1. ASN.1
CNS 4650
Fall 2004
Rev. 2

2. What is ASN.1 Abstract Syntax Notation One
Overcome how different computer systems transmit data
Model parameters exchanged between application entities

3. Why ASN.1? What was available at the time Different hardware
Different development languages

4. “Endian” Problem Taken from Gulliver’s Travels
Whether eggs should be broken on top or bottom
“Little Endian” refers to machines that read bytes right to left (Intel)
“Big Endian” refers to machines that read bytes left to right (Motorola, IBM, SUN)
Data cannot be transferred directly between the two
Tanenbaum’s Stegosaurus”
Tanenbaum’s stegosaurus when transferred directly between endian machine ended up with a very wild name and was 167,772 meters!

5. How to Solve the “Endian” Problem
Marshalling of data
Single “Syntax”
Solutions today
CORBA
ASN.1
Java and .Net (kind of…)

6. Development Languages
Languages do not all contain the same data types
For instance C does not really have a boolean it is usually simulated
C requires ‘\0’ to terminate a string, this is not always the case in other languages
Without “standard” types who has to manipulate the data? The sender or receiver?
Note that C and other languages share many of the same data types today. But that was not the case in former times. For instance the C string is somewhat of a de facto standard for most languages or at least the ability to produce a C style string. This was not always the case.

7. Backus-Naur Format (BNF)
Formal way to describe formal languages
Most often used to describe computer languages
Designed to be unambiguous

8. Built-in Language Types
Boolean
Integer
Octet string
Null
Sequence
Set
Many more

9. Built-in Syntax Definitions
IA5 String
Numeric String
UTC Time
Printable String
IA5 String is International Alphabet No.5 String
Printable string is a string that is printable from a Telex (old…very old)
UTC is Universal Time Coordinate

10. BNF Examples Begin with general then to specifics Uses := { } [ ] |
:= is the left must be replaced by the right
{ } [ ] is for formating and grouping
| is for “or”

11. BNF ASN.1 Example File := SEQUENCE { Owner Owner,
fileName PrintableString,
createDate UTCTime,
contents Any }
Owner := SEQUENCE {
personalName IA5String,
organizationalName IA5String }
Owner is described by the Owner structure below the the file structure

12. Encoding Rules Basic Encoding Rules (BER)
Distinguished Encoding Rules (DER)
Canonical Encoding Rules (CER)
Packet Encoding Rules (PER)
Discuss first two. The others are for information.

13. Basic Encoding Rules (BER)
Represent ASN.1 values as an octect string
Three encoding methods\
Primitive, definite length
Constructed, definite length
Constructed, indefinite length
Simple non-string types employ the primitive, definite-length method; structured types employ either of the constructed methods; and simple string types employ any of the methods, depending on whether the length of the value is known.

14. BER Parts
TLV
Tag octet
Length octet
Value octet

15. Distinguished Encoding Rules (DER)
Subset of BER
Exactly one way to represent the octet string
Restricts certain types (BIT STRING) beyond BER
Used for digital certificates

16. ASN.1 and OIDs Used by LDAP, Kerberos, and SNMP
Value used to uniquely identify every objectclass and attribute
Object Indentifer (OID) is same as ASN.1

17. OIDs Sub-arc No standard on how to delegate number after arc
Example:
No standard on how to delegate number after arc
De-facto standard is to place all objectclasses under a sub-arc and all attributes under a separate sub-arc

18. How to read ASN.1 Numbers