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---
-- ASN.1 functions.
--
-- Large chunks of this code have been ripped right out from <code>snmp.lua</code>.
--
-- @copyright Same as Nmap--See http://nmap.org/book/man-legal.html
--
-- @author Patrik Karlsson
--
-- Version 0.3
-- Created 01/12/2010 - v0.1 - Created by Patrik Karlsson <patrik@cqure.net>
-- Revised 01/28/2010 - v0.2 - Adapted to create a framework for SNMP, LDAP and future protocols
-- Revised 02/02/2010 - v0.3 - Changes: o Re-designed so that ASN1Encoder and ASN1Decoder are separate classes
-- o Each script or library should now create it's own Encoder and Decoder instance
--
module(... or "asn1",package.seeall)
require("bit")
BERCLASS = {
Universal = 0,
Application = 64,
ContextSpecific = 128,
Private = 192
}
--- The decoder class
--
ASN1Decoder = {
new = function(self,o)
o = o or {}
setmetatable(o, self)
self.__index = self
return o
end,
--- Registers the base simple type decoders
--
registerBaseDecoders = function(self)
self.decoder = {}
-- Boolean
self.decoder["01"] = function( self, encStr, elen, pos )
local val = bin.unpack("H", encStr, pos)
if val ~= "FF" then
return pos, true
else
return pos, false
end
end
-- Integer
self.decoder["02"] = function( self, encStr, elen, pos )
return self.decodeInt(encStr, elen, pos)
end
-- Octet String
self.decoder["04"] = function( self, encStr, elen, pos )
return bin.unpack("A" .. elen, encStr, pos)
end
-- Null
self.decoder["05"] = function( self, encStr, elen, pos )
return pos, false
end
-- Object Identifier
self.decoder["06"] = function( self, encStr, elen, pos )
return self:decodeOID( encStr, elen, pos )
end
-- Context specific tags
--
self.decoder["30"] = function( self, encStr, elen, pos )
return self:decodeSeq(encStr, elen, pos)
end
end,
--- Allows for registration of additional tag decoders
--
-- @param tagDecoders table containing decoding functions @see tagDecoders
registerTagDecoders = function(self, tagDecoders)
self:registerBaseDecoders()
for k, v in pairs(tagDecoders) do
self.decoder[k] = v
end
end,
--- Decodes the ASN.1's built-in simple types
--
-- @param encStr Encoded string.
-- @param pos Current position in the string.
-- @return The position after decoding
-- @return The decoded value(s).
decode = function(self, encStr, pos)
local etype, elen
local newpos = pos
newpos, etype = bin.unpack("H1", encStr, newpos)
newpos, elen = self.decodeLength(encStr, newpos)
if self.decoder[etype] then
return self.decoder[etype]( self, encStr, elen, newpos )
else
stdnse.print_debug("no decoder for etype: " .. etype)
return newpos, nil
end
end,
---
-- Decodes length part of encoded value according to ASN.1 basic encoding
-- rules.
-- @param encStr Encoded string.
-- @param pos Current position in the string.
-- @return The position after decoding.
-- @return The length of the following value.
decodeLength = function(encStr, pos)
local elen
pos, elen = bin.unpack('C', encStr, pos)
if (elen > 128) then
elen = elen - 128
local elenCalc = 0
local elenNext
for i = 1, elen do
elenCalc = elenCalc * 256
pos, elenNext = bin.unpack("C", encStr, pos)
elenCalc = elenCalc + elenNext
end
elen = elenCalc
end
return pos, elen
end,
---
-- Decodes a sequence according to ASN.1 basic encoding rules.
-- @param encStr Encoded string.
-- @param len Length of sequence in bytes.
-- @param pos Current position in the string.
-- @return The position after decoding.
