diff --git a/CHANGELOG b/CHANGELOG
index 92b0f8861..9e3db318a 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,8 @@
 # Nmap Changelog ($Id$); -*-text-*-
 
+o [NSE] Added a new script krb5-enum-users.nse that performs user enumeration
+  against Kerberos. [Patrik]
+
 o [NSE] Added a new script http-put.nse that allows uploading of local files
   to remote web servers using the HTTP PUT method. Added HTTP PUT support to
   the http library. [Patrik]
diff --git a/scripts/krb5-enum-users.nse b/scripts/krb5-enum-users.nse
new file mode 100644
index 000000000..74f3fdee7
--- /dev/null
+++ b/scripts/krb5-enum-users.nse
@@ -0,0 +1,396 @@
+description = [[
+Discovers valid usernames by querying the Kerberos service for a TGT.
+When an invalid username is requested the server will responde using the
+Kerberos error code KRB5KDC_ERR_C_PRINCIPAL_UNKNOWN, allowing us to determine
+that the user name was invalid. Valid user names will illicit either the
+TGT in a AS-REP response or the error KRB5KDC_ERR_PREAUTH_REQUIRED, signaling
+that the user is required to perform pre authentication.
+
+The script should work against Active Directory and ?
+It needs a valid Kerberos REALM in order to operate.
+]]
+
+---
+-- @usage
+-- nmap -p 88 --script krb5-enum-users --script-args krb5-enum-users.realm='test'
+--
+-- @output
+-- PORT   STATE SERVICE      REASON
+-- 88/tcp open  kerberos-sec syn-ack
+-- | krb5-enum-users: 
+-- | Discovered Kerberos principals
+-- |     administrator@test
+-- |     mysql@test
+-- |_    tomcat@test
+--
+-- @args krb5-enum-users.realm this argument is required as it supplies the
+--       script with the Kerberos REALM against which to guess the user names.
+--
+
+--
+--
+-- Version 0.1
+-- Created 10/16/2011 - v0.1 - created by Patrik Karlsson <patrik@cqure.net>
+--
+
+author = "Patrik Karlsson"
+license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
+categories = {"auth", "discovery", "safe"}
+
+require 'shortport'
+require 'stdnse'
+require 'asn1'
+require 'unpwdb'
+
+portrule = shortport.port_or_service( 88, {"kerberos-sec"}, {"udp","tcp"}, {"open", "open|filtered"} )
+
+-- This an embryo of a Kerberos 5 packet creation and parsing class. It's very
+-- tiny class and holds only the necessary functions to support this script.
+-- This class be factored out into it's own library, once more scripts make use
+-- of it.
+KRB5 = {
+
+	-- Valid Kerberos message types
+	MessageType = {
+		['AS-REQ'] = 10,
+		['AS-REP'] = 11,
+		['KRB-ERROR'] = 30,
+	},
+
+	-- Some of the used error messages
+	ErrorMessages = {
+		['KRB5KDC_ERR_C_PRINCIPAL_UNKNOWN'] = 6,
+		['KRB5KDC_ERR_PREAUTH_REQUIRED'] = 25,
+		['KDC_ERR_WRONG_REALM'] = 68,
+	},
+
+	-- A list of some ot the encryption types
+	EncryptionTypes = {
+		{ ['aes256-cts-hmac-sha1-96'] = 18 },
+		{ ['aes128-cts-hmac-sha1-96'] = 17 },
+		{ ['des3-cbc-sha1'] = 16 },
+		{ ['rc4-hmac'] = 23 },
+		--	{ ['des-cbc-crc'] = 1 },
+		--	{ ['des-cbc-md5'] = 3 },
+		--	{ ['des-cbc-md4'] = 2 }
+	},
+
+	-- A list of principal name types
+	NameTypes = {
+		['NT-PRINCIPAL'] = 1,
+		['NT-SRV-INST'] = 2,
+	},
+
+	-- Creates a new Krb5 instance
+	-- @return o as the new instance
+	new = function(self)
+		local o = {}
+		setmetatable(o, self)
+		self.__index = self
+		return o
+    end,
+
+	-- A number of custom ASN1 decoders needed to decode the response
+	tagDecoder = {
+
+		["18"] = function( self, encStr, elen, pos )
+		return bin.unpack("A" .. elen, encStr, pos)
+		end,
+
+		["1B"] = function( ... ) return KRB5.tagDecoder["18"](...) end,
+
+		["6B"] = function( self, encStr, elen, pos )
+		local seq
+		pos, seq = self:decodeSeq(encStr, elen, pos)
+		return pos, seq
+		end,
+
+		-- Not really sure what these are, but they all decode sequences
