cal.pub0.org/apps/web/lib/buildNonce.ts

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const BASE64_ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
/*
The buildNonce array allows a randomly generated 22-unsigned-byte array
and returns a 24-ASCII character string that mimics a base64-string.
*/
export const buildNonce = (uint8array: Uint8Array): string => {
// the random uint8array should contain 22 bytes
// 22 bytes mimic the base64-encoded 16 bytes
// base64 encodes 6 bits (log2(64)) with 8 bits (64 allowed characters)
// thus ceil(16*8/6) gives us 22 bytes
if (uint8array.length != 22) {
return "";
}
// for each random byte, we take:
// a) only the last 6 bits (so we map them to the base64 alphabet)
// b) for the last byte, we are interested in two bits
// explaination:
// 16*8 bits = 128 bits of information (order: left->right)
// 22*6 bits = 132 bits (order: left->right)
// thus the last byte has 4 redundant (least-significant, right-most) bits
// it leaves the last byte with 2 bits of information before the redundant bits
// so the bitmask is 0x110000 (2 bits of information, 4 redundant bits)
const bytes = uint8array.map((value, i) => {
if (i < 20) {
return value & 0b111111;
}
return value & 0b110000;
});
const nonceCharacters: string[] = [];
bytes.forEach((value) => {
nonceCharacters.push(BASE64_ALPHABET.charAt(value));
});
// base64-encoded strings can be padded with 1 or 2 `=`
// since 22 % 4 = 2, we pad with two `=`
nonceCharacters.push("==");
// the end result has 22 information and 2 padding ASCII characters = 24 ASCII characters
return nonceCharacters.join("");
};