DER编码和ASN.1

ASN.1抽象语法标记（Abstract Syntax Notation One） ASN.1是一种 ISO/ITU-T 标准，描述了一种对数据进行表示、编码、传输和解码的数据格式。

DER是ASN.1众多编码方案中的一个。

ASN.1 defines the abstract syntax of information but does not restrict the way the information is encoded. Various ASN.1 encoding rules provide the transfer syntax (a concrete representation) of the data values whose abstract syntax is described in ASN.1.

The standard ASN.1 encoding rules include:

• Basic Encoding Rules (BER)
• Canonical Encoding Rules (CER)
• Distinguished Encoding Rules (DER)
• XML Encoding Rules (XER)
• Packed Encoding Rules (PER)
• Generic String Encoding Rules (GSER)

ASN.1 together with specific ASN.1 encoding rules 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.

Application layer protocols such as X.400 electronic mail , X.500 and LDAP directory services , H.323 (VoIP ), BACnet and SNMP use ASN.1 to describe the protocol data units (PDUs) they exchange. It is also extensively used in the Access and Non-Access Strata of UMTS . There are many other application domains of ASN.1 ^{[ 1]} .

A particularly useful new application of ASN.1 is Fast Infoset . Fast Infoset is an international standard that specifies a binary encoding format for the XML Information Set (XML Infoset ) as an alternative to the XML document format. It aims to provide more efficient serialization than the text-based XML format.

Example

Data structures of FooProtocol defined using the ASN.1 notation:

FooProtocol DEFINITIONS ::= BEGIN



FooQuestion ::= SEQUENCE {

trackingNumber INTEGER,

question       IA5String

}



FooAnswer ::= SEQUENCE {

questionNumber INTEGER,

answer         BOOLEAN

}

This could be a specification published by creators of Foo protocol. ASN.1 does not define conversation flows. This is up to the textual description of the protocol.

Assuming a message, which complies with Foo protocol and which will be sent to the receiving party. This particular message (PDU ) is:

myQuestion FooQuestion ::= {

trackingNumber     5,

question           "Anybody there?"

}

To send the above message 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; one of the simplest is Distinguished Encoding Rules (DER) .

The Foo protocol specification should explicitly name one set of encoding rules to use, so that users of the Foo protocol know which one they should use.

[edit ] Example encoded in DER

Below is the data structure shown above encoded in the DER format (all numbers are in hexadecimal):

30 -- tag indicating SEQUENCE

13 -- length in octets



02 -- tag indicating INTEGER

01 -- length in octets

05 -- value



16 -- tag indicating IA5String

0e -- length in octets

41 6e 79 62 6f 64 79 20 74 68 65 72 65 3f -- value 

(Note: DER uses a pattern of tag-length-value triplets)

So what one actually gets is the string of 21 octets:

30 13 02 01 05 16 0e 41 6e 79 62 6f 64 79 20 74 68 65 72 65 3f

The scope of ASN.1 and DER ends here. It is possible to transmit the encoded message to the party by any means (utilizing TCP or any other protocol). The party should be able to decode the octets back using DER.

("Anybody there?" in ASCII)