In Files

Net::BER

Basic Encoding Rules (BER) Support Module

Much of the text below is cribbed from Wikipedia: en.wikipedia.org/wiki/Basic_Encoding_Rules

The ITU Specification is also worthwhile reading: www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf

The Basic Encoding Rules were the original rules laid out by the ASN.1 standard for encoding abstract information into a concrete data stream. The rules, collectively referred to as a transfer syntax in ASN.1 parlance, specify the exact octet sequences which are used to encode a given data item. The syntax defines such elements as: the representations for basic data types, the structure of length information, and the means for defining complex or compound types based on more primitive types. The BER syntax, along with two subsets of BER (the Canonical Encoding Rules and the Distinguished Encoding Rules), are defined by the ITU-T’s X.690 standards document, which is part of the ASN.1 document series.

Encoding

The BER format specifies a self-describing and self-delimiting format for encoding ASN.1 data structures. Each data element is encoded as a type identifier, a length description, the actual data elements, and where necessary, an end-of-content marker. This format allows a receiver to decode the ASN.1 information from an incomplete stream, without requiring any pre-knowledge of the size, content, or semantic meaning of the data.

<Type | Length | Value [| End-of-Content]>

Protocol Data Units (PDU)

Protocols are defined with schema represented in BER, such that a PDU consists of cascaded type-length-value encodings.

Type Tags

BER type tags are represented as single octets (bytes). The lower five bits of the octet are tag identifier numbers and the upper three bits of the octet are used to distinguish the type as native to ASN.1, application-specific, context-specific, or private. See Net::BER::TAG_CLASS and Net::BER::ENCODING_TYPE for more information.

If Class is set to Universal (0b00______), the value is of a type native to ASN.1 (e.g. INTEGER). The Application class (0b01______) is only valid for one specific application. Context_specific (0b10______) depends on the context and private (0b11_______) can be defined in private specifications

If the primitive/constructed bit is zero (0b__0_____), it specifies that the value is primitive like an INTEGER. If it is one (0b__1_____), the value is a constructed value that contains type-length-value encoded types like a SET or a SEQUENCE.

Defined Universal (ASN.1 Native) Types

There are a number of pre-defined universal (native) types.

<table> <tr><th>Name</th><th>Primitive<br />Constructed</th><th>Number</th></tr> <tr><th>EOC (End-of-Content)</th><th>P</th><td>0: 0 (0x0, 0b00000000)</td></tr> <tr><th>BOOLEAN</th><th>P</th><td>1: 1 (0x01, 0b00000001)</td></tr> <tr><th>INTEGER</th><th>P</th><td>2: 2 (0x02, 0b00000010)</td></tr> <tr><th>BIT STRING</th><th>P</th><td>3: 3 (0x03, 0b00000011)</td></tr> <tr><th>BIT STRING</th><th>C</th><td>3: 35 (0x23, 0b00100011)</td></tr> <tr><th>OCTET STRING</th><th>P</th><td>4: 4 (0x04, 0b00000100)</td></tr> <tr><th>OCTET STRING</th><th>C</th><td>4: 36 (0x24, 0b00100100)</td></tr> <tr><th>NULL</th><th>P</th><td>5: 5 (0x05, 0b00000101)</td></tr> <tr><th>OBJECT IDENTIFIER</th><th>P</th><td>6: 6 (0x06, 0b00000110)</td></tr> <tr><th>Object Descriptor</th><th>P</th><td>7: 7 (0x07, 0b00000111)</td></tr> <tr><th>EXTERNAL</th><th>C</th><td>8: 40 (0x28, 0b00101000)</td></tr> <tr><th>REAL (float)</th><th>P</th><td>9: 9 (0x09, 0b00001001)</td></tr> <tr><th>ENUMERATED</th><th>P</th><td>10: 10 (0x0a, 0b00001010)</td></tr> <tr><th>EMBEDDED PDV</th><th>C</th><td>11: 43 (0x2b, 0b00101011)</td></tr> <tr><th>UTF8String</th><th>P</th><td>12: 12 (0x0c, 0b00001100)</td></tr> <tr><th>UTF8String</th><th>C</th><td>12: 44 (0x2c, 0b00101100)</td></tr> <tr><th>RELATIVE-OID</th><th>P</th><td>13: 13 (0x0d, 0b00001101)</td></tr> <tr><th>SEQUENCE and SEQUENCE OF</th><th>C</th><td>16: 48 (0x30, 0b00110000)</td></tr> <tr><th>SET and SET OF</th><th>C</th><td>17: 49 (0x31, 0b00110001)</td></tr> <tr><th>NumericString</th><th>P</th><td>18: 18 (0x12, 0b00010010)</td></tr> <tr><th>NumericString</th><th>C</th><td>18: 50 (0x32, 0b00110010)</td></tr> <tr><th>PrintableString</th><th>P</th><td>19: 19 (0x13, 0b00010011)</td></tr> <tr><th>PrintableString</th><th>C</th><td>19: 51 (0x33, 0b00110011)</td></tr> <tr><th>T61String</th><th>P</th><td>20: 20 (0x14, 0b00010100)</td></tr> <tr><th>T61String</th><th>C</th><td>20: 52 (0x34, 0b00110100)</td></tr> <tr><th>VideotexString</th><th>P</th><td>21: 21 (0x15, 0b00010101)</td></tr> <tr><th>VideotexString</th><th>C</th><td>21: 53 (0x35, 0b00110101)</td></tr> <tr><th>IA5String</th><th>P</th><td>22: 22 (0x16, 0b00010110)</td></tr> <tr><th>IA5String</th><th>C</th><td>22: 54 (0x36, 0b00110110)</td></tr> <tr><th>UTCTime</th><th>P</th><td>23: 23 (0x17, 0b00010111)</td></tr> <tr><th>UTCTime</th><th>C</th><td>23: 55 (0x37, 0b00110111)</td></tr> <tr><th>GeneralizedTime</th><th>P</th><td>24: 24 (0x18, 0b00011000)</td></tr> <tr><th>GeneralizedTime</th><th>C</th><td>24: 56 (0x38, 0b00111000)</td></tr> <tr><th>GraphicString</th><th>P</th><td>25: 25 (0x19, 0b00011001)</td></tr> <tr><th>GraphicString</th><th>C</th><td>25: 57 (0x39, 0b00111001)</td></tr> <tr><th>VisibleString</th><th>P</th><td>26: 26 (0x1a, 0b00011010)</td></tr> <tr><th>VisibleString</th><th>C</th><td>26: 58 (0x3a, 0b00111010)</td></tr> <tr><th>GeneralString</th><th>P</th><td>27: 27 (0x1b, 0b00011011)</td></tr> <tr><th>GeneralString</th><th>C</th><td>27: 59 (0x3b, 0b00111011)</td></tr> <tr><th>UniversalString</th><th>P</th><td>28: 28 (0x1c, 0b00011100)</td></tr> <tr><th>UniversalString</th><th>C</th><td>28: 60 (0x3c, 0b00111100)</td></tr> <tr><th>CHARACTER STRING</th><th>P</th><td>29: 29 (0x1d, 0b00011101)</td></tr> <tr><th>CHARACTER STRING</th><th>C</th><td>29: 61 (0x3d, 0b00111101)</td></tr> <tr><th>BMPString</th><th>P</th><td>30: 30 (0x1e, 0b00011110)</td></tr> <tr><th>BMPString</th><th>C</th><td>30: 62 (0x3e, 0b00111110)</td></tr> </table>

