# module OpenSSL::ASN1

Abstract Syntax Notation One (or ASN.1) is a notation syntax to describe data structures and is defined in ITU-T X.680. ASN.1 itself does not mandate any encoding or parsing rules, but usually ASN.1 data structures are encoded using the Distinguished `Encoding` Rules (DER) or less often the Basic `Encoding` Rules (BER) described in ITU-T X.690. DER and BER encodings are binary Tag-Length-Value (TLV) encodings that are quite concise compared to other popular data description formats such as XML, `JSON` etc. ASN.1 data structures are very common in cryptographic applications, e.g. X.509 public key certificates or certificate revocation lists (CRLs) are all defined in ASN.1 and DER-encoded. ASN.1, DER and BER are the building blocks of applied cryptography. The `ASN1` module provides the necessary classes that allow generation of ASN.1 data structures and the methods to encode them using a DER encoding. The decode method allows parsing arbitrary BER-/DER-encoded data to a Ruby object that can then be modified and re-encoded at will.

## ASN.1 class hierarchy¶ ↑

The base class representing ASN.1 structures is `ASN1Data`. `ASN1Data` offers attributes to read and set the tag, the tag_class and finally the value of a particular ASN.1 item. Upon parsing, any tagged values (implicit or explicit) will be represented by `ASN1Data` instances because their “real type” can only be determined using out-of-band information from the ASN.1 type declaration. Since this information is normally known when encoding a type, all sub-classes of `ASN1Data` offer an additional attribute tagging that allows to encode a value implicitly (`:IMPLICIT`) or explicitly (`:EXPLICIT`).

### `Constructive`¶ ↑

`Constructive` is, as its name implies, the base class for all constructed encodings, i.e. those that consist of several values, opposed to “primitive” encodings with just one single value. The value of an `Constructive` is always an `Array`.

#### `ASN1::Set` and ASN1::Sequence¶ ↑

The most common constructive encodings are SETs and SEQUENCEs, which is why there are two sub-classes of `Constructive` representing each of them.

### `Primitive`¶ ↑

This is the super class of all primitive values. `Primitive` itself is not used when parsing ASN.1 data, all values are either instances of a corresponding sub-class of `Primitive` or they are instances of `ASN1Data` if the value was tagged implicitly or explicitly. Please cf. `Primitive` documentation for details on sub-classes and their respective mappings of ASN.1 data types to Ruby objects.

## Possible values for tagging¶ ↑

When constructing an `ASN1Data` object the ASN.1 type definition may require certain elements to be either implicitly or explicitly tagged. This can be achieved by setting the tagging attribute manually for sub-classes of `ASN1Data`. Use the symbol `:IMPLICIT` for implicit tagging and `:EXPLICIT` if the element requires explicit tagging.

## Possible values for tag_class¶ ↑

It is possible to create arbitrary `ASN1Data` objects that also support a PRIVATE or APPLICATION tag class. Possible values for the tag_class attribute are:

- `:UNIVERSAL` (the default for untagged values)

- `:CONTEXT_SPECIFIC` (the default for tagged values)

- `:APPLICATION`

- `:PRIVATE`

## Tag constants¶ ↑

There is a constant defined for each universal tag:

- OpenSSL::ASN1::EOC (0)

- OpenSSL::ASN1::BOOLEAN (1)

- OpenSSL::ASN1::INTEGER (2)

- OpenSSL::ASN1::BIT_STRING (3)

- OpenSSL::ASN1::OCTET_STRING (4)

- OpenSSL::ASN1::NULL (5)

- OpenSSL::ASN1::OBJECT (6)

- OpenSSL::ASN1::ENUMERATED (10)

- OpenSSL::ASN1::UTF8STRING (12)

- OpenSSL::ASN1::SEQUENCE (16)

- OpenSSL::ASN1::SET (17)

- OpenSSL::ASN1::NUMERICSTRING (18)

- OpenSSL::ASN1::PRINTABLESTRING (19)

- OpenSSL::ASN1::T61STRING (20)

- OpenSSL::ASN1::VIDEOTEXSTRING (21)

- OpenSSL::ASN1::IA5STRING (22)

- OpenSSL::ASN1::UTCTIME (23)

- OpenSSL::ASN1::GENERALIZEDTIME (24)

- OpenSSL::ASN1::GRAPHICSTRING (25)

- OpenSSL::ASN1::ISO64STRING (26)

- OpenSSL::ASN1::GENERALSTRING (27)

- OpenSSL::ASN1::UNIVERSALSTRING (28)

- OpenSSL::ASN1::BMPSTRING (30)

## `UNIVERSAL_TAG_NAME` constant¶ ↑

An `Array` that stores the name of a given tag number. These names are the same as the name of the tag constant that is additionally defined, e.g. `UNIVERSAL_TAG_NAME[2] = "INTEGER"` and `OpenSSL::ASN1::INTEGER = 2`.