-- @return The decoded sequence as a table.
decodeSeq = function(self, encStr, len, pos)
local seq = {}
local sPos = 1
local sStr
pos, sStr = bin.unpack("A" .. len, encStr, pos)
while (sPos < len) do
local newSeq
sPos, newSeq = self:decode(sStr, sPos)
table.insert(seq, newSeq)
end
return pos, seq
end,
-- Decode one component of an OID from a byte string. 7 bits of the component
-- are stored in each octet, most significant first, with the eigth bit set in
-- all octets but the last. These encoding rules come from
-- http://luca.ntop.org/Teaching/Appunti/asn1.html, section 5.9 OBJECT
-- IDENTIFIER.
decode_oid_component = function(encStr, pos)
local octet
local n = 0
repeat
pos, octet = bin.unpack("C", encStr, pos)
n = n * 128 + bit.band(0x7F, octet)
until octet < 128
return pos, n
end,
--- Decodes an OID from a sequence of bytes.
--
-- @param encStr Encoded string.
-- @param len Length of sequence in bytes.
-- @param pos Current position in the string.
-- @return The position after decoding.
-- @return The OID as an array.
decodeOID = function(self, encStr, len, pos)
local last
local oid = {}
local octet
last = pos + len - 1
if pos <= last then
oid._snmp = '06'
pos, octet = bin.unpack("C", encStr, pos)
oid[2] = math.mod(octet, 40)
octet = octet - oid[2]
oid[1] = octet/40
end
while pos <= last do
local c
pos, c = self.decode_oid_component(encStr, pos)
oid[#oid + 1] = c
end
return pos, oid
end,
---
-- Decodes length part of encoded value according to ASN.1 basic encoding
-- rules.
-- @param encStr Encoded string.
-- @param pos Current position in the string.
-- @return The position after decoding.
-- @return The length of the following value.
decodeLength = function(encStr, pos)
local elen
pos, elen = bin.unpack('C', encStr, pos)
if (elen > 128) then
elen = elen - 128
local elenCalc = 0
local elenNext
for i = 1, elen do
elenCalc = elenCalc * 256
pos, elenNext = bin.unpack("C", encStr, pos)
elenCalc = elenCalc + elenNext
end
elen = elenCalc
end
return pos, elen
end,
---
-- Decodes an Integer according to ASN.1 basic encoding rules.
-- @param encStr Encoded string.
-- @param len Length of integer in bytes.
-- @param pos Current position in the string.
-- @return The position after decoding.
-- @return The decoded integer.
decodeInt = function(encStr, len, pos)
local hexStr
pos, hexStr = bin.unpack("H" .. len, encStr, pos)
local value = tonumber(hexStr, 16)
if (value >= math.pow(256, len)/2) then
value = value - math.pow(256, len)
end
return pos, value
end,
---
-- Decodes an SNMP packet or a part of it according to ASN.1 basic encoding
-- rules.
-- @param encStr Encoded string.
-- @param pos Current position in the string.
-- @return The decoded value(s).
dec = function(self, encStr, pos)
local result
local _
_, result = self:decode(encStr, pos)
return result
end,
}
--- The encoder class
--
ASN1Encoder = {
new = function(self,o)
o = o or {}
setmetatable(o, self)
self.__index = self
return o
end,
---
-- Encodes an ASN1 sequence
encodeSeq = function(self, seqData)
return bin.pack('HAA' , '30', self.encodeLength(string.len(seqData)), seqData)
end,
---
-- Encodes a given value according to ASN.1 basic encoding rules for SNMP
-- packet creation.
-- @param val Value to be encoded.
-- @return Encoded value.
