问机器翻译API用http get方式时签名验证不通过？

local function transform(txt,target)
    local xhr = cc.XMLHttpRequest:new() 
    xhr.responseType = cc.XMLHTTPREQUEST_RESPONSE_STRING 
    local url = "https://tmt.tencentcloudapi.com/"
    local timestamp = ServerTimeUtil.getServerTime()
    local nonce = math.random(10,1000)
    local SecretId = "自己的id"
    local SecretKey = "自己的key"
    local singParam = "Action=TextTranslate&Nonce="..nonce.."&ProjectId=1300328761&Region=ap-guangzhou&SecretId="..SecretId.."&Source=auto&SourceText="..txt.."&Target="..target.."&Timestamp="..timestamp.."&Version=2018-03-21"
    local param = "?"..singParam
    -- local signStr = "GET".."&"..string.urlencode(url).."&"..string.urlencode(singParam)
    local signStr = "GETtmt.tencentcloudapi.com/?"..singParam
    local simpleParam = "ProjectId=1300328761&Source=auto&SourceText="..txt.."&Target="..target
    local simpleParam = '{"SourceText":"hello","Source":"auto","Target":"zh","ProjectId":1300328761}' 
    print("before sign:",signStr)
    local md5 = require("Game.Net.md5")
    local sha1 = require('Game.Net.sha1.init')
    local sha2 = require('Game.Net.sha1.sha2')
    local hmac = sha2.hmac
    -- local signMid = encodeBase64(sha1.hmac(SecretKey, signStr))
    -- local signMid = encodeBase64(md5.sumhexa(signStr))
    -- local signMid = md5.sumhexa(signStr)
    local signMid = hmac(sha2.sha256, SecretKey, simpleParam)
    --string.urlencode(signMid)
    print("after sign:",signMid)
    print("transform tecent:",url..param.."&Signature="..signMid)
    xhr:open("GET", url..param.."&Signature="..signMid) 
    --设置消息头
    xhr:setRequestHeader("Content-Type", "application/json")
    local function onReadyStateChange() 
        print("onReadyStateChange",xhr.readyState,xhr.status)
        if xhr.readyState == 4 and (xhr.status >= 200 and xhr.status < 207) then 
            local statusString = "Http Status Code:"..xhr.statusText
            print("请求返回状态码"..statusString)
            local xhrStr = xhr.response
            print("xhrStr:",xhrStr)
            local xhrObj = json.decode(xhrStr)
            xhr:abort()
        else
            xhr:abort()
        end
    end
    xhr:registerScriptHandler(onReadyStateChange) 
    xhr:send()
end