## Example usage¶ ↑

### Decoding and viewing a DER-encoded file¶ ↑

    require 'openssl'
    require 'pp'
    der = File.binread('data.der')
    asn1 = OpenSSL::ASN1.decode(der)
    pp der

### Creating an ASN.1 structure and DER-encoding it¶ ↑

    require 'openssl'
    version = OpenSSL::ASN1::Integer.new(1)
    # Explicitly 0-tagged implies context-specific tag class
    serial = OpenSSL::ASN1::Integer.new(12345, 0, :EXPLICIT, :CONTEXT_SPECIFIC)
    name = OpenSSL::ASN1::PrintableString.new('Data 1')
    sequence = OpenSSL::ASN1::Sequence.new( [ version, serial, name ] )
    der = sequence.to_der

### Constants

CLASS_TAG_MAP
UNIVERSAL_TAG_NAME

  `Array` storing tag names at the tag’s index.

### Public Class Methods

OpenSSL::ASN1.decode(der) → ASN1Data
Source

    static VALUE
    ossl_asn1_decode(VALUE self, VALUE obj)
    {
        VALUE ret;
        unsigned char *p;
        VALUE tmp;
        long len, read = 0, offset = 0;

        obj = ossl_to_der_if_possible(obj);
        tmp = rb_str_new4(StringValue(obj));
        p = (unsigned char *)RSTRING_PTR(tmp);
        len = RSTRING_LEN(tmp);
        ret = ossl_asn1_decode0(&p, len, &offset, 0, 0, &read);
        RB_GC_GUARD(tmp);
        int_ossl_decode_sanity_check(len, read, offset);
        return ret;
    }

Decodes a BER- or DER-encoded value and creates an `ASN1Data` instance. der may be a `String` or any object that features a `.to_der` method transforming it into a BER-/DER-encoded String+

## Example¶ ↑

    der = File.binread('asn1data')
    asn1 = OpenSSL::ASN1.decode(der)

OpenSSL::ASN1.decode_all(der) → Array of ASN1Data
Source

    static VALUE
    ossl_asn1_decode_all(VALUE self, VALUE obj)
    {
        VALUE ary, val;
        unsigned char *p;
        long len, tmp_len = 0, read = 0, offset = 0;
        VALUE tmp;

        obj = ossl_to_der_if_possible(obj);
        tmp = rb_str_new4(StringValue(obj));
        p = (unsigned char *)RSTRING_PTR(tmp);
        len = RSTRING_LEN(tmp);
        tmp_len = len;
        ary = rb_ary_new();
        while (tmp_len > 0) {
            long tmp_read = 0;
            val = ossl_asn1_decode0(&p, tmp_len, &offset, 0, 0, &tmp_read);
            rb_ary_push(ary, val);
            read += tmp_read;
            tmp_len -= tmp_read;
        }
        RB_GC_GUARD(tmp);
        int_ossl_decode_sanity_check(len, read, offset);
        return ary;
    }

Similar to decode with the difference that decode expects one distinct value represented in der. decode_all on the contrary decodes a sequence of sequential BER/DER values lined up in der and returns them as an array.

## Example¶ ↑

    ders = File.binread('asn1data_seq')
    asn1_ary = OpenSSL::ASN1.decode_all(ders)

OpenSSL::ASN1.traverse(asn1) → nil
Source

    static VALUE
    ossl_asn1_traverse(VALUE self, VALUE obj)
    {
        unsigned char *p;
        VALUE tmp;
        long len, read = 0, offset = 0;

        obj = ossl_to_der_if_possible(obj);
        tmp = rb_str_new4(StringValue(obj));
        p = (unsigned char *)RSTRING_PTR(tmp);
        len = RSTRING_LEN(tmp);
        ossl_asn1_decode0(&p, len, &offset, 0, 1, &read);
        RB_GC_GUARD(tmp);
        int_ossl_decode_sanity_check(len, read, offset);
        return Qnil;
    }

If a block is given, it prints out each of the elements encountered. Block parameters are (in that order):

- depth: The recursion depth, plus one with each constructed value being encountered (`Integer`)

- offset: Current byte offset (`Integer`)

- header length: Combined length in bytes of the Tag and Length headers. (`Integer`)

- length: The overall remaining length of the entire data (`Integer`)

- constructed: Whether this value is constructed or not (`Boolean`)

- tag_class: Current tag class (`Symbol`)

- tag: The current tag number (`Integer`)

## Example¶ ↑

    der = File.binread('asn1data.der')
    OpenSSL::ASN1.traverse(der) do | depth, offset, header_len, length, constructed, tag_class, tag|
      puts "Depth: #{depth} Offset: #{offset} Length: #{length}"
      puts "Header length: #{header_len} Tag: #{tag} Tag class: #{tag_class} Constructed: #{constructed}"
    end