static
nsresult
Base64urlEncode(const PRUint8* aBytes,
PRUint32 aNumBytes,
nsCString& _result)
{
// SetLength does not set aside space for NULL termination. PL_Base64Encode
// will not NULL terminate, however, nsCStrings must be NULL terminated. As a
// result, we set the capacity to be one greater than what we need, and the
// length to our desired length.
PRUint32 length = (aNumBytes + 2) / 3 * 4; // +2 due to integer math.
NS_ENSURE_TRUE(_result.SetCapacity(length + 1), NS_ERROR_OUT_OF_MEMORY);
_result.SetLength(length);
(void)PL_Base64Encode(reinterpret_cast<const char*>(aBytes), aNumBytes,
_result.BeginWriting());
// base64url encoding is defined in RFC 4648. It replaces the last two
// alphabet characters of base64 encoding with '-' and '_' respectively.
_result.ReplaceChar('+', '-');
_result.ReplaceChar('/', '_');
return NS_OK;
}