Tags

ASN.1 types formally differ from each other by carrying distinct tags. A tag is essentially an integer exhibiting certain inner structure.

Individual tags are usually combined into a collection known as TagSet. Tags and tag sets in pyasn1 are immutable objects assigned to ASN.1 types as the tagSet attribute.

Tags can be appended to one another (in EXPLICIT tagging mode) or overridden (IMPLICIT tagging mode) ultimately creating a new ASN.1 subtype.

class Counter64(Integer):
   """
   ASN.1 specification:

   Counter64 ::=
       [APPLICATION 6]
           IMPLICIT INTEGER
   """
   tagSet = Integer.tagSet.tagImplicitly(
       Tag(tagClassApplication, tagFormatSimple, 6)
   )

# alternatively
counter64 = Integer().subtype(
    implicitTag=Tag(tagClassApplication, tagFormatSimple, 6)
)

ASN.1 types can be related to each other via the .isSameTypeWith(), .isSuperTypeOf() and .isSubTypeOf() methods. Internally, the .tagSet of the types are compared along with the value constraints (e.g. .subtypeSpec).

assert Counter64().isSubTypeOf(Integer()) == True
assert Counter64().isSameTypeWith(Integer()) == False

• Solitary tag
• Tag set
• Tag->type map