encode = function(self, val)
local vtype = type(val)
if self.encoder[vtype] then
return self.encoder[vtype](self,val)
else
return nil
end
return ''
end,
--- Allows for registration of additional tag encoders
--
-- @param tagEncoders table containing encoding functions @see tagEncoders
registerTagEncoders = function(self, tagEncoders)
self:registerBaseEncoders()
for k, v in pairs(tagEncoders) do
self.encoder[k] = v
end
end,
-- ASN.1 Simple types encoders
registerBaseEncoders = function(self)
self.encoder = {}
-- Bolean encoder
self.encoder['boolean'] = function( self, val )
if val then
return bin.pack('H','01 01 FF')
else
return bin.pack('H', '01 01 00')
end
end
-- Integer encoder
self.encoder['number'] = function( self, val )
local ival = self.encodeInt(val)
local len = self.encodeLength(string.len(ival))
return bin.pack('HAA', '02', len, ival)
end
-- Octet String encoder
self.encoder['string'] = function( self, val )
local len = self.encodeLength(string.len(val))
return bin.pack('HAA', '04', len, val)
end
-- Null encoder
self.encoder['nil'] = function( self, val )
return bin.pack('H', '05 00')
end
end,
-- Encode one component of an OID as a byte string. 7 bits of the component are
-- stored in each octet, most significant first, with the eigth bit set in all
-- octets but the last. These encoding rules come from
-- http://luca.ntop.org/Teaching/Appunti/asn1.html, section 5.9 OBJECT
-- IDENTIFIER.
encode_oid_component = function(n)
local parts = {}
parts[1] = string.char(bit.mod(n, 128))
while n >= 128 do
n = bit.rshift(n, 7)
parts[#parts + 1] = string.char(bit.mod(n, 128) + 0x80)
end
return string.reverse(table.concat(parts))
end,
---
-- Encodes an Integer according to ASN.1 basic encoding rules.
-- @param val Value to be encoded.
-- @return Encoded integer.
encodeInt = function(val)
local lsb = 0
if val > 0 then
local valStr = ""
while (val > 0) do
lsb = math.mod(val, 256)
valStr = valStr .. bin.pack("C", lsb)
val = math.floor(val/256)
end
if lsb > 127 then -- two's complement collision
valStr = valStr .. bin.pack("H", "00")
end
return string.reverse(valStr)
elseif val < 0 then
local i = 1
local tcval = val + 256 -- two's complement
while tcval <= 127 do
tcval = tcval + (math.pow(256, i) * 255)
i = i+1
end
local valStr = ""
while (tcval > 0) do
lsb = math.mod(tcval, 256)
valStr = valStr .. bin.pack("C", lsb)
tcval = math.floor(tcval/256)
end
return string.reverse(valStr)
else -- val == 0
return bin.pack("x")
end
end,
---
-- Encodes the length part of a ASN.1 encoding triplet using the "primitive,
-- definite-length" method.
-- @param len Length to be encoded.
-- @return Encoded length value.
encodeLength = function(len)
if len < 128 then
return string.char(len)
else
local parts = {}
while len > 0 do
parts[#parts + 1] = string.char(bit.mod(len, 256))
len = bit.rshift(len, 8)
end
assert(#parts < 128)
return string.char(#parts + 0x80) .. string.reverse(table.concat(parts))
end
end
}
---
-- Converts a BER encoded type to a numeric value
-- This allows it to be used in the encoding function
--
-- @param class number - see <code>BERCLASS<code>
-- @param constructed boolean (true if constructed, false if primitive)
-- @param number numeric
-- @return number to be used with <code>encode</code>
function BERtoInt(class, constructed, number)
local asn1_type = class + number
if constructed == true then
asn1_type = asn1_type + 32
end
return asn1_type
end
---
-- Converts an integer to a BER encoded type table
--
-- @param i number containing the value to decode
-- @return table with the following entries <code>class</code>, <code>constructed</code>,
-- <code>primitive</code> and <code>number</code>
function intToBER( i )
local ber = {}
if bit.band( i, BERCLASS.Application ) == BERCLASS.Application then
ber.class = BERCLASS.Application
elseif bit.band( i, BERCLASS.ContextSpecific ) == BERCLASS.ContextSpecific then
ber.class = BERCLASS.ContextSpecific
elseif bit.band( i, BERCLASS.Private ) == BERCLASS.Private then
ber.class = BERCLASS.Private
else
ber.class = BERCLASS.Universal
end
if bit.band( i, 32 ) == 32 then
ber.constructed = true
ber.number = i - ber.class - 32
else
ber.primitive = true
ber.number = i - ber.class
end
return ber
end
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