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An [ISO]/[ITU-T]/[CCITT] standard for transmitting structured data on networks. |
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Used by [SSL] Certificates, and many protocols such as [SNMP], [LDAP], various [IBM] Protocols, Z39.50, etc. |
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ASN.1 is a standard for transmitting structured binary data over networks. ASN.1 also is a grammer that can be used to define PDU's to be sent over a network. It's frequently seen as [SNMP] [MIB]s. |
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ASN.1 is made up of "encoding" of some chunk of data. An encoding is either: |
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|Identifier Octets|Length Octets|Contents |
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(called the "Definite form") |
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or |
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|Identifier Octets|Length Octets|Contents|End of contets octets |
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(called the "Indefinite form") |
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Definate form is in general used when you know the size of something before hand, indefinate form is used when the size of what you are encoding is unknown. |
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The Identifier Octets are built up like so: |
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|1|2|3|4|5|6|7|8 |
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||Class|Constructed|||||Tag |
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where class is |
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|Value|Class |
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|00b|Universal |
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|01b|Application |
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|10b|Context specific |
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|11b|Private |
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If constructed is 1, then it means that this is a constructed type, not a primative one. |
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The tag says which type it is, if the tag is under <=31, then it's value is used directly, otherwise the tag bits above are set to "11111" and then a sequence of octets that represent the number. The 8th bit of each octet is set to 1 to mean "more octets after this one". The low bits are the 7 bits of the tag, MSB first. |
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"Choice" values encode which choice was taken directly into the tag bits. I don't know why. |
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In definate encodings, if the 8th bit is 0, then it's a "short" encoding of the length, and the length is the lower 7 bits. If the 8th bit is set, and the lower 7 bits are how many __octets__ are used to encode the length(!). This allows for lengths up to 2**1008. The lower 7 bits being set is reserved for future expansion. |
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End of contents is encoded as |
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|Type|Length|Value |
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|00|00| Absent |
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Boolean is encoded as an octet, 0 for false, non-zero for true. |
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Integers are encoded as a sequence of bytes, [MSB] first. |
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Sequences are encoded as a series of encodings. |
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PerryLorier |
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Encodings: |
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|Number|Name |
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|Universal 0|Reserved for encoding rules |
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|Universal 1|Boolean |
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|Universal 2|Integer |
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|Universal 3|Bitstring |
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|Universal 4|Octet String |
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|Universal 5|Null type |
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|Universal 6|Object identifier type |
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|Universal 7|Object descriptor type |
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|Universal 8|External type/Instance of type |
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|Universal 9|Real |
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|Universal 10|Enumerated Type |
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|Universal 11|Embedded PDV (Presentation Data Value) Type |
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|Universal 12-15|Reserved |
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|Universal 16|Sequence/Sequence-of types |
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|Universal 17|Set/Set-of types |
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|Universal 18-22|Charactor String types |
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|Universal 23-24|Time types |
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|Universal 25-30|Charactor String types |
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|Universal 31-..|Reserved |