Constants

ENCODING_TYPE

BER encoding type is kept in bit 6 of the tag type octet.

<table> <tr><th>Bitmask</th><th>Definition</th></tr> <tr><th>0b__0_____</th><td>Primitive</td></tr> <tr><th>0b__1_____</th><td>Constructed</td></tr> </table>

MAX_FIXNUM_SIZE

Used for BER-encoding the length and content bytes of a Fixnum integer values.

Null

The default BerIdentifiedNull object.

TAG_CLASS

BER tag classes are kept in bits seven and eight of the tag type octet.

<table> <tr><th>Bitmask</th><th>Definition</th></tr> <tr><th>0b00______</th><td>Universal (ASN.1 Native) Types</td></tr> <tr><th>0b01______</th><td>Application Types</td></tr> <tr><th>0b10______</th><td>Context-Specific Types</td></tr> <tr><th>0b11______</th><td>Private Types</td></tr> </table>

VERSION

Public Class Methods

compile_syntax(syntax) click to toggle source

Accepts a hash of hashes describing a BER syntax and converts it into a byte-keyed object for fast BER conversion lookup. The resulting “compiled” syntax is used by Net::BER::BERParser.

This method should be called only by client classes of Net::BER (e.g., Net::LDAP and Net::SNMP) and not by clients of those classes.

The hash-based syntax uses TAG_CLASS keys that contain hashes of ENCODING_TYPE keys that contain tag numbers with object type markers.

:<TAG_CLASS> => {
  :<ENCODING_TYPE> => {
    <number> => <object-type>
  },
},

Permitted Object Types

:string

A string value, represented as BerIdentifiedString.

:integer

An integer value, represented with Fixnum.

:oid

An Object Identifier value; see X.690 section 8.19. Currently represented with a standard array, but may be better represented as a BerIdentifiedOID object.

:array

A sequence, represented as BerIdentifiedArray.

:boolean

A boolean value, represented as true or false.

:null

A null value, represented as BerIdentifiedNull.

Example

Net::LDAP defines its ASN.1 BER syntax something like this:

class Net::LDAP
  AsnSyntax = Net::BER.compile_syntax({
    :application => {
      :primitive => {
        2 => :null,
      },
      :constructed => {
        0 => :array,
        # ...
      },
    },
    :context_specific => {
      :primitive => {
        0 => :string,
        # ...
      },
      :constructed => {
        0 => :array,
        # ...
      },
    }
    })
end

NOTE

For readability and formatting purposes, Net::LDAP and its siblings actually construct their syntaxes more deliberately, as shown below. Since a hash is passed in the end in any case, the format does not matter.

primitive = { 2 => :null }
constructed = {
  0 => :array,
  # ...
}
application = {
  :primitive => primitive,
  :constructed => constructed
}

primitive = {
  0 => :string,
  # ...
}
constructed = {
  0 => :array,
  # ...
}
context_specific = {
  :primitive => primitive,
  :constructed => constructed
}
AsnSyntax = Net::BER.compile_syntax(:application => application,
                                    :context_specific => context_specific)
# File lib/net/ber.rb, line 230
def self.compile_syntax(syntax)
  # TODO 20100327 AZ: Should we be allocating an array of 256 values
  # that will either be +nil+ or an object type symbol, or should we
  # allocate an empty Hash since unknown values return +nil+ anyway?
  out = [ nil ] * 256
  syntax.each do |tag_class_id, encodings|
    tag_class = TAG_CLASS[tag_class_id]
    encodings.each do |encoding_id, classes|
      encoding = ENCODING_TYPE[encoding_id]
      object_class = tag_class + encoding
      classes.each do |number, object_type|
        out[object_class + number] = object_type
      end
    end
  end
  out
end

[Validate]

Generated with the Darkfish Rdoc Generator 2.