ASN.1 encoding rules are sets of rules used to transform data specified in the ASN.1 language into a standard format that can be decoded on any system that has a decoder based on the same set of rules. ASN.1 and its encoding rules were once part of the same standard. They have since been separated, but it is still common for the terms ASN.1 and BER (Basic Encoding Rules) to be used to mean the same thing, though this is not the case. Different encoding rules can be applied to a given ASN.1 definition. The choice of encoding rules used is an option of the protocol designer.

The ASN.1 encoding rules currently standardized are: Basic Encoding Rules (BER), Distinguished Encoding Rules (DER), Canonical Encoding Rules (CER), Packed Encoding Rules (PER), XML Encoding Rules (XER) and Extended XML Encoding Rules (E-XER).

BER was created in the early 1980s and is used in a wide range of applications, such as Simple Network Management Protocol (SNMP) for management of the Internet; Message Handling Services (MHS) for exchange of electronic mail and TSAPI for control of telephone/computer interactions.

DER is a specialized form of BER that is used in security-conscious applications. These applications, such as electronic commerce, typically involve cryptography, and require that there be one and only one way to encode and decode a message.

CER is another specialized form of BER that is similar to DER, but is meant for use with messages so huge that it is easiest to start encoding them before their entire value is fully available. CER is rarely used, as the industry has locked onto DER as the preferred means of encoding values for use in secure exchanges.

PER is more recent than the above sets of encoding rules and is noted for its efficient algorithms that result in faster and more compact encodings than BER. PER is used in applications that are bandwidth or CPU starved, such as air traffic control and audiovisual telecommunications.

XER (XML Encoding Rules) allow you to encode a message that has been defined via ASN.1 using XML. You can now add visibility to your ASN.1-described messages via XML.

E-XER (Extended XML Encoding Rules) is an amendment to the ITU-T Rec. X.693 (23002) ASN.1 Encoding Rules: Specification of XML Encoding Rules (XER). Extended-XER encoding makes ASN.1 an XML schema notation as powerful as XSD, with the simplicity of ASN.1.