Attempt to index ? (a nil value)

os.loadAPI("sha")

local passPath = "passwords" --Change this if you want a different password file

if not fs.exists(passPath) then --Create a password file
	local f = fs.open(passPath, "w")
	f.write(textutils.serialize({}))
	f.close()
end

local function secureInput( mask, prestring )
	local l = true
	term.setCursorBlink(true)
	local prestring = prestring or ""
	local str = ""
	local sx, sy = term.getCursorPos()
	while l do
		local e, a, b, c, d = os.pullEventRaw()
		if e == "char" then
			str = str .. a
		elseif e == "key" then
			if a == 14 then
				str = str:sub(1, -2)
			elseif a == 28 then
				l = false
				term.setBackgroundColour( colours.black )
				term.setTextColour( colours.white )
				print()
				return str
			end
		end
		term.setCursorPos(sx, sy)
		term.setBackgroundColour( colours.white )
		term.setTextColour( colours.black )
		term.clearLine()
		if mask then
			write( prestring .. " > " .. string.rep(mask, #str) )
		else
			write( prestring .. " > " .. str )
		end
	end
end

local function getPass( usr )
	local f = fs.open(passPath,"r")
	local pwds = textutils.unserialize( f.readAll() )
	f.close()
	if pwds[ usr ] then
		return pwds[ usr ]
	else
		return false
	end
end

local function login( usr, pwd )
	local info = getPass( usr )
	if info then
		if sha.sha256( pwd .. info.salt ) == info.pwd then
			return true
		else
			return false
		end
	else
		return false
	end
end

--Login script--
local loop = true
while loop do
	local username = secureInput( nil, "username" )
	local password = secureInput( "x", "password" )
	if login( username, password ) then
		print("Login successfull!")
		loop = false
		break
		--DO SOMETHING
	else
		print("Login failed.")
		--REMOVE these 2 lines below so that the loop continues after incorrect login, once proven program works.
		loop = false
		break
	end
end