# asn(n) 0.8 asn "ASN.1 processing"

## Name

asn - ASN.1 BER encoder/decoder

## Table Of Contents

## Synopsis

- package require Tcl 8.4
- package require asn ?0.8.4?

- ::asn::asnSequence evalue...
- ::asn::asnSequenceFromList elist
- ::asn::asnSet evalue...
- ::asn::asnSetFromList elist
- ::asn::asnApplicationConstr appNumber evalue...
- ::asn::asnApplication appNumber data
- ::asn::asnChoice appNumber evalue...
- ::asn::asnChoiceConstr appNumber evalue...
- ::asn::asnInteger number
- ::asn::asnEnumeration number
- ::asn::asnBoolean bool
- ::asn::asnContext context data
- ::asn::asnContextConstr context evalue...
- ::asn::asnObjectIdentifier idlist
- ::asn::asnUTCTime utcstring
- ::asn::asnNull
- ::asn::asnBitString string
- ::asn::asnOctetString string
- ::asn::asnNumericString string
- ::asn::asnPrintableString string
- ::asn::asnIA5String string
- ::asn::asnBMPString string
- ::asn::asnUTF8String string
- ::asn::asnString string
- ::asn::defaultStringType ?type?
- ::asn::asnPeekByte data_var byte_var
- ::asn::asnGetLength data_var length_var
- ::asn::asnGetResponse chan data_var
- ::asn::asnGetInteger data_var int_var
- ::asn::asnGetEnumeration data_var enum_var
- ::asn::asnGetOctetString data_var string_var
- ::asn::asnGetString data_var string_var ?type_var?
- ::asn::asnGetNumericString data_var string_var
- ::asn::asnGetPrintableString data_var string_var
- ::asn::asnGetIA5String data_var string_var
- ::asn::asnGetBMPString data_var string_var
- ::asn::asnGetUTF8String data_var string_var
- ::asn::asnGetUTCTime data_var utc_var
- ::asn::asnGetBitString data_var bits_var
- ::asn::asnGetObjectIdentifier data_var oid_var
- ::asn::asnGetBoolean data_var bool_var
- ::asn::asnGetNull data_var
- ::asn::asnGetSequence data_var sequence_var
- ::asn::asnGetSet data_var set_var
- ::asn::asnGetApplication data_var appNumber_var ?content_var? ?encodingType_var?
- ::asn::asnGetContext data_var contextNumber_var ?content_var? ?encodingType_var?
- ::asn::asnPeekTag data_var tag_var tagtype_var constr_var
- ::asn::asnTag tagnumber ?class? ?tagstyle?
- ::asn::asnRetag data_var newTag

## Description

The asn package provides partial de- and encoder commands for BER encoded ASN.1 data. It can also be used for decoding DER, which is a restricted subset of BER.

ASN.1 is a standard Abstract Syntax Notation, and BER are its Basic Encoding Rules.

See http://asn1.elibel.tm.fr/en/standards/index.htm for more information about the standard.

Also see http://luca.ntop.org/Teaching/Appunti/asn1.html for A Layman's Guide to a Subset of ASN.1, BER, and DER, an RSA Laboratories Technical Note by Burton S. Kaliski Jr. (Revised November 1, 1993). A text version of this note is part of the module sources and should be read by any implementor.

## PUBLIC API

### ENCODER

::asn::asnSequence evalue...

  Takes zero or more encoded values, packs them into an ASN sequence and returns its encoded binary form.

::asn::asnSequenceFromList elist

  Takes a list of encoded values, packs them into an ASN sequence and returns its encoded binary form.

::asn::asnSet evalue...

  Takes zero or more encoded values, packs them into an ASN set and returns its encoded binary form.

::asn::asnSetFromList elist

  Takes a list of encoded values, packs them into an ASN set and returns its encoded binary form.

::asn::asnApplicationConstr appNumber evalue...

  Takes zero or more encoded values, packs them into an ASN application construct and returns its encoded binary form.

::asn::asnApplication appNumber data

  Takes a single encoded value data, packs it into an ASN application construct and returns its encoded binary form.

::asn::asnChoice appNumber evalue...

  Takes zero or more encoded values, packs them into an ASN choice construct and returns its encoded binary form.

::asn::asnChoiceConstr appNumber evalue...

  Takes zero or more encoded values, packs them into an ASN choice construct and returns its encoded binary form.

::asn::asnInteger number

  Returns the encoded form of the specified integer number.

::asn::asnEnumeration number

  Returns the encoded form of the specified enumeration id number.

::asn::asnBoolean bool

  Returns the encoded form of the specified boolean value bool.

::asn::asnContext context data

  Takes an encoded value and packs it into a constructed value with application tag, the context number.

