SubtleCrypto 的非加密用途

<h3>Demonstration of hashing a file with SHA256</h3>

<label
  >Choose file(s) to hash <input type="file" id="file" name="file" multiple
/></label>
<output style="display:block;font-family:monospace;"></output>

const output = document.querySelector("output");
const file = document.getElementById("file");

// Run the hashing function when the user selects one or more file
file.addEventListener("change", hashTheseFiles);

// The digest function is asynchronous, it returns a promise
// We use the async/await syntax to simplify the code.
async function fileHash(file) {
  const arrayBuffer = await file.arrayBuffer();

  // Use the subtle crypto API to perform a SHA256 Sum of the file's
  // Array Buffer. The resulting hash is stored in an array buffer
  const hashAsArrayBuffer = await crypto.subtle.digest("SHA-256", arrayBuffer);

  // To display it as a string we will get the hexadecimal value of
  // each byte of the array buffer. This gets us an array where each byte
  // of the array buffer becomes one item in the array
  const uint8ViewOfHash = new Uint8Array(hashAsArrayBuffer);
  // We then convert it to a regular array so we can convert each item
  // to hexadecimal strings, where characters of 0-9 or a-f represent
  // a number between 0 and 15, containing 4 bits of information,
  // so 2 of them is 8 bits (1 byte).
  const hashAsString = Array.from(uint8ViewOfHash)
    .map((b) => b.toString(16).padStart(2, "0"))
    .join("");
  return hashAsString;
}

async function hashTheseFiles(e) {
  let outHTML = "";
  // iterate over each file in file select input
  for (const file of this.files) {
    // calculate its hash and list it in the output element.
    outHTML += `${file.name}    ${await fileHash(file)}\n`;
  }
  output.innerText = outHTML;
}

<h3>Demonstration of how git uses SHA1 for files</h3>

<label
  >Choose file(s) to hash <input type="file" id="file" name="file" multiple
/></label>

<output style="display:block;font-family:monospace;"></output>

const output = document.querySelector("output");
const file = document.getElementById("file");
file.addEventListener("change", hashTheseFiles);

async function fileHash(file) {
  const arrayBuffer = await file.arrayBuffer();

  // Git prepends the null terminated text 'blob 1234' where 1234
  // represents the file size before hashing so we are going to reproduce that

  // first we work out the Byte length of the file
  const uint8View = new Uint8Array(arrayBuffer);
  const length = uint8View.length;

  // Git in the terminal uses UTF8 for its strings; the Web uses UTF16.
  // We need to use an encoder because different binary representations
  // of the letters in our message will result in different hashes
  const encoder = new TextEncoder();
  // Null-terminated means the string ends in the null character which
  // in JavaScript is '\0'
  const view = encoder.encode(`blob ${length}\0`);

  // We then combine the 2 Array Buffers together into a new Array Buffer.
  const newBlob = new Blob([view.buffer, arrayBuffer], {
    type: "text/plain",
  });
  const arrayBufferToHash = await newBlob.arrayBuffer();

  // Finally we perform the hash this time as SHA1 which is what Git uses.
  // Then we return it as a string to be displayed.
  return hashToString(await crypto.subtle.digest("SHA-1", arrayBufferToHash));
}

function hashToString(arrayBuffer) {
  const uint8View = new Uint8Array(arrayBuffer);
  return Array.from(uint8View)
    .map((b) => b.toString(16).padStart(2, "0"))
    .join("");
}

// like before we iterate over the files
async function hashTheseFiles(e) {
  let outHTML = "";
  for (const file of this.files) {
    outHTML += `${file.name}    ${await fileHash(file)}\n`;
  }
  output.innerText = outHTML;
}