oracular (3) Convert::BER.3pm.gz
NAME
Convert::BER - ASN.1 Basic Encoding Rules
SYNOPSIS
use Convert::BER;
$ber = new Convert::BER;
$ber->encode(
INTEGER => 1,
SEQUENCE => [
BOOLEAN => 0,
STRING => "Hello",
],
REAL => 3.7,
);
$ber->decode(
INTEGER => \$i,
SEQUENCE => [
BOOLEAN => \$b,
STRING => \$s,
],
REAL => \$r,
);
DESCRIPTION
WARNING this module is no longer supported, See Convert::ASN1
"Convert::BER" provides an OO interface to encoding and decoding data using the ASN.1 Basic Encoding
Rules (BER), a platform independent way of encoding structured binary data together with the structure.
METHODS
new
new ( BUFFER )
new ( opList )
"new" creates a new "Convert::BER" object.
encode ( opList )
Encode data in opList appending to the data in the buffer.
decode ( opList )
Decode the data in the buffer as described by opList, starting where the last decode finished or
position set by "pos".
buffer ( [ BUFFER ] )
Return the buffer contents. If BUFFER is specified set the buffer contents and reset pos to zero.
pos ( [ POS ] )
Without any arguments "pos" returns the offset where the last decode finished, or the last offset set
by "pos". If POS is specified then POS will be where the next decode starts.
tag ( )
Returns the tag at the current position in the buffer.
length ( )
Returns the length of the buffer.
error ( )
Returns the error message associated with the last method, if any. This value is not automatically
reset. If "encode" or "decode" returns undef, check this.
dump ( [ FH ] )
Dump the buffer to the filehandle "FH", or STDERR if not specified. The output contains the hex dump
of each element, and an ASN.1-like text representation of that element.
hexdump ( [ FH ] )
Dump the buffer to the filehandle "FH", or STDERR if not specified. The output is hex with the
possibly-printable text alongside.
IO METHODS
read ( IO )
write ( IO )
recv ( SOCK )
send ( SOCK [, ADDR ] )
OPLIST
An opList is a list of operator-value pairs. An operator can be any of those defined below, or any
defined by sub-classing "Convert::BER", which will probably be derived from the primitives given here.
The values depend on whether BER is being encoded or decoded:
Encoding
If the value is a scalar, just encode it. If the value is a reference to a list, then encode each
item in the list in turn. If the value is a code reference, then execute the code. If the returned
value is a scalar, encode that value. If the returned value is a reference to a list, encode each
item in the list in turn.
Decoding
If the value is a reference to a scalar, decode the value into the scalar. If the value is a
reference to a list, then decode all the items of this type into the list. Note that there must be at
least one item to decode, otherwise the decode will fail. If the value is a code reference, then
execute the code and decode the value into the reference returned from the evaluated code.
PRIMITIVE OPERATORS
These operators encode and decode the basic primitive types defined by BER.
BOOLEAN
A BOOLEAN value is either true or false.
Encoding
The value is tested for boolean truth, and encoded appropriately.
# Encode a TRUE value
$ber->encode(
BOOLEAN => 1,
) or die;
Decoding
The decoded values will be either 1 or 0.
# Decode a boolean value into $bval
$ber->decode(
BOOLEAN => \$bval,
) or die;
INTEGER
An INTEGER value is either a positive whole number, or a negative whole number, or zero. Numbers can
either be native perl integers, or values of the "Math::BigInt" class.
Encoding
The value is the integer value to be encoded.
$ber->encode(
INTEGER => -123456,
) or die;
Decoding
The value will be the decoded integer value.
$ber->decode(
INTEGER => \$ival,
) or die;
STRING
This is an OCTET STRING, which is an arbitrarily long binary value.
Encoding
The value contains the binary value to be encoded.
$ber->encode(
STRING => "\xC0First character is hex C0",
) or die;
Decoding
The value will be the binary bytes.
$ber->decode(
STRING => \$sval,
) or die;
NULL
There is no value for NULL. You often use NULL in ASN.1 when you want to denote that something else is
absent rather than just not encoding the 'something else'.
Encoding
The values are ignored, but must be present.
$ber->encode(
NULL => undef,
) or die;
Decoding
Dummy values are stored in the returned values, as though they were present in the encoding.
