module 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> <tr><th>ExtendedResponse</th><th>C</th><td>107: 139 (0x8b, 0b010001011)</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
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
orfalse
. :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 233 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