问OCR签名失败 问题，{"code":14,"message":"sign no pass"}？

var req = require('request');
var crypto = require('crypto');

var secretId = ' ',
    secretKey = ' ',
    appid = ' ',
    pexpired = 86400,
    userid = 0;

var now = parseInt(Date.now() / 1000),
    rdm = parseInt(Math.random() * Math.pow(2, 32)),
    plainText = 'a=' + appid + '&k=' + secretId + '&e=' + (now + pexpired) + '&t=' + now + '&r=' + rdm + userid + '&f=',
    data = new Buffer(plainText, 'utf8'),
    res = crypto.createHmac('sha1', secretKey).update(data).digest(),
    bin = Buffer.concat([res, data]);

var sign = bin.toString('base64');
console.log('sign : ' + sign);
req.post({
        url: 'http://recognition.image.myqcloud.com/ocr/businesscard' + '?appid=' + appid + '&bucket=test1&url_list=["http://yoututest-1251966477.cossh.myqcloud.com/mingpian.jpg"]',
        headers: {
            'content-type': 'application/json',
            'Authorization': sign, 
            'host': 'recognition.image.myqcloud.com'
        },
        method: 'POST'
    },
    function(e, r, body) {
        console.log(e);
        console.log(body);
    });