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>