---
-- DICOM library
--
-- This library implements (partially) the DICOM protocol. This protocol is used to
-- capture, store and distribute medical images.
--
-- From Wikipedia:
-- The core application of the DICOM standard is to capture, store and distribute
-- medical images. The standard also provides services related to imaging such as
-- managing imaging procedure worklists, printing images on film or digital media
-- like DVDs, reporting procedure status like completion of an imaging acquisition,
-- confirming successful archiving of images, encrypting datasets, removing patient
-- identifying information from datasets, organizing layouts of images for review,
-- saving image manipulations and annotations, calibrating image displays, encoding
-- ECGs, encoding CAD results, encoding structured measurement data, and storing
-- acquisition protocols.
--
-- OPTIONS:
-- *<code>called_aet</code> - If set it changes the called Application Entity Title
--                            used in the requests. Default: ANY-SCP
-- *<code>calling_aet</code> - If set it changes the calling Application Entity Title
--                            used in the requests. Default: ECHOSCU
--
-- @args dicom.called_aet Called Application Entity Title. Default: ANY-SCP
-- @args dicom.calling_aet Calling Application Entity Title. Default: ECHOSCU
-- 
-- @author Paulino Calderon <paulino@calderonpale.com>
-- @copyright Same as Nmap--See https://nmap.org/book/man-legal.html
---

local nmap = require "nmap"
local stdnse = require "stdnse"
local string = require "string"
local table = require "table"

_ENV = stdnse.module("dicom", stdnse.seeall)

local MIN_SIZE_ASSOC_REQ = 68
local MAX_SIZE_PDU = 128000
local MIN_HEADER_LEN = 6
local PDU_NAMES = {}
local PDU_CODES = {}

PDU_CODES =
{
  ASSOCIATE_REQUEST  = 0x01,
  ASSOCIATE_ACCEPT   = 0x02,
  ASSOCIATE_REJECT   = 0x03,
  DATA               = 0x04,
  RELEASE_REQUEST    = 0x05,
  RELEASE_RESPONSE   = 0x06,
  ABORT              = 0x07
}

for i, v in pairs(PDU_CODES) do
  PDU_NAMES[v] = i
end

---
-- start_connection(host, port) starts socket to DICOM service
--
-- @param host Host object
-- @param port Port table
-- @return (status, socket) If status is true, socket of DICOM object is set.
--                          If status is false, socket is the error message.
---
function start_connection(host, port)
  local dcm = {}
  local status, err
  dcm['socket'] = nmap.new_socket()

  status, err = dcm['socket']:connect(host, port, "tcp")

  if not status then
    return false, "DICOM: Failed to connect to host: " .. err
  end

  return true, dcm
end

---
-- send(dcm, data) Sends DICOM packet over established socket
--
-- @param dcm DICOM object
-- @param data Data to send
-- @return status True if data was sent correctly, otherwise false and error message is returned.
---
function send(dcm, data) 
  local status, err
  stdnse.debug2("DICOM: Sending DICOM packet (%d)", #data)
  if dcm['socket'] then
    status, err = dcm['socket']:send(data)
    if not status then
      return false, err
    end
  else 
    return false, "No socket found. Check your DICOM object"
  end
  return true
end

---
-- receive(dcm) Reads DICOM packets over an established socket
--
-- @param dcm DICOM object
-- @return (status, data) Returns data if status true, otherwise data is the error message.
---
function receive(dcm)
  local status, data = dcm['socket']:receive()
  if not status then
    return false, data
  end
  stdnse.debug1("DICOM: receive() read %d bytes", #data)
  return true, data
end

---
-- pdu_header_encode(pdu_type, length) encodes the DICOM PDU header
--
-- @param pdu_type PDU type as ann unsigned integer
-- @param length Length of the DICOM message
-- @return (status, dcm) If status is true, the DICOM object with the header set is returned.
--                       If status is false, dcm is the error message.
---
function pdu_header_encode(pdu_type, length)
  -- Some simple sanity checks, we do not check ranges to allow users to create malformed packets.
  if not(type(pdu_type)) == "number" then
    return false, "PDU Type must be an unsigned integer. Range:0-7"
  end
  if not(type(length)) == "number" then
    return false, "Length must be an unsigned integer."
  end

  local header = string.pack("<B >B I4",
                            pdu_type, -- PDU Type ( 1 byte - unsigned integer in Big Endian )
                            0,        -- Reserved section ( 1 byte that should be set to 0x0 )
                            length)   -- PDU Length ( 4 bytes - unsigned integer in Little Endian)
  if #header < MIN_HEADER_LEN then
    return false, "Header must be at least 6 bytes. Something went wrong."
  end
   return true, header
end

