NAME
    Convert::BER::XS - *very* low level BER en-/decoding

SYNOPSIS
     use Convert::BER::XS ':all';

     my $ber = ber_decode $buf
        or die "unable to decode SNMP v1/v2c Message";

     # the above results in a data structure consisting of (class, tag,
     # constructed, data) tuples. here is such a message, SNMPv1 trap
     # with a cisoc mac change notification

     [ ASN_UNIVERSAL, ASN_SEQUENCE, 1,
       [
          [ ASN_UNIVERSAL, ASN_INTEGER32, 0, 0 ], # snmp version 1
          [ ASN_UNIVERSAL, 4, 0, "public" ], # community
          [ ASN_CONTEXT, 4, 1, # CHOICE, constructed
             [
                [ ASN_UNIVERSAL, ASN_OBJECT_IDENTIFIER, 0, "1.3.6.1.4.1.9.9.215.2" ], # enterprise oid
                [ ASN_APPLICATION, 0, 0, "\x0a\x00\x00\x01" ], # SNMP IpAddress, 10.0.0.1
                [ ASN_UNIVERSAL, ASN_INTEGER32, 0, 6 ], # generic trap
                [ ASN_UNIVERSAL, ASN_INTEGER32, 0, 1 ], # specific trap
                [ ASN_APPLICATION, ASN_TIMETICKS, 0, 1817903850 ], # SNMP TimeTicks
                [ ASN_UNIVERSAL, ASN_SEQUENCE, 1, # the varbindlist
                   [
                      [ ASN_UNIVERSAL, ASN_SEQUENCE, 1, # a single varbind, "key value" pair
                         [
                            [ ASN_UNIVERSAL, ASN_OBJECT_IDENTIFIER, 0, "1.3.6.1.4.1.9.9.215.1.1.8.1.2.1" ], # the oid
                            [ ASN_UNIVERSAL, ASN_OCTET_STRING, 0, "...data..." # the value
                            ]
                         ]
                      ],
                      ...

     # let's decode it a bit with some helper functions

     my $msg = ber_is_seq $ber
        or die "SNMP message does not start with a sequence";

     ber_is $msg->[0], ASN_UNIVERSAL, ASN_INTEGER32, 0
        or die "SNMP message does not start with snmp version\n";

     # message is SNMP v1 or v2c?
     if ($msg->[0][BER_DATA] == 0 || $msg->[0][BER_DATA] == 1) {

        # message is v1 trap?
        if (ber_is $msg->[2], ASN_CONTEXT, 4, 1) {
           my $trap = $msg->[2][BER_DATA];

           # check whether trap is a cisco mac notification mac changed message
           if (
              (ber_is_oid $trap->[0], "1.3.6.1.4.1.9.9.215.2") # cmnInterfaceObjects
              and (ber_is_i32 $trap->[2], 6)
              and (ber_is_i32 $trap->[3], 1) # mac changed msg
           ) {
              ... and so on

     # finally, let's encode it again and hope it results in the same bit pattern

     my $buf = ber_encode $ber;

DESCRIPTION
    This module implements a *very* low level BER/DER en-/decoder.

    If is tuned for low memory and high speed, while still maintaining some
    level of user-friendlyness.

    Currently, not much is documented, as this is an initial release to
    reserve CPAN namespace, stay tuned for a few days.

  ASN.1/BER/DER/... BASICS
    ASN.1 is a strange language that can be sed to describe protocols and
    data structures. It supports various mappings to JSON, XML, but most
    importantly, to a various binary encodings such as BER, that is the
    topic of this module, and is used in SNMP or LDAP for example.

    While ASN.1 defines a schema that is useful to interpret encoded data,
    the BER encoding is actually somehat self-describing: you might not know
    whether something is a string or a number or a sequence or something
    else, but you can nevertheless decode the overall structure, even if you
    end up with just a binary blob for the actual value.

    This works because BER values are tagged with a type and a namespace,
    and also have a flag that says whther a value consists of subvalues (is
    "constructed") or not (is "primitive").

    Tags are simple integers, and ASN.1 defines a somewhat weird assortment
    of those - for example, you have 32 bit signed integers and 16(!)
    different string types, but there is no unsigned32 type for example.
    Different applications work around this in different ways, for example,
    SNMP defines application-specific Gauge32, Counter32 and Unsigned32,
    which are mapped to two different tags: you can distinguish between
    Counter32 and the others, but not between Gause32 and Unsigned32,
    without the ASN.1 schema.

    Ugh.

  DECODED BER REPRESENTATION
    This module represents every BER value as a 4-element tuple (actually an
    array-reference):

       [CLASS, TAG, CONSTRUCTED, DATA]

    *CLASS* is something like a namespace for *TAG*s - there is the
    "ASN_UNIVERSAL" namespace which defines tags common to all ASN.1
    implementations, the "ASN_APPLICATION" namespace which defines tags for
    specific applications (for example, the SNMP "Unsigned32" type is in
    this namespace), a special-purpose context namespace ("ASN_CONTEXT",
    used e.g. for "CHOICE") and a private namespace ("ASN_PRIVATE").

    The meaning of the *TAG* depends on the namespace, and defines a
    (partial) interpretation of the data value. For example, right now, SNMP
    application namespace knowledge ix hardcoded into this module, so it
    knows that SNMP "Unsigned32" values need to be decoded into actual perl
    integers.

    The most common tags in the "ASN_UNIVERSAL" namespace are
    "ASN_INTEGER32", "ASN_BIT_STRING", "ASN_NULL", "ASN_OCTET_STRING",
    "ASN_OBJECT_IDENTIFIER", "ASN_SEQUENCE", "ASN_SET" and "ASN_IA5_STRING".

    The most common tags in SNMP's "ASN_APPLICATION" namespace are
    "SNMP_IPADDRESS", "SNMP_COUNTER32", "SNMP_UNSIGNED32", "SNMP_TIMETICKS",
    "SNMP_OPAQUE" and "SNMP_COUNTER64".

    The *CONSTRUCTED* flag is really just a boolean - if it is false, the
    the value is "primitive" and contains no subvalues, kind of like a
    non-reference perl scalar. IF it is true, then the value is
    "constructed" which just means it contains a list of subvalues which
    this module will en-/decode as BER tuples themselves.

    The *DATA* value is either a reference to an array of further tuples (if
    the value is *CONSTRUCTED*), some decoded representation of the value,
    if this module knows how to decode it (e.g. for the integer types above)
    or a binary string with the raw octets if this module doesn't know how
    to interpret the namespace/tag.

    Thus, you can always decode a BER data structure and at worst you get a
    string in place of some nice decoded value.

    See the SYNOPSIS for an example of such an encoded tuple representation.

  RELATIONSHIP TO Convert::BER and Convert::ASN1
    This module is *not* the XS version of Convert::BER, but a different
    take at doing the same thing. I imagine this module would be a good base
    for speeding up either of these, or write a similar module, or write
    your own LDAP or SNMP module for example.

  BUGS / SHORTCOMINGs
    This module does have a number of SNMPisms hardcoded, such as the SNMP
    tags for Unsigned32 and so on. More configurability is needed, and, if
    ever implemented, will come in a form similar to how JSON::XS and
    CBOR::XS respresent things, namely with an object-oriented interface.

AUTHOR
     Marc Lehmann <schmorp@schmorp.de>
     http://software.schmorp.de/pkg/Convert-BER-XS