$ber->decode(
NULL => \$nval,
) or die;
OBJECT_ID
An OBJECT_ID value is an OBJECT IDENTIFIER (also called an OID). This is a hierarchically structured
value that is used in protocols to uniquely identify something. For example, SNMP (the Simple Network
Management Protocol) uses OIDs to denote the information being requested, and LDAP (the Lightweight
Directory Access Protocol, RFC 2251) uses OIDs to denote each attribute in a directory entry.
Each level of the OID hierarchy is either zero or a positive integer.
Encoding
The value should be a dotted-decimal representation of the OID.
$ber->encode(
OBJECT_ID => '2.5.4.0', # LDAP objectClass
) or die;
Decoding
The value will be the dotted-decimal representation of the OID.
$ber->decode(
OBJECT_ID => \$oval,
) or die;
ENUM
The ENUMERATED type is effectively the same as the INTEGER type. It exists so that friendly names can be
assigned to certain integer values. To be useful, you should sub-class this operator.
BIT_STRING
The BIT STRING type is an arbitrarily long string of bits - 0's and 1's.
Encoding
The value is a string of arbitrary 0 and 1 characters. As these are packed into 8-bit octets when
encoding and there may not be a multiple of 8 bits to be encoded, trailing padding bits are added in
the encoding.
$ber->encode(
BIT_STRING => '0011',
) or die;
Decoding
The value will be a string of 0 and 1 characters. The string will have the same number of bits as
were encoded (the padding bits are ignored.)
$ber->decode(
BIT_STRING => \$bval,
) or die;
BIT_STRING8
This is a variation of the BIT_STRING operator, which is optimized for writing bit strings which are
multiples of 8-bits in length. You can use the BIT_STRING operator to decode BER encoded with the
BIT_STRING8 operator (and vice-versa.)
Encoding
The value should be the packed bits to encode, not a string of 0 and 1 characters.
$ber->encode(
BIT_STRING8 => pack('B8', '10110101'),
) or die;
Decoding
The value will be the decoded packed bits.
$ber->decode(
BIT_STRING8 => \$bval,
) or die;
REAL
The REAL type encodes an floating-point number. It requires the POSIX module.
Encoding
The value should be the number to encode.
$ber->encode(
REAL => 3.14159265358979,
) or die;
Decoding
The value will be the decoded floating-point value.
$ber->decode(
REAL => \$rval,
);
ObjectDescriptor
The ObjectDescriptor type encodes an ObjectDescriptor string. It is a sub-class of "STRING".
UTF8String
The UTF8String type encodes a string encoded in UTF-8. It is a sub-class of "STRING".
NumericString
The NumericString type encodes a NumericString, which is defined to only contain the characters 0-9 and
space. It is a sub-class of "STRING".
PrintableString
The PrintableString type encodes a PrintableString, which is defined to only contain the characters A-Z,
a-z, 0-9, space, and the punctuation characters ()-+=:',./?. It is a sub-class of "STRING".
TeletexString/T61String
The TeletexString type encodes a TeletexString, which is a string containing characters according to the
T.61 character set. Each T.61 character may be one or more bytes wide. It is a sub-class of "STRING".
T61String is an alternative name for TeletexString.
VideotexString
The VideotexString type encodes a VideotexString, which is a string. It is a sub-class of "STRING".
IA5String
The IA5String type encodes an IA5String. IA5 (International Alphabet 5) is equivalent to US-ASCII. It is
a sub-class of "STRING".
UTCTime
The UTCTime type encodes a UTCTime value. Note this value only represents years using two digits, so it
is not recommended in Y2K-compliant applications. It is a sub-class of "STRING".
UTCTime values must be strings like:
yymmddHHMM[SS]Z
or:
yymmddHHMM[SS]sHHMM
Where yy is the year, mm is the month (01-12), dd is the day (01-31), HH is the hour (00-23), MM is the
minutes (00-60). SS is the optional seconds (00-61).
The time is either terminated by the literal character Z, or a timezone offset. The "Z" character
indicates Zulu time or UTC. The timezone offset specifies the sign s, which is + or -, and the difference
in hours and minutes.
GeneralizedTime
The GeneralizedTime type encodes a GeneralizedTime value. Unlike "UTCTime" it represents years using 4
digits, so is Y2K-compliant. It is a sub-class of "STRING".
GeneralizedTime values must be strings like:
yyyymmddHHMM[SS][.U][Z]
or:
yyyymmddHHMM[SS][.U]sHHMM
Where yyyy is the year, mm is the month (01-12), dd is the day (01-31), HH is the hour (00-23), MM is the
minutes (00-60). SS is the optional seconds (00-61). U is the optional fractional seconds value; a comma
is permitted instead of a dot before this value.