::asn::asnContextConstr context evalue...

  Takes zero or more encoded values and packs them into a constructed value with application tag, the context number.

::asn::asnObjectIdentifier idlist

  Takes a list of at least 2 integers describing an object identifier (OID) value, and returns the encoded value.

::asn::asnUTCTime utcstring

  Returns the encoded form of the specified UTC time string.

::asn::asnNull

  Returns the NULL encoding.

::asn::asnBitString string

  Returns the encoded form of the specified string.

::asn::asnOctetString string

  Returns the encoded form of the specified string.

::asn::asnNumericString string

  Returns the string encoded as ASN.1 NumericString. Raises an error if the string contains characters other than decimal numbers and space.

::asn::asnPrintableString string

  Returns the string encoding as ASN.1 PrintableString. Raises an error if the string contains characters which are not allowed by the Printable String datatype. The allowed characters are A-Z, a-z, 0-9, space, apostrophe, colon, parentheses, plus, minus, comma, period, forward slash, question mark, and the equals sign.

::asn::asnIA5String string

  Returns the string encoded as ASN.1 IA5String. Raises an error if the string contains any characters outside of the US-ASCII range.

::asn::asnBMPString string

  Returns the string encoded as ASN.1 Basic Multilingual Plane string (Which is essentialy big-endian UCS2).

::asn::asnUTF8String string

  Returns the string encoded as UTF8 String. Note that some legacy applications such as Windows CryptoAPI do not like UTF8 strings. Use BMPStrings if you are not sure.

::asn::asnString string

  Returns an encoded form of string, choosing the most restricted ASN.1 string type possible. If the string contains non-ASCII characters, then there is more than one string type which can be used. See ::asn::defaultStringType.

::asn::defaultStringType ?type?

  Selects the string type to use for the encoding of non-ASCII strings. Returns current default when called without argument. If the argument type is supplied, it should be either UTF8 or BMP to choose UTF8String or BMPString respectively.

### DECODER

General notes:

1. Nearly all decoder commands take two arguments. These arguments are variable names, except for ::asn::asnGetResponse. The first variable contains the encoded ASN value to decode at the beginning, and more, and the second variable is where the value is stored to. The remainder of the input after the decoded value is stored back into the datavariable.

2. After extraction the data variable is always modified first, before by writing the extracted value to its variable. This means that if both arguments refer to the same variable, it will always contain the extracted value after the call, and not the remainder of the input.

::asn::asnPeekByte data_var byte_var

  Retrieve the first byte of the data, without modifing data_var. This can be used to check for implicit tags.

::asn::asnGetLength data_var length_var

  Decode the length information for a block of BER data. The tag has already to be removed from the data.

::asn::asnGetResponse chan data_var

  Reads an encoded ASN sequence from the channel chan and stores it into the variable named by data_var.

::asn::asnGetInteger data_var int_var

  Assumes that an encoded integer value is at the front of the data stored in the variable named data_var, extracts and stores it into the variable named by int_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetEnumeration data_var enum_var

  Assumes that an enumeration id is at the front of the data stored in the variable named data_var, and stores it into the variable named by enum_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetOctetString data_var string_var

  Assumes that a string is at the front of the data stored in the variable named data_var, and stores it into the variable named by string_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetString data_var string_var ?type_var?

  Decodes a user-readable string. This is a convenience function which is able to automatically distinguish all supported ASN.1 string types and convert the input value appropriately. See ::asn::asnGetPrintableString, ::asnGetIA5String, etc. below for the type-specific conversion commands.

  If the optional third argument type_var is supplied, then the type of the incoming string is stored in the variable named by it.

  The function throws the error "Invalid command name asnGetSomeUnsupportedString" if the unsupported string type Unsupported is encountered. You can create the appropriate function "asn::asnGetSomeUnsupportedString" in your application if neccessary.

::asn::asnGetNumericString data_var string_var

  Assumes that a numeric string value is at the front of the data stored in the variable named data_var, and stores it into the variable named by string_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetPrintableString data_var string_var

  Assumes that a printable string value is at the front of the data stored in the variable named data_var, and stores it into the variable named by string_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetIA5String data_var string_var

  Assumes that a IA5 (ASCII) string value is at the front of the data stored in the variable named data_var, and stores it into the variable named by string_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetBMPString data_var string_var

  Assumes that a BMP (two-byte unicode) string value is at the front of the data stored in the variable named data_var, and stores it into the variable named by string_var, converting it into a proper Tcl string. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetUTF8String data_var string_var

  Assumes that a UTF8 string value is at the front of the data stored in the variable named data_var, and stores it into the variable named by string_var, converting it into a proper Tcl string. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetUTCTime data_var utc_var

