BER ENCODING Basic Encoding Rules

Basic Encoding Rules What is it?  BER is the original rules laid out by the ASN.1 standard for encoding information into octet string.

Basic Encoding Rules What it does?  Facilitates the exchange of structured data especially between application programs over networks by describing data structures in a way that is independent of machine architecture and implementation language.  To send data through the network one needs to encode it to a string of bits. ASN.1 defines various algorithms to accomplish that task, called Encoding rules. There are plenty of them; the standard is the Basic Encoding Rules (BER).

Basically, BER is used for Encoding abstract information into concrete data stream independent to the machine architecture and implementation language.

Basic Encoding Rules How it works?  BER uses a form of encoding commonly known as Tag-Length-Value or Type-Length-Value. Each item is encoded as a tag, indicating what type it is, a length indicating the size of the object, and a value, which contains the actual contents of the object.  There are three methods to encode an ASN.1 value under BER, the choice of which depends on the type of value and whether the length of the value is known.

BER encoding methods:  Primitive, definite-length encoding  Constructed, definite- length encoding  Constructed, indefinite-length encoding Every method has 3-4 parts:  Identifier octets - These identify the class and tag number of the ASN.1 value, and indicate whether the method is primitive or constructed.  Length octets. - For the definite-length methods, these give the number of contents octets. For the constructed, indefinite-length method, these indicate that the length is indefinite.  Contents octets. - For the primitive, definite-length method, these give a concrete representation of the value. For the constructed methods, these give the concatenation of the BER encodings of the components of the value  End-of-contents octets. - For the constructed, indefinite- length method, these denote the end of the contents. For the other methods, these are absent.

Example encoding the instance “CPE”,51,”A” 1A5String sampleString1= “CPE” 1A5String sampleString2= “A”,” INTEGER sampleInt = 51 I L CI L CI L C CPE51A Identifier octets 16(hex) = (binary) Identifier octets Type Length octets Length Contents octets Value 8bit7bit6bit5bit4bit3bit2bit1bit Class 0 0 P/C 0 Tag Number

Classbit 8bit 7description Universal00 The type is native to ASN.1 Application01 valid for one specific application Context- specific 10 depends on the context Private11 Defined in private specifications P/Cbit 6 Primitive0 Constructed1 8bit7bit6bit5bit4bit3bit2bit1bit Class 0 0 P/C 0 Tag Number