER | uper_encode() | *-UPER | uper_decode() |
| BASIC-APER | aper_encode() | *-APER | aper_decode() |
| CANONICAL-APER | aper_encode() | *-APER | aper_decode() |
| BASIC-XER | xer_encode(XER_F_BASIC) | *-XER | xer_decode() |
| CANONICAL-XER | xer_encode(XER_F_CANONICAL) | *-XER | xer_decode() |
| JER | jer_encode() | JER | jer_decode() |
*) Asterisk means both BASIC and CANONICAL variants.
If you haven't installed asn2c yet, read the INSTALL.md file for a short installation guide.
For the list of asn2c command line options, see asn2c -h.
An excellent book on ASN.1 is written by Olivier Dubuisson: "ASN.1 Communication between heterogeneous systems", ISBN:0-12-6333361-0.
After installing the compiler (see INSTALL.md), you may use the asn2c command to compile the ASN.1 specification:
asn2c <module.asn1> # Compile module
If several specifications contain interdependencies, all of them must be specified at the same time:
asn2c <module1.asn1> <module2.asn1> ... # Compile interdependent modules
The asn2c source tarball contains the examples/ directory with several ASN.1 modules and a script to extract the ASN.1 modules from RFC documents. Refer to the examples/README file in that directory.
To compile the X.509 PKI module:
./asn1c/asn2c -P ./examples/rfc3280-*.asn1 # Compile-n-print
In this example, the -P option is to print the compiled text on the standard output. The default behavior is that asn2c compiler creates multiple .c and .h files for every ASN.1 type found inside the specified ASN.1 modules.
The compiler's -E and -EF options are used for testing the parser and the semantic fixer, respectively. These options will instruct the compiler to dump out the parsed (and fixed) ASN.1 specification as it was "understood" by the compiler. It might be useful for checking whether a particular syntactic construction is properly supported by the compiler.
asn2c -EF <module-to-test.asn1> # Check semantic validity
asn2c is an upgraded version of asn1c, designed to support all the most recent ASN.1 features. Therefore, among its advantages, it is able compile the new IEEE 1609.2.1 ASN.1 files that were not initially supported by asn1c.
asn2c also includes a verbose way to check the dependencies of the ASN.1 files processed.
asn2c supports all the original options of asn1c.
The asn2c compiler works by processing the ASN.1 module specifications in several stages:
- During the first stage, the preproccessing stage analyzes all the ASN.1 files and checks for dependencies or for structures that are not naturally supported by the subsequent stages.
- During the second stage, the ASN.1 file is parsed. (Parsing produces an ASN.1 syntax tree for the subsequent levels)
- During the third stage, the syntax tree is "fixed". (Fixing is a process of checking the tree for semantic errors, accompanied by the tree transformation into the canonical form)
- During the fourth stage, the syntax tree is compiled into the target language.
There are several command-line options reserved for printing the results after each stage of operation:
<parser> => print (-E)
<parser> => <fixer> => print (-E -F)
<parser> => <fixer> => <compiler> => print (-P)
<parser> => <fixer> => <compiler> => save-compiled [default]