Error Checking
CRC
GF(2) Math
GF(2) is pronounced as “Galois Field 2”. Often used in cryptography, it’s a binary field that contains only 0 and 1 for error correction.
Some properties of GF(2):
- Subtraction is actually XOR here.
- Addition is XOR too.
- Multiplication is and
- Division?
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0 + 0 = 0
1 + 0 = 1
0 + 1 = 1
1 + 1 = 0
0 - 0 = 0
1 - 0 = 1
0 - 1 = 1
1 - 1 = 0
0 * 0 = 0
1 * 0 = 0
0 * 1 = 0
1 * 1 = 1
When we have larger numbers, we use an abstract placeholder variable x to form a polynomial. For example:
And we apply the same addition subtraction rules (which is XOR):
For multiplication, we need: $x^ax^b = x^{a+b}$. and reduce coefficients mod 2 (1+1=0) So we have:
\[\begin{gather*} \begin{aligned} & 1011 \times 0110 = (x^3 + x + 1)(x^2 + x) \\ & = x^5 + x^3 + x^2 + x^4 + x^2 + x = \\ & = x^5 + x^4 + x^3 + x \end{aligned} \end{gather*}\]For division:
- Align the divisor with the highest degree of the dividend
- Subtract the two
- Shift the divident to the right, subtract, until the degree of the remainder is less than that of the divisor.
For example
\[\begin{gather*} \begin{aligned} & (x^4 + x^3 + x^2 + 1)/(x^3 + x^2 +1) = x + y + remainder \\ & \rightarrow (x^4 + x^3 + x^2 + 1) - (x^3 + x^2 +1)x = x^2 + x + 1 \\ & \text{Since: } \\ & (x^2 + x + 1) = 0 \times (x^3 + x^2 +1) + (x^2 + x + 1) \\ & \text{Therefore: } \\ & \rightarrow y = 0, remainder = x^2 + x + 1 \end{aligned} \end{gather*}\]CRC Math
CRC (Cyclic redundancy check) is a small value calculated on a piece of data to ensure data integrity. It was first proposed in 1961. Here, we walk through a simple CRC-3 example:
- Choose our generator polynomial (the Divisor above) to be 3rd order polynomial (since it’s CRC -3): $G(x) = x^3 + x + 1 (1011)$
- Append input data
1101by the highest degree of the generator (which is 3):1101 -> 1101000 - Divide the appended input by the generator polynomial:
So the remainder is 100, we choose that as the “checksum” of this message. We send the message with this checksum, then on the receiver end, we calculate the checksum of the received message, and compare that checksum with this checksum.
In reality, it’s common to use CRC-16 or CRC-32.
Encoding & Decoding
Base 64
Base 64 is to encode binary data into a text-safe format. Email, JSON, HTTP, XML are all designed to work with text. This is great for avoiding issues caused by
- Line breaks such as
\r,\n. Such delimeters could prematurely terminate transmission, e.g., JSON.- File systems / databases may reserve
/for directories, NULL. So really we can run into misinterpretation issues here.
- File systems / databases may reserve
- Special characters
<,>, could be misinterpreted as commands, codes, etc. E.g., in XML,&,<must be escaped because they interfere with XML tags
Base 64 replaces binary data with with a safe standardized char set: a-z, A-Z, 0-9, +-/.
So a sample workflow is:
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Binary file (e.g., an image) is encoded as Base64. -> Transmitted over a protocol like email or API -> Decoded back into the original binary format by the receiver.
