Strange Base64 encode/decode problem

Whatever populates params expects the request to be a URL-encoded form (specifically, application/x-www-form-urlencoded, where "+" means space), but you didn't URL-encode it. I don't know what functions your language provides, but in pseudo code, queryString should be constructed from

concat(uri_escape("data"), "=", uri_escape(base64_encode(rawBytes)))

which simplifies to

concat("data=", uri_escape(base64_encode(rawBytes)))

The "+" characters will be replaced with "%2B".

You have to use a special base64encode which is also url-safe. The problem is that standard base64encode includes +, / and = characters which are replaced by the percent-encoded version.

http://en.wikipedia.org/wiki/Base64#URL_applications

I'm using the following code in php:

    /**
     * Custom base64 encoding. Replace unsafe url chars
     *
     * @param string $val
     * @return string
     */
    static function base64_url_encode($val) {

        return strtr(base64_encode($val), '+/=', '-_,');

    }

    /**
     * Custom base64 decode. Replace custom url safe values with normal
     * base64 characters before decoding.
     *
     * @param string $val
     * @return string
     */
    static function base64_url_decode($val) {

        return base64_decode(strtr($val, '-_,', '+/='));

    }