```ts
export function lineTag(body: string): string {
  if (typeof body !== "string") {
    throw new Error("lineTag expects a string");
  }
  let sum = 0;
  for (const ch of body) {
    sum = (sum + ch.codePointAt(0)!) % 256;
  }
  return sum.toString(16).padStart(2, "0");
}

export function checkLine(line: string): string {
  if (typeof line !== "string" || line.length < 3) {
    throw new Error("Invalid line format");
  }

  const tildeIndex = line.lastIndexOf('~');
  if (tildeIndex === -1 || tildeIndex < 2) {
    throw new Error("Invalid line format");
  }

  const tag = line.slice(tildeIndex + 1);
  const body = line.slice(0, tildeIndex);

  if (tag.length !== 2) {
    throw new Error("Invalid tag length");
  }

  const expectedTag = lineTag(body);
  if (expectedTag !== tag) {
    throw new Error("Tag mismatch");
  }

  return body;
}
```