ASN.1
=====

_Abstract Syntax Notation One_ (ASN.1) allows defining data structures
that can be serialized and deserialized in a cross-platform way.

_Basic Encoding Rules_ (BER) specify a self-describing and
self-delimiting format for encoding ASN.1 data structures.

_Distinguished Encoding Rules_ (DER) is a restricted variant of BER thas
is unequivocal.

DER is a _tag-length-value_ (TLV) encoding:

-   tag (two forms):
    -   _low-tag-number form_: One octet. Bits 8 and 7 specify the
        class, bit 6 has value 0 indicating that the encoding is
        primitive, and bits 5-1 give the tag number.
    -   _high-tag-number form_: Two or more octets. First octet is as in
        low-tag-number form, except that bits 5-1 all have value 1
        Second and following octets give the tag number, base 128, most
        significant digit first, with as few digits as possible, and
        with the bit 8 of each octet except the last set to 1. If bit 6
        has value 1, it indicates that the encoding is constructed.
-   length (two forms):
    -   _Short form_: One octet. Bit 8 has value 0 and bits 7-1 give the
        length.
    -   _Long form_: Two to 127 octets. Bit 8 of first octet has value 1
        and bits 7-1 give the number of additional length octets. Second
        and following octets give the length, base 256, most significant
        digit first.
-   value: actual value as byte array of defined length.

Class encodings:

| Class            | Bit 8 | Bit 7 |
|------------------|-------|-------|
| universal        | 0     | 0     |
| application      | 0     | 1     |
| context-specific | 1     | 0     |
| private          | 1     | 1     |

References
----------

-   [Go asn1 package]
-   [A Layman's Guide to a Subset of ASN.1, BER, and DER]

  [Go asn1 package]: https://golang.org/pkg/encoding/asn1/
  [A Layman's Guide to a Subset of ASN.1, BER, and DER]: http://luca.ntop.org/Teaching/Appunti/asn1.html