+		["7E"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A0"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A1"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A2"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A3"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A4"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A5"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A6"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A7"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A8"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,
+		["A9"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,	
+		["AA"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,	
+		["AC"] = function( ... ) return KRB5.tagDecoder["6B"](...) end,	
+
+	},
+
+	-- A few Kerberos ASN1 encoders
+	tagEncoder = {
+
+		['table'] = function(self, val)
+
+			local types = {
+				['GeneralizedTime'] = 0x18,
+				['GeneralString'] = 0x1B,
+			}
+
+			local len = asn1.ASN1Encoder.encodeLength(#val[1])
+
+			if ( val._type and types[val._type] ) then
+				return bin.pack("CAA", types[val._type], len, val[1])
+			elseif ( val._type and 'number' == type(val._type) ) then
+				return bin.pack("CAA", val._type, len, val[1])
+			end
+
+		end,
+	},
+
+	-- Encodes a sequence using a custom type
+	-- @param encoder class containing an instance of a ASN1Encoder
+	-- @param seqtype number the sequence type to encode
+	-- @param seq string containing the sequence to encode 
+	encodeSequence = function(self, encoder, seqtype, seq)
+		return encoder:encode( { _type = seqtype, seq } )
+	end,
+
+	-- Encodes a Kerberos Principal
+	-- @param encoder class containing an instance of ASN1Encoder
+	-- @param name_type number containing a valid Kerberos name type
+	-- @param names table containing a list of names to encode
+	-- @return princ string containing an encoded principal
+	encodePrincipal = function(self, encoder, name_type, names )
+		local princ = ""
+
+		for _, n in ipairs(names) do
+			princ = princ .. encoder:encode( { _type = 'GeneralString', n } ) 
+		end
+
+		princ = self:encodeSequence(encoder, 0x30, princ)
+		princ = self:encodeSequence(encoder, 0xa1, princ)
+		princ = encoder:encode( name_type ) .. princ
+
+		-- not sure about how this works, but apparently it does
+		princ = bin.pack("H", "A003") .. princ
+		princ = self:encodeSequence(encoder,0x30, princ)
+
+		return princ
+	end,
+
+	-- Encodes the Kerberos AS-REQ request
+	-- @param realm string containing the Kerberos REALM
+	-- @param user string containing the Kerberos principal name
+	-- @param protocol string containing either of "tcp" or "udp"
+	-- @return data string containing the encoded request
+	encodeASREQ = function(self, realm, user, protocol)
+
+		assert(protocol == "tcp" or protocol == "udp",
+			"Protocol has to be either \"tcp\" or \"udp\"")
+
+		local encoder = asn1.ASN1Encoder:new()
+		encoder:registerTagEncoders(KRB5.tagEncoder)
+
+		local data = ""
+
+		-- encode encryption types
+		for _,enctype in ipairs(KRB5.EncryptionTypes) do
+			for k, v in pairs( enctype ) do
+				data = data .. encoder:encode(v)
+			end
+		end
+
+		data = self:encodeSequence(encoder, 0x30, data )
+		data = self:encodeSequence(encoder, 0xA8, data )
+
+		-- encode nonce
+		local nonce = 155874945
+		data = self:encodeSequence(encoder, 0xA7, encoder:encode(nonce) ) .. data
+
+		-- encode from/to
+		local fromdate = os.time() + 10 * 60 * 60
+		local from = os.date("%Y%m%d%H%M%SZ", fromdate)
+		data = self:encodeSequence(encoder, 0xA5, encoder:encode( { from, _type='GeneralizedTime' })) .. data
+
+		local names = { "krbtgt", realm }
+		local sname = self:encodePrincipal( encoder, KRB5.NameTypes['NT-SRV-INST'], names )
+		sname = self:encodeSequence(encoder, 0xA3, sname)
+		data = sname .. data
+
+		-- realm
+		data = self:encodeSequence(encoder, 0xA2, encoder:encode( { _type = 'GeneralString', realm })) .. data
+
+		local cname = self:encodePrincipal(encoder, KRB5.NameTypes['NT-PRINCIPAL'], { user })
