A piece of ASN.1 with tags
{ product-based [0] NULL ,
Notes:
This slide is not specifically about tagging - but includes a common use of tags. The text discusses the tags in the last paragraph - skip to there if you wish!
This is a data structure capable of communicating information about an employee of a firm. Notice the use of “SET” (fields transmitted in any order). This was common in early ASN.1 specifications, but is today frowned on - encoder’s options are today considered bad as they produce both security and testing problems.
The “identification” is “Name” in red, which means that it is a user-defined type (presumably more complex than a simple character string) defined earlier or later in the specification. “title” is “UTF8String”, another (more modern) character string type capable of holding any of the characters in the world. UTF8String is discussed fully near the end of this course.
We then have the “division”, which is a choice of three possibilities - the “marketing” division, the “research” division, and the “production” division.
In the case of the “research” division, we can indicate “product-based” or “basic” research, but wish to transmit no additional information, so the built-in type “NULL” is used here.
Finally, we must mention the numbers in square brackets. These are called “tags”, and were a very visible feature of early ASN.1 specifications (and still appear today). Post-1994, it is not necessary to include tags (you can have them automatically inserted where needed), but many writers of ASN.1 take a pride in understanding tagging, and it is this that this tutorial addresses.