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
-
tagequal to 1 -
tag_classequal to:CONTEXT_SPECIFIC -
valueequal to aStringthat 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
-
tagequal to 1 -
tag_classequal to:CONTEXT_SPECIFIC -
valueequal to anArraywith 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