Base252 is an encoding scheme primarily intended to turn binary data into valid C strings with minimal overhead while allowing flexible optional escaping of any byte value.
The Base252 encoding draws inspiration from UTF-8.
| Dec | Hex | Name | Escape Code |
|---|---|---|---|
| 0 | 0x00 | NUL | 245 128 |
| 245 | 0xF5 | ESC_0 | 248 245 |
| 246 | 0xF6 | ESC_64 | 248 246 |
| 247 | 0xF7 | ESC_128 | 248 247 |
| 248 | 0xF8 | ESC_192 | 248 248 |
To keep things simple for starters I'll explain how to turn binary data into a C string. A C string exists of a sequence of bytes with values between 1 and 255. The 0 value is reserved as the string terminator and is also known as the NUL byte.
In order to turn binary data into a Base252 string each byte with the 0 value needs to be converted into a two byte sequence with the values 245 128.
This conversion process is known as escaping, which I'll describe in detail below.
Previously we saw the escaping of 0 to 245 128. However, in many cases programming languages have special characters that pose processing or security issues when they are not escaped. The \ and " characters come to mind, but in theory any character can be a special character that needs to be escaped.
In order to escape any ASCII character Base252 reserves character value 245, 246, 247 and 248. The math to escape is simple and similar to UTF-8.
245 + char / 64
128 + char % 64Besides the 0 byte each occurance of 245, 246, 247 and 248 in the data needs to be escaped using the formula above.
Characters 128 through 191 cannot be as easily fully escaped since they are typically used for the second byte of each escaped code. However, since the second byte needs to be modulated by 64, a value between 64 and 127, or between 192 and 255, could be used to encode the second byte. All character values are valid for the second byte, with the exception of 0.
Subsequently there are three valid ways to fully escape the 0 value: 245 64, 245 128, and 245 192.
Similarly there are four ways to escape the 248 value: 248 248, 248 184, 248 120, or 248 56.
The example below escapes the 5 required codes as well as the '\' character.
for (cnt = 0 ; cnt < size ; cnt++)
{
switch (input[cnt])
{
case 0:
*output++ = 245;
*output++ = 128 + input[cnt] % 64;
break;
case '\\':
*output++ = 246;
*output++ = 128 + input[cnt] % 64;
break;
case 245:
case 246:
case 247:
case 248:
*output++ = 248;
*output++ = 128 + input[cnt] % 64;
break;
default:
*output++ = input[cnt];
break;
}
}
*output++ = 0;Translating Base252 data back to its original format is even easier and the code below should translate any proper Base252 string.
while (input[cnt] != 0)
{
switch (input[cnt])
{
case 245:
*output++ = 0 + input[++cnt] % 64;
break;
case 246:
*output++ = 64 + input[++cnt] % 64;
break;
case 247:
*output++ = 128 + input[++cnt] % 64;
break;
case 248:
*output++ = 192 + input[++cnt] % 64;
break;
default:
*output++ = input[cnt++];
continue;
}
if (input[cnt] != 0)
{
cnt++;
}
}
*output++ = 0;A Base252 string needs to be terminated with a NUL byte.
After Base252 conversion we have a best case overhead of 0% (notably when turning ASCII or UTF-8 into Base252) and a worst case overhead of 100%. The average case should be an overhead of 1.7% and this should also be the typical case for compressed / encrypted data.
The primary usecase is to embed binary data in a typeless string based data storage system.
Base252 cannot easily be used in combination with JSON because the JSON specification requires data to contain valid unicode codepoints. However, if you are willing to ignore that requirement and use JSON in a closed environment, Base252 can be used if your JSON parser permits it.
Base252 was designed to work in conjunction with VTON, a typeless object notation.