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