---
-- associate(host, port) Attempts to associate to a DICOM Service Provider by sending an A-ASSOCIATE request.
--
-- @param host Host object
-- @param port Port object
-- @return (status, dcm) If status is true, the DICOM object is returned.
--                       If status is false, dcm is the error message.
---

function associate(host, port, calling_aet, called_aet)
  local application_context = ""
  local presentation_context = ""
  local userinfo_context = ""
  
  local status, dcm = start_connection(host, port)
  if not status then
    return false, dcm
  end
  
  local application_context_name = "1.2.840.10008.3.1.1.1"
  application_context = string.pack(">B B s2",
                                    0x10,
                                    0x0,
                                    application_context_name)
  
  local abstract_syntax_name = "1.2.840.10008.1.1"
  local transfer_syntax_name = "1.2.840.10008.1.2"
  presentation_context = string.pack(">B B I2 B B B B B B s2 B B s2",
                                    0x20, -- Presentation context type ( 1 byte )
                                    0x0,  -- Reserved ( 1 byte )
                                    0x2e,   -- Item Length ( 2 bytes )
                                    0x1,  -- Presentation context id ( 1 byte )
                                    0x0,0x0,0x0,  -- Reserved ( 3 bytes )
                                    0x30, -- Abstract Syntax Tree ( 1 byte )
                                    0x0,  -- Reserved ( 1 byte )
                                    abstract_syntax_name,
                                    0x40, -- Transfer Syntax ( 1 byte )
                                    0x0,  -- Reserved ( 1 byte )
                                    transfer_syntax_name)
                                    
  local implementation_id = "1.2.276.0.7230010.3.0.3.6.2"
  local implementation_version = "OFFIS_DCMTK_362"
  userinfo_context = string.pack(">B B I2 B B I2 I4 B B s2 B B s2",
                                0x50,    -- Type 0x50 (1 byte)
                                0x0,     -- Reserved ( 1 byte )
                                0x3a,    -- Length ( 2 bytes )
                                0x51,    -- Type 0x51 ( 1 byte) 
                                0x0,     -- Reserved ( 1 byte)
                                0x04,     -- Length ( 2 bytes )
                                0x4000,   -- DATA ( 4 bytes )
                                0x52,    -- Type 0x52 (1 byte)
                                0x0,
                                implementation_id,
                                0x55,
                                0x0,
                                implementation_version)
  
  local called_ae_title = called_aet or stdnse.get_script_args("dicom.called_aet") or "ANY-SCP"
  local calling_ae_title = calling_aet or stdnse.get_script_args("dicom.calling_aet") or "ECHOSCU"
  if #called_ae_title > 16 or #calling_ae_title > 16 then
    return false, "Calling/Called Application Entity Title must be less than 16 bytes"
  end
  called_ae_title = ("%-16s"):format(called_ae_title)
  calling_ae_title = ("%-16s"):format(calling_ae_title)

 -- ASSOCIATE request
  local assoc_request = string.pack(">I2 I2 c16 c16 c32",
                                  0x1, -- Protocol version ( 2 bytes )
                                  0x0, -- Reserved section ( 2 bytes that should be set to 0x0 )
                                  called_ae_title, -- Called AE title ( 16 bytes)
                                  calling_ae_title, -- Calling AE title ( 16 bytes)
                                  "") -- Reserved section ( 32 bytes set to 0x0 )
                                  .. application_context
                                  .. presentation_context
                                  .. userinfo_context
 
  local status, header = pdu_header_encode(PDU_CODES["ASSOCIATE_REQUEST"], #assoc_request)

  -- Something might be wrong with our header
  if not status then
    return false, header
  end

  assoc_request = header .. assoc_request
 
  stdnse.debug2("PDU len minus header:%d", #assoc_request-#header)
  if #assoc_request < MIN_SIZE_ASSOC_REQ then
    return false, string.format("ASSOCIATE request PDU must be at least %d bytes and we tried to send %d.", MIN_SIZE_ASSOC_REQ, #assoc_request)
  end 
  local status, err = send(dcm, assoc_request)
  if not status then
    return false, string.format("Couldn't send ASSOCIATE request:%s", err)
  end
  status, err = receive(dcm)
  if not status then
    return false, string.format("Couldn't read ASSOCIATE response:%s", err)
  end

  local resp_type, _, resp_length = string.unpack(">B B I4", err)
  stdnse.debug1("PDU Type:%d Length:%d", resp_type, resp_length)
  if resp_type == PDU_CODES["ASSOCIATE_ACCEPT"] then
    stdnse.debug1("ASSOCIATE ACCEPT message found!")
    return true, dcm
  elseif resp_type == PDU_CODES["ASSOCIATE_REJECT"] then
    stdnse.debug1("ASSOCIATE REJECT message found!")
    return false, "ASSOCIATE REJECT received"
  else
    return false, "Received unknown response"
  end
end

function send_pdata(dicom, data)
  local status, header = pdu_header_encode(PDU_CODES["DATA"], #data)
  if not status then
    return false, header
  end
  local err
  status, err = send(dicom, header .. data)
  if not status then
    return false, err
  end
end

return _ENV