+		cname = self:encodeSequence(encoder, 0xA1, cname)
+		data = cname .. data
+
+		-- forwardable
+		local kdc_options = 0x40000000
+		data = bin.pack(">I", kdc_options) .. data
+
+		-- add padding
+		data = bin.pack("C", 0) .. data
+
+		-- hmm, wonder what this is
+		data = bin.pack("H", "A0070305") .. data
+		data = self:encodeSequence(encoder, 0x30, data)
+		data = self:encodeSequence(encoder, 0xA4, data)
+		data = self:encodeSequence(encoder, 0xA2, encoder:encode(KRB5.MessageType['AS-REQ'])) .. data
+
+		local pvno = 5
+		data = self:encodeSequence(encoder, 0xA1, encoder:encode(pvno) ) .. data
+
+		data = self:encodeSequence(encoder, 0x30, data)
+		data = self:encodeSequence(encoder, 0x6a, data)
+
+		if ( protocol == "tcp" ) then
+			data = bin.pack(">I", #data) .. data
+		end
+
+		return data
+	end,
+
+	-- Parses the result from the AS-REQ
+	-- @param data string containing the raw unparsed data
+	-- @return status boolean true on success, false on failure
+	-- @return msg table containing the fields <code>type</code> and
+	--         <code>error_code</code> if the type is an error.
+	parseResult = function(self, data)
+
+		local decoder = asn1.ASN1Decoder:new()
+		decoder:registerTagDecoders(KRB5.tagDecoder)
+		decoder:setStopOnError(true)
+		local pos, result = decoder:decode(data)
+		local msg = {}
+
+
+		if ( #result == 0 or #result[1] < 2 or #result[1][2] < 1 ) then
+			return false, nil
+		end
+
+		msg.type = result[1][2][1]
+
+		if ( msg.type == KRB5.MessageType['KRB-ERROR'] ) then
+			if ( #result[1] < 5 and #result[1][5] < 1 ) then
+				return false, nil
+			end
+
+			msg.error_code = result[1][5][1]
+			return true, msg
+			elseif ( msg.type == KRB5.MessageType['AS-REP'] ) then
+				return true, msg
+			end
+
+			return false, nil
+		end,
+
+}
+
+-- Checks whether the user exists or not
+-- @param host table as received by the action method
+-- @param port table as received by the action method
+-- @param realm string containing the Kerberos REALM
+-- @param user string containing the Kerberos principal
+-- @return status boolean, true on success, false on failure
+-- @return state VALID or INVALID or error message if status was false
+local function checkUser( host, port, realm, user )
+
+	local krb5 = KRB5:new()
+	local data = krb5:encodeASREQ(realm, user, port.protocol)
+	local socket = nmap.new_socket()
+	local status = socket:connect(host, port)
+
+	if ( not(status) ) then
+		return false, "ERROR: Failed to connect to Kerberos service"
+	end
+
+	socket:send(data)
+	status, data = socket:receive()
+	
+	if ( port.protocol == 'tcp' ) then data = data:sub(5) end
+	
+	if ( not(status) ) then
+		return false, "ERROR: Failed to receive result from Kerberos service"
+	end
+	socket:close()
+	
+	local msg
+	status, msg = krb5:parseResult(data)
+
+	if ( not(status) ) then
+		return false, "ERROR: Failed to parse the result returned from the Kerberos service"
+	end
+	
+	if ( msg and msg.error_code ) then
+		if ( msg.error_code == KRB5.ErrorMessages['KRB5KDC_ERR_PREAUTH_REQUIRED'] ) then
+			return true, "VALID"
+		elseif ( msg.error_code == KRB5.ErrorMessages['KDC_ERR_WRONG_REALM'] ) then
+			return false, "Invalid Kerberos REALM"
+		end
+	elseif ( msg.type == KRB5.MessageType['AS-REP'] ) then
+		return true, "VALID"
+	end
+	return true, "INVALID"
+end
+
+-- Checks whether the Kerberos REALM exists or not
+-- @param host table as received by the action method
+-- @param port table as received by the action method
+-- @param realm string containing the Kerberos REALM
+-- @return status boolean, true on success, false on failure
+local function isValidRealm( host, port, realm )
+	return checkUser( host, port, realm, "nmap")
+end
+
+-- Wraps the checkUser function so that it is suitable to be called from
+-- a thread. Adds a user to the result table in case it's valid.