  Assumes that a UTC time value is at the front of the data stored in the variable named data_var, and stores it into the variable named by utc_var. The UTC time value is stored as a string, which has to be decoded with the usual clock scan commands. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetBitString data_var bits_var

  Assumes that a bit string value is at the front of the data stored in the variable named data_var, and stores it into the variable named by bits_var as a string containing only 0 and 1. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetObjectIdentifier data_var oid_var

  Assumes that a object identifier (OID) value is at the front of the data stored in the variable named data_var, and stores it into the variable named by oid_var as a list of integers. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetBoolean data_var bool_var

  Assumes that a boolean value is at the front of the data stored in the variable named data_var, and stores it into the variable named by bool_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

::asn::asnGetNull data_var

  Assumes that a NULL value is at the front of the data stored in the variable named data_var and removes the bytes used to encode it from the data.

::asn::asnGetSequence data_var sequence_var

  Assumes that an ASN sequence is at the front of the data stored in the variable named data_var, and stores it into the variable named by sequence_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

  The data in sequence_var is encoded binary and has to be further decoded according to the definition of the sequence, using the decoder commands here.

::asn::asnGetSet data_var set_var

  Assumes that an ASN set is at the front of the data stored in the variable named data_var, and stores it into the variable named by set_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands.

  The data in set_var is encoded binary and has to be further decoded according to the definition of the set, using the decoder commands here.

::asn::asnGetApplication data_var appNumber_var ?content_var? ?encodingType_var?

  Assumes that an ASN application construct is at the front of the data stored in the variable named data_var, and stores its id into the variable named by appNumber_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands. If a content_var is specified, then the command places all data associated with it into the named variable, in the binary form which can be processed using the decoder commands of this package. If a encodingType_var is specified, then that var is set to 1 if the encoding is constructed and 0 if it is primitive.

  Otherwise it is the responsibility of the caller to decode the remainder of the application construct based on the id retrieved by this command, using the decoder commands of this package.

::asn::asnGetContext data_var contextNumber_var ?content_var? ?encodingType_var?

  Assumes that an ASN context tag construct is at the front of the data stored in the variable named data_var, and stores its id into the variable named by contextNumber_var. Additionally removes all bytes associated with the value from the data for further processing by the following decoder commands. If a content_var is specified, then the command places all data associated with it into the named variable, in the binary form which can be processed using the decoder commands of this package. If a encodingType_var is specified, then that var is set to 1 if the encoding is constructed and 0 if it is primitive.

  Otherwise it is the responsibility of the caller to decode the remainder of the construct based on the id retrieved by this command, using the decoder commands of this package.

### HANDLING TAGS

Working with ASN.1 you often need to decode tagged values, which use a tag thats different from the universal tag for a type. In those cases you have to replace the tag with the universal tag used for the type, to decode the value. To decode a tagged value use the ::asn::asnRetag to change the tag to the appropriate type to use one of the decoders for primitive values. To help with this the module contains three functions:

::asn::asnPeekTag data_var tag_var tagtype_var constr_var

  The ::asn::asnPeekTag command can be used to take a peek at the data and decode the tag value, without removing it from the data. The tag_var gets set to the tag number, while the tagtype_var gets set to the class of the tag. (Either UNIVERSAL, CONTEXT, APPLICATION or PRIVATE). The constr_var is set to 1 if the tag is for a constructed value, and to 0 for not constructed. It returns the length of the tag.

::asn::asnTag tagnumber ?class? ?tagstyle?

  The ::asn::asnTag can be used to create a tag value. The tagnumber gives the number of the tag, while the class gives one of the classes (UNIVERSAL,CONTEXT,APPLICATION or PRIVATE). The class may be abbreviated to just the first letter (U,C,A,P), default is UNIVERSAL. The tagstyle is either C for Constructed encoding, or P for primitve encoding. It defaults to P. You can also use 1 instead of C and 0 instead of P for direct use of the values returned by ::asn::asnPeekTag.

::asn::asnRetag data_var newTag

  Replaces the tag in front of the data in data_var with newTag. The new Tag can be created using the ::asn::asnTag command.

## EXAMPLES

Examples for the usage of this package can be found in the implementation of package ldap.

## BUGS, IDEAS, FEEDBACK

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category asn of the Tcllib SF Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

## Keywords

asn, ber, cer, der, internet, protocol, x.208, x.209

## Category

Networking

## Copyright

Copyright © 2004 Andreas Kupries <andreas_kupries@users.sourceforge.net>
Copyright © 2004 Jochen Loewer <loewerj@web.de>
Copyright © 2004-2011 Michael Schlenker <mic42@users.sourceforge.net>