[boost] [serialization] Base64 padding in basic_text_oprimitive

From: Vyacheslav E. Andrejev (mortituris_at_[hidden])
Date: 2008-06-09 05:16:47

Hello,

Code that pads base64 output with '=' looks in basic_text_oprimitive.ipp 
like following:

    std::size_t padding = 2 - count % 3;
    if(padding > 1)
        *oi = '=';
        if(padding > 2)
            *oi = '=';

It will give the following transformations of a few simple strings:

"012" -> "MDEy="
"0123" -> "MDEyMw"
"01234" -> "MDEyMzQ"
"012345" -> "MDEyMzQ1="

Which violates the principle of the base 64 that the encoded output length 
is multiple of 4 bytes. Right output for the samples above should be:

"012" -> "MDEy"
"0123" -> "MDEyMw=="
"01234" -> "MDEyMzQ="
"012345" -> "MDEyMzQ1"

The problem with the code above is that the condition (padding > 2) is never 
satisfied. I think that the proper code should look like

        std::size_t padding = 2 - count % 3;
        if (padding < 2) {
          *oi = '=';
          if (padding > 0)
            *oi = '=';
        }

--
-- Vyacheslav Andrejev
-- System Architect, Optech International, Inc.