base64.urlsafe_b64encode and the equal character

Gabriel Genellina gagsl-py2 at yahoo.com.ar
Fri Mar 28 17:56:34 CET 2008


En Fri, 28 Mar 2008 13:22:06 -0300, Clodoaldo <clodoaldo.pinto at gmail.com>  
escribió:
> On Mar 28, 12:09 pm, "Gabriel Genellina" <gagsl-... at yahoo.com.ar> wrote:
>> En Fri, 28 Mar 2008 10:54:49 -0300, Clodoaldo  
>> <clodoaldo.pi... at gmail.com> escribió:
>>
>> > What i need to know is where can an equal char appear in a
>> > urlsafe_b64encoded string?:
>>
>> > a)only at end;
>> > b)both at the end and at the begginig;
>> > c)anywhere in the string;
>>
>> > A sure answer will make my regexp safer.
>>
>> Only at the end. The encoded string has 4*n chars when the input string
>> has 3*n chars; when the input length is 3*n+1 or 3*n+2, the output has
>> 4*(n+1) chars right padded with 2 or 1 "=" chars.
>> If your input has 3n chars, the output won't have any "="
>
> Thanks. But I'm not sure i get it. What is n?

(Any nonnegative integer...)
I mean: For base64 encoding, the length of the output depends solely of  
the length of the input. If the input string length is a multiple of 3,  
the output length is a multiple of 4, with no "=". If the input length is  
one more than a multiple of 3, the output has two "==" at the end. If the  
input length is two more than a multiple of 3, the output has only one "="  
at the end. In all cases, the output length is a multiple of 4.

[base64 uses 64=2**6 characters so it encodes 6 bits per character; to  
encode 3 bytes=3*8=24 bits one requires 24/6=4 characters]

> A md5 digest will always be 16 bytes length. So if i understand it
> correctly (not sure) the output will always be 22 chars plus two
> trailing equal chars. Right?

Exactly.

-- 
Gabriel Genellina




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