The time may be terminated by the literal character Z, or a timezone offset. The "Z" character indicates
Zulu time or UTC. The timezone offset specifies the sign s, which is + or -, and the difference in hours
and minutes. If there is timezone specified UTC is assumed.
GraphicString
The GraphicString type encodes a GraphicString value. It is a sub-class of "STRING".
VisibleString/ISO646String
The VisibleString type encodes a VisibleString value, which is a value using the ISO646 character set. It
is a sub-class of "STRING".
ISO646String is an alternative name for VisibleString.
GeneralString
The GeneralString type encodes a GeneralString value. It is a sub-class of "STRING".
UniversalString/CharacterString
The UniveralString type encodes a UniveralString value, which is a value using the ISO10646 character
set. Each character in ISO10646 is 4-bytes wide. It is a sub-class of "STRING".
CharacterString is an alternative name for UniversalString.
BMPString
The BMPString type encodes a BMPString value, which is a value using the Unicode character set. Each
character in the Unicode character set is 2-bytes wide. It is a sub-class of "STRING".
CONSTRUCTED OPERATORS
These operators are used to build constructed types, which contain values in different types, like a C
structure.
SEQUENCE
A SEQUENCE is a complex type that contains other types, a bit like a C structure. Elements inside a
SEQUENCE are encoded and decoded in the order given.
Encoding
The value should be a reference to an array containing another opList which defines the elements
inside the SEQUENCE.
$ber->encode(
SEQUENCE => [
INTEGER => 123,
BOOLEAN => [ 1, 0 ],
]
) or die;
Decoding
The value should a reference to an array that contains the opList which decodes the contents of the
SEQUENCE.
$ber->decode(
SEQUENCE => [
INTEGER => \$ival,
BOOLEAN => \@bvals,
]
) or die;
SET
A SET is an complex type that contains other types, rather like a SEQUENCE. Elements inside a SET may be
present in any order.
Encoding
The value is the same as for the SEQUENCE operator.
$ber->encode(
SET => [
INTEGER => 13,
STRING => 'Hello',
]
) or die;
Decoding
The value should be a reference to an equivalent opList to that used to encode the SET. The ordering
of the opList should not matter.
$ber->decode(
SET => [
STRING => \$sval,
INTEGER => \$ival,
]
) or die;
SEQUENCE_OF
A SEQUENCE_OF is an ordered list of other types.
Encoding
The value is a ref followed by an opList. The ref must be a reference to a list or a hash: if it is
to a list, then the opList will be repeated once for every element in the list. If it is to a hash,
then the opList will be repeated once for every key in the hash (note that ordering of keys in a hash
is not guaranteed by perl.)
The remaining opList will then usually contain values which are code references. If the ref is to a
list, then the contents of that item in the list are passed as the only argument to the code
reference. If the ref is to a hash, then only the key is passed to the code.
@vals = ( [ 10, 'Foo' ], [ 20, 'Bar' ] ); # List of refs to lists
$ber->encode(
SEQUENCE_OF => [ \@vals,
SEQUENCE => [
INTEGER => sub { $_[0][0] }, # Passed a ref to the inner list
STRING => sub { $_[0][1] }, # Passed a ref to the inner list
]
]
) or die;
%hash = ( 40 => 'Baz', 30 => 'Bletch' ); # Just a hash
$ber->decode(
SEQUENCE_OF => [ \%hash,
SEQUENCE => [
INTEGER => sub { $_[0] }, # Passed the key
STRING => sub { $hash{$_[0]} }, # Passed the key
]
]
);
Decoding
The value must be a reference to a list containing a ref and an opList. The ref must always be a
reference to a scalar. Each value in the <opList> is usually a code reference. The code referenced is
called with the value of the ref (dereferenced); the value of the ref is incremented for each item in
the SEQUENCE_OF.
$ber->decode(
SEQUENCE_OF => [ \$count,
# In the following subs, make space at the end of an array, and
# return a reference to that newly created space.
SEQUENCE => [
INTEGER => sub { $ival[$_[0]] = undef; \$ival[-1] },
STRING => sub { $sval[$_[0]] = undef; \$sval[-1] },
]
]
) or die;
SET_OF
A SET_OF is an unordered list. This is treated in an identical way to a SEQUENCE_OF, except that no
ordering should be inferred from the list passed or returned.
