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  • openssl/ossl_asn1.c



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The top-level class representing any ASN.1 object. When parsed by OpenSSL::ASN1.decode, tagged values are always represented by an instance of ASN1Data.

The role of ASN1Data for parsing tagged values

When encoding an ASN.1 type it is inherently clear what original type (e.g. INTEGER, OCTET STRING etc.) this value has, regardless of its tagging. But opposed to the time an ASN.1 type is to be encoded, when parsing them it is not possible to deduce the "real type" of tagged values. This is why tagged values are generally parsed into ASN1Data instances, but with a different outcome for implicit and explicit tagging.

Example of a parsed implicitly tagged value

An implicitly 1-tagged INTEGER value will be parsed as an ASN1Data with

  • tag equal to 1

  • tag_class equal to :CONTEXT_SPECIFIC

  • value equal to a String that carries the raw encoding of the INTEGER.

This implies that a subsequent decoding step is required to completely decode implicitly tagged values.

Example of a parsed explicitly tagged value

An explicitly 1-tagged INTEGER value will be parsed as an ASN1Data with

  • tag equal to 1

  • tag_class equal to :CONTEXT_SPECIFIC

  • value equal to an Array with one single element, an instance of OpenSSL::ASN1::Integer, i.e. the inner element is the non-tagged primitive value, and the tagging is represented in the outer ASN1Data

Example - Decoding an implicitly tagged INTEGER

int = OpenSSL::ASN1::Integer.new(1, 0, :IMPLICIT) # implicit 0-tagged
seq = OpenSSL::ASN1::Sequence.new( [int] )
der = seq.to_der
asn1 = OpenSSL::ASN1.decode(der)
# pp asn1 => #<OpenSSL::ASN1::Sequence:0x87326e0
#              @infinite_length=false,
#              @tag=16,
#              @tag_class=:UNIVERSAL,
#              @tagging=nil,
#              @value=
#                [#<OpenSSL::ASN1::ASN1Data:0x87326f4
#                   @infinite_length=false,
#                   @tag=0,
#                   @tag_class=:CONTEXT_SPECIFIC,
#                   @value="\x01">]>
raw_int = asn1.value[0]
# manually rewrite tag and tag class to make it an UNIVERSAL value
raw_int.tag = OpenSSL::ASN1::INTEGER
raw_int.tag_class = :UNIVERSAL
int2 = OpenSSL::ASN1.decode(raw_int)
puts int2.value # => 1

Example - Decoding an explicitly tagged INTEGER

int = OpenSSL::ASN1::Integer.new(1, 0, :EXPLICIT) # explicit 0-tagged
seq = OpenSSL::ASN1::Sequence.new( [int] )
der = seq.to_der
asn1 = OpenSSL::ASN1.decode(der)
# pp asn1 => #<OpenSSL::ASN1::Sequence:0x87326e0
#              @infinite_length=false,
#              @tag=16,
#              @tag_class=:UNIVERSAL,
#              @tagging=nil,
#              @value=
#                [#<OpenSSL::ASN1::ASN1Data:0x87326f4
#                   @infinite_length=false,
#                   @tag=0,
#                   @tag_class=:CONTEXT_SPECIFIC,
#                   @value=
#                     [#<OpenSSL::ASN1::Integer:0x85bf308
#                        @infinite_length=false,
#                        @tag=2,
#                        @tag_class=:UNIVERSAL
#                        @tagging=nil,
#                        @value=1>]>]>
int2 = asn1.value[0].value[0]
puts int2.value # => 1



Never nil. A Boolean indicating whether the encoding was infinite length (in the case of parsing) or whether an infinite length encoding shall be used (in the encoding case). In DER, every value has a finite length associated with it. But in scenarios where large amounts of data need to be transferred it might be desirable to have some kind of streaming support available. For example, huge OCTET STRINGs are preferably sent in smaller-sized chunks, each at a time. This is possible in BER by setting the length bytes of an encoding to zero and by this indicating that the following value will be sent in chunks. Infinite length encodings are always constructed. The end of such a stream of chunks is indicated by sending a EOC (End of Content) tag. SETs and SEQUENCEs may use an infinite length encoding, but also primitive types such as e.g. OCTET STRINGS or BIT STRINGS may leverage this functionality (cf. ITU-T X.690).


A Number representing the tag number of this ASN1Data. Never nil.


A Symbol representing the tag class of this ASN1Data. Never nil. See ASN1Data for possible values.


Carries the value of a ASN.1 type. Please confer Constructive and Primitive for the mappings between ASN.1 data types and Ruby classes.

Public Class Methods

OpenSSL::ASN1::ASN1Data.new(value, tag, tag_class) => ASN1Data click to toggle source

value: Please have a look at Constructive and Primitive to see how Ruby types are mapped to ASN.1 types and vice versa.

tag: A Number indicating the tag number.

tag_class: A Symbol indicating the tag class. Please cf. ASN1 for possible values.


asn1_int = OpenSSL::ASN1Data.new(42, 2, :UNIVERSAL) # => Same as OpenSSL::ASN1::Integer.new(42)
tagged_int = OpenSSL::ASN1Data.new(42, 0, :CONTEXT_SPECIFIC) # implicitly 0-tagged INTEGER
               static VALUE
ossl_asn1data_initialize(VALUE self, VALUE value, VALUE tag, VALUE tag_class)
        ossl_raise(eASN1Error, "invalid tag class");
    if((SYM2ID(tag_class) == sUNIVERSAL) && NUM2INT(tag) > 31)
        ossl_raise(eASN1Error, "tag number for Universal too large");
    ossl_asn1_set_tag(self, tag);
    ossl_asn1_set_value(self, value);
    ossl_asn1_set_tag_class(self, tag_class);
    ossl_asn1_set_infinite_length(self, Qfalse);

    return self;

Public Instance Methods

to_der => DER-encoded String click to toggle source

Encodes this ASN1Data into a DER-encoded String value. The result is DER-encoded except for the possibility of infinite length encodings. Infinite length encodings are not allowed in strict DER, so strictly speaking the result of such an encoding would be a BER-encoding.

               static VALUE
ossl_asn1data_to_der(VALUE self)
    VALUE value, der, inf_length;
    int tag, tag_class, is_cons = 0;
    long length;
    unsigned char *p;

    value = ossl_asn1_get_value(self);
    if(rb_obj_is_kind_of(value, rb_cArray)){
        is_cons = 1;
        value = join_der(value);

    tag = ossl_asn1_tag(self);
    tag_class = ossl_asn1_tag_class(self);
    inf_length = ossl_asn1_get_infinite_length(self);
    if (inf_length == Qtrue) {
        is_cons = 2;
    if((length = ossl_asn1_object_size(is_cons, RSTRING_LENINT(value), tag)) <= 0)
        ossl_raise(eASN1Error, NULL);
    der = rb_str_new(0, length);
    p = (unsigned char *)RSTRING_PTR(der);
    ossl_asn1_put_object(&p, is_cons, RSTRING_LENINT(value), tag, tag_class);
    memcpy(p, RSTRING_PTR(value), RSTRING_LEN(value));
    p += RSTRING_LEN(value);
    ossl_str_adjust(der, p);

    return der;

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