+-- @param host table as received by the action method
+-- @param port table as received by the action method
+-- @param realm string containing the Kerberos REALM
+-- @param user string containing the Kerberos principal
+-- @param result table to which all discovered users are added
+local function checkUserThread( host, port, realm, user, result )
+	local condvar = nmap.condvar(result)
+	local status, state = checkUser(host, port, realm, user)
+	if ( status and state == "VALID" ) then
+		table.insert(result, ("%s@%s"):format(user,realm))
+	end
+	condvar "signal"
+end
+
+action = function( host, port )
+
+	local realm = stdnse.get_script_args("krb5-enum-users.realm")
+	local result = {}
+	local condvar = nmap.condvar(result)
+
+	-- did the user supply a realm
+	if ( not(realm) ) then
+		return "ERROR: No Kerberos REALM was supplied, aborting ..."
+	end
+
+	-- does the realm appear to exist
+	if ( not(isValidRealm(host, port, realm)) ) then
+		return "ERROR: Invalid Kerberos REALM, aborting ..."
+	end
+
+	-- load our user database from unpwdb
+	local status, usernames = unpwdb.usernames()
+	if( not(status) ) then return "ERROR: Failed to load unpwdb usernames" end
+		
+	-- start as many threads as there are names in the list
+	local threads = {}
+	for user in usernames do
+		local co = stdnse.new_thread( checkUserThread, host, port, realm, user, result )
+		threads[co] = true
+	end
+	
+	-- wait for all threads to finish up
+	repeat
+		condvar "wait"
+		for t in pairs(threads) do
+			if ( coroutine.status(t) == "dead" ) then threads[t] = nil end
+		end
+	until( next(threads) == nil )
+	
+	if ( #result > 0 ) then
+		result = { name = "Discovered Kerberos principals", result }
+	end
+	return stdnse.format_output(true, result)
+end
\ No newline at end of file
diff --git a/scripts/script.db b/scripts/script.db
index fa2d98e05..17b63d8e9 100644
--- a/scripts/script.db
+++ b/scripts/script.db
@@ -124,6 +124,7 @@ Entry { filename = "irc-unrealircd-backdoor.nse", categories = { "exploit", "int
 Entry { filename = "iscsi-brute.nse", categories = { "brute", "intrusive", } }
 Entry { filename = "iscsi-info.nse", categories = { "default", "discovery", "safe", } }
 Entry { filename = "jdwp-version.nse", categories = { "version", } }
+Entry { filename = "krb5-enum-users.nse", categories = { "auth", "discovery", "safe", } }
 Entry { filename = "ldap-brute.nse", categories = { "brute", "intrusive", } }
 Entry { filename = "ldap-novell-getpass.nse", categories = { "discovery", "safe", } }
 Entry { filename = "ldap-rootdse.nse", categories = { "discovery", "safe", } }