SPECIAL OPERATORS
BER
It is sometimes useful to construct or deconstruct BER encodings in several pieces. The BER operator lets
you do this.
Encoding
The value should be another "Convert::BER" object, which will be inserted into the buffer. If value
is undefined then nothing is added.
$tmp->encode(
SEQUENCE => [
INTEGER => 20,
STRING => 'Foo',
]
);
$ber->encode(
BER => $tmp,
BOOLEAN => 1
);
Decoding
value should be a reference to a scalar, which will contain a "Convert::BER" object. This object will
contain the remainder of the current sequence or set being decoded.
# After this, ber2 will contain the encoded INTEGER B<and> STRING.
# sval will be ignored and left undefined, but bval will be decoded. The
# decode of ber2 will return the integer and string values.
$ber->decode(
SEQUENCE => [
BER => \$ber2,
STRING => \$sval,
],
BOOLEAN => \$bval,
);
$ber2->decode(
INTEGER => \$ival,
STRING => \$sval2,
);
ANY
This is like the "BER" operator except that when decoding only the next item is decoded and placed into
the "Convert::BER" object returned. There is no difference when encoding.
Decoding
value should be a reference to a scalar, which will contain a "Convert::BER" object. This object will
only contain the next single item in the current sequence being decoded.
# After this, ber2 will decode further, and ival and sval
# will be decoded.
$ber->decode(
INTEGER = \$ival,
ANY => \$ber2,
STRING => \$sval,
);
OPTIONAL
This operator allows you to specify that an element is absent from the encoding.
Encoding
The value should be a reference to another list with another opList. If all of the values of the
inner opList are defined, the entire OPTIONAL value will be encoded, otherwise it will be omitted.
$ber->encode(
SEQUENCE => [
INTEGER => 16, # Will be encoded
OPTIONAL => [
INTEGER => undef, # Will not be encoded
],
STRING => 'Foo', # Will be encoded
]
);
Decoding
The contents of value are decoded if possible, if not then decode continues at the next
operator-value pair.
$ber->decode(
SEQUENCE => [
INTEGER => \$ival1,
OPTIONAL => [
INTEGER => \$ival2,
],
STRING => \$sval,
]
);
CHOICE
The opList is a list of alternate operator-value pairs. Only one will be encoded, and only one will be
decoded.
Encoding
A scalar at the start of the opList identifies which opList alternative to use for encoding the
value. A value of 0 means the first one is used, 1 means the second one, etc.
# Encode the BMPString alternate of the CHOICE
$ber->encode(
CHOICE => [ 2,
PrintableString => 'Printable',
TeletexString => 'Teletex/T61',
BMPString => 'BMP/Unicode',
UniversalString => 'Universal/ISO10646',
]
) or die;
Decoding
A reference to a scalar at the start of the opList is used to store which alternative is decoded (0
for the first one, 1 for the second one, etc.) Pass undef instead of the ref if you don't care about
this, or you store all the alternate values in different variables.
# Decode the above.
# Afterwards, $alt will be set to 2, $str will be set to 'BMP/Unicode'.
$ber->decode(
CHOICE => [ \$alt,
PrintableString => \$str,
TeletexString => \$str,
BMPString => \$str,
UniversalString => \$str,
]
) or die;
TAGS
In BER everything being encoded has a tag, a length, and a value. Normally the tag is derived from the
operator - so INTEGER has a different tag from a BOOLEAN, for instance.
In some applications it is necessary to change the tags used. For example, a SET may need to contain two
different INTEGER values. Tags may be changed in two ways, either IMPLICITly or EXPLICITly. With IMPLICIT
tagging, the new tag completely replaces the old tag. With EXPLICIT tagging, the new tag is used as well
as the old tag.
"Convert::BER" supports two ways of using IMPLICIT tagging. One method is to sub-class "Convert::BER",
which is described in the next section. For small applications or those that think sub-classing is just
too much then the operator may be passed an arrayref. The array must contain two elements, the first is
the usual operator name and the second is the tag value to use, as shown below.
$ber->encode(
[ SEQUENCE => 0x34 ] => [
INTEGER => 10,
STRING => "A"
]
) or die;
This will encode a sequence, with a tag value of 0x34, which will contain and integer and a string which
will have their default tag values.
