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 message"; # The above results in a data structure consisting of (class, tag, # constructed, data) tuples. Below is such a message, SNMPv1 trap # with a Cisco mac change notification. # Did you know that Cisco is in the news almost every week because # of some backdoor password or other extremely stupid security bug? [ 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 - trap PDU [ [ 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 WARNING: Before release 1.0, the API is not considered stable in any way. 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] To avoid non-descriptive hardcoded array index numbers, this module defines symbolic constants to access these members: "BER_CLASS", "BER_TAG", "BER_CONSTRUCTED" and "BER_DATA". Also, the first three members are integers with a little caveat: for performance reasons, these are readonly and shared, so you must not modify them (increment, assign to them etc.) in any way. You may modify the *DATA* member, and you may re-assign the array itself, e.g.: $ber = ber_decode $binbuf; # the following is NOT legal: $ber->[BER_CLASS] = ASN_PRIVATE; # ERROR, readonly(!) # but all of the following are fine: $ber->[BER_DATA] = "string"; $ber->[BER_DATA] = [ASN_UNIVERSAL, ASN_INTEGER32, 0, 123]; @$ber = (ASN_APPLICATION, SNMP_TIMETICKS, 1000); *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. DECODING AND ENCODING $tuple = ber_decoded $bindata Decodes binary BER data in $bindata and returns the resulting BER tuple. Croaks on any decoding error, so the returned $tuple is always valid. $bindata = ber_encode $tuple Encodes the BER tuple into a BER/DER data structure. HELPER FUNCTIONS Working with a 4-tuple for every value can be annoying. Or, rather, *is* annoying. To reduce this a bit, this module defines a number of helper functions, both to match BER tuples and to conmstruct BER tuples: MATCH HELPERS Thse functions accept a BER tuple as first argument and either paertially or fully match it. They often come in two forms, one which exactly matches a value, and one which only matches the type and returns the value. They do check whether valid tuples are passed in and croak otherwise. As a ease-of-use exception, they usually also accept "undef" instead of a tuple reference. in which case they silently fail to match. $bool = ber_is $tuple, $class, $tag, $constructed, $data This takes a BER $tuple and matches its elements agains the privded values, all of which are optional - values that are either missing or "undef" will be ignored, the others will be matched exactly (e.g. as if you used "==" or "eq" (for $data)). Some examples: ber_is $tuple, ASN_UNIVERSAL, ASN_SEQUENCE, 1 orf die "tuple is not an ASN SEQUENCE"; ber_is $tuple, ASN_UNIVERSAL, ASN_NULL or die "tuple is not an ASN NULL value"; ber_is $tuple, ASN_UNIVERSAL, ASN_INTEGER32, 0, 50 or die "BER integer must be 50"; $seq = ber_is_seq $tuple Returns the sequence members (the array of subvalues) if the $tuple is an ASN SEQUENCE, i.e. the "BER_DATA" member. If the $tuple is not a sequence it returns "undef". For example, SNMP version 1/2c/3 packets all consist of an outer SEQUENCE value: my $ber = ber_decode $snmp_data; my $snmp = ber_is_seq $ber or die "SNMP packet invalid: does not start with SEQUENCE"; # now we know $snmp is a sequence, so decode the SNMP version my $version = ber_is_i32 $snmp->[0] or die "SNMP packet invalid: does not start with version number"; $bool = ber_is_i32 $tuple, $i32 Returns a true value if the $tuple represents an ASN INTEGER32 with the value $i32. $i32 = ber_is_i32 $tuple Returns true (and extracts the integer value) if the $tuple is an ASN INTEGER32. For 0, this function returns a special value that is 0 but true. $bool = ber_is_oid $tuple, $oid_string Returns true if the $tuple represents an ASN_OBJECT_IDENTIFIER that exactly matches C$oid_string>. Exmaple: ber_is_oid $tuple, "1.3.6.1.4" or die "oid must be 1.3.6.1.4"; $oid = ber_is_oid $tuple Returns true (and extracts the OID string) if the $tuple is an ASN OBJECT IDENTIFIER. Otherwise, it returns "undef". CONSTRUCTION HELPERS $tuple = ber_i32 $value Constructs a new "ASN_INTEGER32" tuple. 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 http://software.schmorp.de/pkg/Convert-BER-XS