TypeScript implementation of steganography using variant selectors

I wanted to actually try out Steganography using Unicode variant selectors: How to secretly hide secret strings in text #JavaScript - Qiita, but I wanted to implement it in TypeScript and make it work properly.

So I implemented it.

Code

class Steganography {

  static #components = {
    VS_BEGIN: 0xfe00,
    STR_EMPTY: "",
    REPLACER: /\uFEFF[\uFE00-\uFE0F]+/u,
    toHex(value: number) {
      return value.toString(16);
    },
    toHexDecimal(str: string): number {
      return Number.parseInt(str, 16);
    },
    encode: {
      PREFIX: "\u{FEFF}"
    }
  }


  static encode(body: string, hidden: string): string {
    const { VS_BEGIN, STR_EMPTY, encode, toHex, toHexDecimal } = this.#components;
    return encode.PREFIX + [...hidden].flatMap(c => [...toHex(c.charCodeAt(0))])
      .map((s) => String.fromCodePoint(VS_BEGIN + toHexDecimal(s)))
      .join(STR_EMPTY) + body
  }

  static decode(value: string): [body: string, hidden: string] {
    const { VS_BEGIN, STR_EMPTY, REPLACER, toHex, toHexDecimal } = this.#components;

    let hidden = STR_EMPTY;
    const body = value.replace(REPLACER, ([_, ...variationSelectors]) => {
      hidden += variationSelectors
        .map((c) => toHex(c.codePointAt(0)! - VS_BEGIN))
        .reduce<string[][]>((acc, _, index, arr) => {
          return index % 2 === 0 ? [...acc, arr.slice(index, index + 2)] : acc;
        }, [])
        .map(([a, b]) => String.fromCharCode(toHexDecimal(a + b)))
        .join(STR_EMPTY);
      return STR_EMPTY;
    });
    return [body, hidden]
  }
}

Points

Variation Selector: Use the range U+FE00-U+FE0F
ZERO WIDTH NO-BREAK SPACE(U+FEFF): Hidden string start marker
Encoding: Convert characters to HEX and express with variant selector
Decode: Extract with regular expression and restore original string

Usage example

const body = "公開されるテキスト";
const hidden = "秘密のメッセージ";

const encoded = Steganography.encode(body, hidden);
console.log(encoded); // 見た目は「公開されるテキスト」

const [decodedBody, decodedHidden] = Steganography.decode(encoded);
console.log(decodedBody);   // "公開されるテキスト"
console.log(decodedHidden); // "秘密のメッセージ"

summary

By leveraging Unicode’s glyph selector, you can embed secret strings into text in a way that is visually undetectable.

Regardless of its practicality, it’s a good subject to learn how Unicode works.