You may wish to construct your tags using some pre-defined functions such as
&Convert::BER::BER_APPLICATION, &Convert::BER::BER_CONTEXT, etc, instead of calculating the tag values
yourself.
To use EXPLICIT tagging, enclose the original element in a SEQUENCE, and just override the SEQUENCE's tag
as above. Don't forget to set the constructed bit using &Convert::BER::BER_CONSTRUCTOR. For example, the
ASN.1 definition:
Foo ::= SEQUENCE {
[0] EXPLICIT INTEGER,
INTEGER
}
might be encoded using this:
$ber->encode(
SEQUENCE => [
[ SEQUENCE => &Convert::BER::BER_CONTEXT |
&Convert::BER::BER_CONSTRUCTOR | 0 ] => [
INTEGER => 10,
],
INTEGER => 11,
],
) or die;
SUB-CLASSING
For large applications where operators with non default tags are used a lot the above mechanism can be
very error-prone. For this reason, "Convert::BER" may be sub-classed.
To do this the sub-class must call a static method "define". The arguments to "define" is a list of
arrayrefs. Each arrayref will define one new operator. Each arrayref contains three values, the first is
the name of the operator, the second is how the data is encoded and the third is the tag value. To aid
with the creation of these arguments "Convert::BER" exports some variables and constant subroutines.
For each operator defined by "Convert::BER", or a "Convert::BER" sub-class, a scalar variable with the
same name is available for import, for example $INTEGER is available from "Convert::BER". And any
operators defined by a new sub-class will be available for import from that class. One of these
variables may be used as the second element of each arrayref.
"Convert::BER" also exports some constant subroutines that can be used to create the tag value. The
subroutines exported are:
BER_BOOLEAN
BER_INTEGER
BER_BIT_STR
BER_OCTET_STR
BER_NULL
BER_OBJECT_ID
BER_SEQUENCE
BER_SET
BER_UNIVERSAL
BER_APPLICATION
BER_CONTEXT
BER_PRIVATE
BER_PRIMITIVE
BER_CONSTRUCTOR
"Convert::BER" also provides a subroutine called "ber_tag" to calculate an integer value that will be
used to represent a tag. For tags with values less than 30 this is not needed, but for tags >= 30 then
tag value passed for an operator definition must be the result of "ber_tag"
"ber_tag" takes two arguments, the first is the tag class and the second is the tag value.
Using this information a sub-class of Convert::BER can be created as shown below.
package Net::LDAP::BER;
use Convert::BER qw(/^(\$|BER_)/);
use strict;
use vars qw($VERSION @ISA);
@ISA = qw(Convert::BER);
$VERSION = "1.00";
Net::LDAP::BER->define(
# Name Type Tag
########################################
[ REQ_UNBIND => $NULL,
BER_APPLICATION | 0x02 ],
[ REQ_COMPARE => $SEQUENCE,
BER_APPLICATION | BER_CONSTRUCTOR | 0x0E ],
[ REQ_ABANDON => $INTEGER,
ber_tag(BER_APPLICATION, 0x10) ],
);
This will create a new class "Net::LDAP::BER" which has three new operators available. This class then
may be used as follows
$ber = new Net::LDAP::BER;
$ber->encode(
REQ_UNBIND => 0,
REQ_COMPARE => [
REQ_ABANDON => 123,
]
);
$ber->decode(
REQ_UNBIND => \$var,
REQ_COMPARE => [
REQ_ABANDON => \$num,
]
);
Which will encode or decode the data using the formats and tags defined in the "Net::LDAP::BER" sub-
class. It also helps to make the code more readable.
DEFINING NEW PACKING OPERATORS
As well as defining new operators which inherit from existing operators it is also possible to define a
new operator and how data is encoded and decoded. The interface for doing this is still changing but will
be documented here when it is done. To be continued ...
LIMITATIONS
Convert::BER cannot support tags that contain more bits than can be stored in a scalar variable,
typically this is 32 bits.
Convert::BER cannot support items that have a packed length which cannot be stored in 32 bits.
BUGS
The "SET" decode method fails if the encoded order is different to the opList order.
AUTHOR
Graham Barr <gbarr@pobox.com> Significant POD updates from Chris Ridd <Chris.Ridd@messagingdirect.com>
COPYRIGHT
Copyright (c) 1995-2000 Graham Barr. All rights reserved. This program is free software; you can
redistribute it and/or modify it under the same terms as Perl itself.