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, "" ], # enterprise oid [ ASN_APPLICATION, 0, 0, "\x0a\x00\x00\x01" ], # SNMP IpAddress, [ 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, "" ], [ ASN_UNIVERSAL, ASN_OCTET_STRING, 0, "" # 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], "") # 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. ASN.1/BER/DER/... BASICS ASN.1 is a strange language that can be used 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 somewhat 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 whether 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, CLASS/TAG/CONSTRUCTED are READ ONLY(!) # 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, 0, 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 $oid_string. Example: ber_is_oid $tuple, "" or die "oid must be"; $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. PROFILES While any BER data can be correctly encoded and decoded out of the box, it can be inconvenient to have to manually decode some values into a "better" format: for instance, SNMP TimeTicks values are decoded into the raw octet strings of their BER representation, which is quite hard to decode. With profiles, you can change which class/tag combinations map to which decoder function inside "ber_decode" (and of course also which encoder functions are used in "ber_encode"). This works by mapping specific class/tag combinations to an internal "ber type". The default profile supports the standard ASN.1 types, but no application-specific ones. This means that class/tag combinations not in the base set of ASN.1 are decoded into their raw octet strings. "Convert::BER::XS" defines two profile variables you can use out of the box: $Convert::BER::XS::DEFAULT_PROFILE This is the default profile, i.e. the profile that is used when no profile is specified for de-/encoding. You can modify it, but remember that this modifies the defaults for all callers that rely on the default profile. $Convert::BER::XS::SNMP_PROFILE A profile with mappings for SNMP-specific application tags added. This is useful when de-/encoding SNMP data. Example: $ber = ber_decode $data, $Convert::BER::XS::SNMP_PROFILE; The Convert::BER::XS::Profile class $profile = new Convert::BER::XS::Profile Create a new profile. The profile will be identical to the default profile. $profile->set ($class, $tag, $type) Sets the mapping for the given $class/$tag combination to $type, which must be one of the "BER_TYPE_*" constants. Note that currently, the mapping is stored in a flat array, so large values of $tag will consume large amounts of memory. Example: $profile = new Convert::BER::XS::Profile; $profile->set (ASN_APPLICATION, SNMP_COUNTER32, BER_TYPE_INT); $ber = ber_decode $data, $profile; $type = $profile->get ($class, $tag) Returns the BER type mapped to the given $class/$tag combination. BER TYPES This lists the predefined BER types - you can map any "CLASS"/"TAG" combination to any "BER_TYPE_*". "BER_TYPE_BYTES" The raw octets of the value. This is the default type for unknown tags and de-/encodes the value as if it were an octet string, i.e. by copying the raw bytes. "BER_TYPE_UTF8" Like "BER_TYPE_BYTES", but decodes the value as if it were a UTF-8 string (without validation!) and encodes a perl unicode string into a UTF-8 BER string. "BER_TYPE_UCS2" Similar to "BER_TYPE_UTF8", but treats the BER value as UCS-2 encoded string. "BER_TYPE_UCS4" Similar to "BER_TYPE_UTF8", but treats the BER value as UCS-4 encoded string. "BER_TYPE_INT" Encodes and decodes a BER integer value to a perl integer scalar. This should correctly handle 64 bit signed and unsigned values. "BER_TYPE_OID" Encodes and decodes an OBJECT IDENTIFIER into dotted form without leading dot, e.g. "BER_TYPE_RELOID" Same as "BER_TYPE_OID" but uses relative object identifier encoding: ASN.1 has this hack of encoding the first two OID components into a single integer in a weird attempt to save an insignificant amount of space in an otherwise wasteful encoding, and relative OIDs are basically OIDs without this hack. The practical difference is that the second component of an OID can only have the values 1..40, while relative OIDs do not have this restriction. "BER_TYPE_NULL" Decodes an "ASN_NULL" value into "undef", and always encodes a "ASN_NULL" type, regardless of the perl value. "BER_TYPE_BOOL" Decodes an "ASN_BOOLEAN" value into 0 or 1, and encodes a perl boolean value into an "ASN_BOOLEAN". "BER_TYPE_REAL" Decodes/encodes a BER real value. NOT IMPLEMENTED. "BER_TYPE_IPADDRESS" Decodes/encodes a four byte string into an IPv4 dotted-quad address string in Perl. Given the obsolete nature of this type, this is a low-effort implementation that simply uses "sprintf" and "sscanf"-style conversion, so it won't handle all string forms supported by "inet_aton" for example. "BER_TYPE_CROAK" Always croaks when encountered during encoding or decoding - the default behaviour when encountering an unknown type is to treat it as "BER_TYPE_BYTES". When you don't want that but instead prefer a hard error for some types, then "BER_TYPE_CROAK" is for you. LIMITATIONS This module can only en-/decode 64 bit signed and unsigned integers, and only when your perl supports those. OBJECT IDENTIFIEERs cannot have unlimited length, although the limit is much larger than e.g. the one imposed by SNMP or other protocols. REAL values are not supported and will croak. This module has undergone little to no testing so far. ITHREADS SUPPORT This module is unlikely to work when the (officially discouraged) ithreads are in use. AUTHOR Marc Lehmann