how to tell if a string is base64 or not

Question

I have many emails coming in from different sources. they all have attachments, many of them have attachment names in chinese, so these names are converted to base64 by their email clients.

When I receive these emails, I wish to decode the name. but there are other names which are not base64. How can I differentiate whether a string is base64 or not, using the jython programming language?

Ie.

First attachment:

------=_NextPart_000_0091_01C940CC.EF5AC860
Content-Type: application/vnd.ms-excel;
 name="Copy of Book1.xls"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
 filename="Copy of Book1.xls"

second attachment:

------=_NextPart_000_0091_01C940CC.EF5AC860
Content-Type: application/vnd.ms-excel;
 name="=?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?="
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
 filename="=?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?="  

Please note both "Content-Transfer-Encoding" have base64

Answer 1

The header value tells you this:

=?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?=

"=?"     introduces an encoded value
"gb2312" denotes the character encoding of the original value
"B"      denotes that B-encoding (equal to Base64) was used (the alternative 
         is "Q", which refers to something close to quoted-printable)
"?"      functions as a separator
"uLG..." is the actual value, encoded using the encoding specified before
"?="     ends the encoded value

So splitting on "?" actually gets you this (JSON notation)

["=", "gb2312", "B", "uLGxvmhlbrixsb5nLnhscw==", "="]

In the resulting array, if "B" is on position 2, you face a base-64 encoded string on position 3. Once you decoded it, be sure to pay attention to the encoding on position 1, probably it would be best to convert the whole thing to UTF-8 using that info.

Answer 2

Please note both Content-Transfer-Encoding have base64

Not relevant in this case, the Content-Transfer-Encoding only applies to the body payload, not to the headers.

=?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?=

That's an RFC2047-encoded header atom. The stdlib function to decode it is email.header.decode_header. It still needs a little post-processing to interpret the outcome of that function though:

import email.header
x= '=?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?='
try:
    name= u''.join([
        unicode(b, e or 'ascii') for b, e in email.header.decode_header(x)
    ])
except email.Errors.HeaderParseError:
    pass # leave name as it was

However...

Content-Type: application/vnd.ms-excel;
 name="=?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?="

This is simply wrong. What mailer created it? RFC2047 encoding can only happen in atoms, and a quoted-string is not an atom. RFC2047 §5 explicitly denies this:

• An 'encoded-word' MUST NOT appear within a 'quoted-string'.

The accepted way to encode parameter headers when long string or Unicode characters are present is RFC2231, which is a whole new bag of hurt. But you should be using a standard mail-parsing library which will cope with that for you.

So, you could detect the '=?' in filename parameters if you want, and try to decode it via RFC2047. However, the strictly-speaking-correct thing to do is to take the mailer at its word and really call the file =?gb2312?B?uLGxvmhlbrixsb5nLnhscw==?=!