How does [0] and [3] wơrk in ASN1?

Question

I\'m decoding ASN1 (as used in X.509 for HTTPS certificates). I\'m doing pretty well, but there is a thing that I just cannot find and understandable documentation for.

Answer 1

It sounds like you need an introduction to ASN.1 tagging. There are two angles to approach this from. X.690 defines BER/CER/DER encoding rules. As such, it answers the question of how tags are encoded. X.680 defines ASN.1 itself. As such, it defines the syntax and rules for tagging. Both specifications can be found on the ITU-T website. I'll give you a quick overview.

Tags are used in BER/DER/CER to identify types. They are especially useful for distinguishing the components of a SEQUENCE and the alternatives of a CHOICE.

A tag combines a tag class and a tag number. The tag classes are UNIVERSAL, APPLICATION, PRIVATE, and CONTEXT-SPECIFIC. The UNIVERSAL class is basically used for the built-in types. APPLICATION is typically used for user-defined types. CONTEXT-SPECIFIC is typically used for the components inside constructed types (SEQUENCE, CHOICE, SEQUENCE OF). Syntactically, when tags are specified in an ASN.1 module, they are written inside brackets: [ tag_class tag_number ]; for CONTEXT-SPECIFIC, the tag_class is omitted. Thus, [APPLICATION 10] or [0].

While every ASN.1 type has an associated tag, syntactically, there is also the "TaggedType", which is used by an ASN.1 author to specify the tag to encode a type with. Basically, a TaggedType puts a tag prefix ahead of a type. For example:

MyType ::= SEQUENCE {
  field_with_tagged_type [0] UTF8String
}

The tag in a TaggedType is either explicit or implicit. If explicit, this means that I want the original tag to be explicitly encoded. If implicit, this means I am happy to have only the tag that I specified be encoded. In the explicit case, the BER encoding results in a nested TLV (tag-length-value): the outer tag ([0] in the example above), the length, and then another TLV as the value. In the example, this inner TLV would have a tag of [UNIVERSAL 12] for the UTF8String.

Whether the tag is explicit or implicit depends upon how you write the tag and the tagging environment. For example:

MyType2 ::= SEQUENCE {
  field_with_explicit_tag [0] EXPLICIT UTF8String OPTIONAL,
  field_with_implicit_tag [1] IMPLICIT UTF8String OPTIONAL,
  field_with_tag [2] UTF8String OPTIONAL
}

If you specify neither IMPLICIT nor EXPLICIT, there are some rules that define whether the tag is explicit or implicit (see X.680 31). These rules take into consideration the tagging environment defined for the ASN.1 module. The ASN.1 module may specify the tagging environment as IMPLICIT TAGS, EXPLICIT TAGS, or AUTOMATIC TAGS. Roughly speaking, if you don't specify IMPLICIT or EXPLICIT for a tag, the tag will be explicit if the tagging environment is EXPLICIT and implicit if the tagging environment is IMPLICIT or AUTOMATIC. An automatic tagging environment is basically the same as an IMPLICIT tagging environment, except that unique tags are automatically assigned for members of SEQUENCE and CHOICE types.

Note that in the above example, the three components of MyType2 are all optional. In BER/CER/DER, a decoder will know what component is present based on the encoded tag (which obviously better be unique).