Gmail API decoding messages in Javascript

Question

I am having serious problems decoding the message body of the emails I get using the Gmail API. I want to grab the message content and put the content in a div. I am using a

Answer 1

Use atob to decode the messages in JavaScript (see ref). For accessing your message payload, you can write a function:

var extractField = function(json, fieldName) {
  return json.payload.headers.filter(function(header) {
    return header.name === fieldName;
  })[0].value;
};
var date = extractField(response, "Date");
var subject = extractField(response, "Subject");

referenced from my previous SO Question and

var part = message.parts.filter(function(part) {
  return part.mimeType == 'text/html';
});
var html = atob(part.body.data);

If the above does not decode 100% properly, the comments by @cgenco on this answer below may apply to you. In that case, do

var html = atob(part.body.data.replace(/-/g, '+').replace(/_/g, '/'));

Answer 2

Thank @ento 's answer. I explain more why you need to replace '-' and '_' character to '+' and '/' before decode.

Wiki Base64 Variants summary table shows:

• RFC 4648 section 4: base64 (standard): use '+' and '/'
• RFC 4648 section 5: base64url (URL-safe and filename-safe standard): use '-' and '_'

In short, Gmail API use base64url (urlsafe) format('-' and '_'), But JavaScript atob function or other JavaScript libraries use base64 (standard) format('+' and '/').

For Gmail API, the document says body use base64url format, see below links:

• string/bytes type
• MessagePartBody
• RAW

For Web atob/btoa standards, see below links:

• The algorithm used by atob() and btoa() is specified in RFC 4648, section 4
• 8.3 Base64 utility methods
• Forgiving base64

Answer 3

For a prototype app I'm writing, the following code is working for me:

var base64 = require('js-base64').Base64;
// js-base64 is working fine for me.

var bodyData = message.payload.body.data;
// Simplified code: you'd need to check for multipart.

base64.decode(bodyData.replace(/-/g, '+').replace(/_/g, '/'));
// If you're going to use a different library other than js-base64,
// you may need to replace some characters before passing it to the decoder.

Caution: these points are not explicitly documented and could be wrong:

1. The users.messages: get API returns "parsed body content" by default. This data seems to be always encoded in UTF-8 and Base64, regardless of the Content-Type and Content-Transfer-Encoding header.

   For example, my code had no problem parsing an email with these headers: Content-Type: text/plain; charset=ISO-2022-JP, Content-Transfer-Encoding: 7bit.

2. The mapping table of the Base64 encoding varies among various implementations. Gmail API uses - and _ as the last two characters of the table, as defined by RFC 4648's "URL and Filename safe Alphabet"¹.

   Check if your Base64 library is using a different mapping table. If so, replace those characters with the ones your library accepts before passing the body to the decoder.

---

¹ There is one supportive line in the documentation: the "raw" format returns "body content as a base64url encoded string". (Thanks Eric!)

Answer 4

I can easily decode using another tool at https://simplycalc.com/base64-decode.php

In JS: https://www.npmjs.com/package/base64url

In Python 3:

import base64
base64.urlsafe_b64decode(coded_string)

Answer 5

I was also annoyed by this point. I discovered a solution through looking at an extension for VSCode. The solution is really simple:

const body = response.data.payload.body; // the base64 encoded body of a message
 body = Buffer.alloc(
        body.data.length,
        body.data,
        "base64"
      ).toString();  // the decoded message

It worked for me as I was using gmail.users.messages.get() call of Gmail API.

Answer 6

Here is the solution: Gmail API - "Users.messages: get" method has in response message.payload.body.data parted base64 data, it's separated by "-" symbol. It's not entire base64 encoded text, it's parts of base64 text. You have to try to decode every single part of this or make one mono string by unite and replace "-" symbol. After this you can easily decode it to human text. You can manually check every part here https://www.base64decode.org