SubtleCrypto.unwrapKey()

const result = crypto.subtle.unwrapKey(
    format,
    wrappedKey,
    unwrappingKey,
    unwrapAlgo,
    unwrappedKeyAlgo,
    extractable,
    keyUsages
);

/*
Salt that is to be used in derivation of the key-wrapping key,
alongside the password the user supplies.
This must match the salt value that was originally used to derive
the key.
*/
const saltBytes = [89,113,135,234,168,204,21,36,55,93,1,132,242,242,192,156];

/*
The wrapped key itself.
*/
const wrappedKeyBytes = [171,223,14,36,201,233,233,120,164,68,217,192,226,80,
    224,39,199,235,239,60,212,169,100,23,61,54,244,197,160,80,109,230,207,
    225,57,197,175,71,80,209];

/*
Convert an array of byte values to an ArrayBuffer.
*/
function bytesToArrayBuffer(bytes) {
  const bytesAsArrayBuffer = new ArrayBuffer(bytes.length);
  const bytesUint8 = new Uint8Array(bytesAsArrayBuffer);
  bytesUint8.set(bytes);
  return bytesAsArrayBuffer;
}

/*
Get some key material to use as input to the deriveKey method.
The key material is a password supplied by the user.
*/
function getKeyMaterial() {
  let password = window.prompt("Enter your password");
  let enc = new TextEncoder();
  return window.crypto.subtle.importKey(
    "raw",
    enc.encode(password),
    {name: "PBKDF2"},
    false,
    ["deriveBits", "deriveKey"]
  );
}

/*
Derive an AES-KW key using PBKDF2.
*/
async function getUnwrappingKey() {
  // 1. get the key material (user-supplied password)
  const keyMaterial = await getKeyMaterial();
  // 2 initialize the salt parameter.
  // The salt must match the salt originally used to derive the key.
  // In this example it's supplied as a constant "saltBytes".
  const saltBuffer = bytesToArrayBuffer(saltBytes);
  // 3 derive the key from key material and salt
  return window.crypto.subtle.deriveKey(
    {
      "name": "PBKDF2",
      salt: saltBuffer,
      "iterations": 100000,
      "hash": "SHA-256"
    },
    keyMaterial,
    { "name": "AES-KW", "length": 256},
    true,
    [ "wrapKey", "unwrapKey" ]
  );
}

/*
Unwrap an AES secret key from an ArrayBuffer containing the raw bytes.
Takes an array containing the bytes, and returns a Promise
that will resolve to a CryptoKey representing the secret key.
*/
async function unwrapSecretKey(wrappedKey) {
  // 1. get the unwrapping key
  const unwrappingKey = await getUnwrappingKey();
  // 2. initialize the wrapped key
  const wrappedKeyBuffer = bytesToArrayBuffer(wrappedKey);
  // 3. unwrap the key
  return window.crypto.subtle.unwrapKey(
    "raw",                 // import format
    wrappedKeyBuffer,      // ArrayBuffer representing key to unwrap
    unwrappingKey,         // CryptoKey representing key encryption key
    "AES-KW",              // algorithm identifier for key encryption key
    "AES-GCM",             // algorithm identifier for key to unwrap
    true,                  // extractability of key to unwrap
    ["encrypt", "decrypt"] // key usages for key to unwrap
  );
}

/*
Salt that is to be used in derivation of the key-wrapping key,
alongside the password the user supplies.
This must match the salt value that was originally used to derive
the key.
*/
const saltBytes = [180,253,62,216,47,35,90,55,218,233,103,10,172,143,161,177];

/*
IV that is to be used in decrypting the key to unwrap.
This must the same IV that was originally used to encrypt
the key.
*/
const ivBytes = [212,187,26,247,172,51,37,151,27,177,249,142];

/*
The wrapped key itself.
*/
const wrappedKeyBytes = [6,155,182,208,7,141,44,18,3,151,58,126,68,100,252,
  225,241,11,25,201,153,171,102,174,150,29,62,195,110,138,106,109,14,6,108,
  148,104,221,22,93,102,221,146,25,65,112,4,140,79,194,164,163,156,250,108,
  11,14,220,78,194,161,17,14,57,121,70,13,28,220,210,78,32,46,217,36,165,220,
  170,244,152,214,150,83,2,138,128,11,251,227,213,72,100,158,10,162,40,195,
  60,248,77,37,156,34,10,213,171,67,147,73,231,31,63,80,176,103,206,187,164,
  214,250,49,223,185,5,48,241,17,1,253,59,185,181,209,255,42,223,175,90,159,
  174,169,205,156,120,195,1,135,165,226,46,119,27,97,183,23,197,227,85,138,
  235,79,158,167,59,62,194,34,210,214,240,215,101,233,63,138,53,87,253,189,
  27,66,150,76,242,76,102,174,179,163,184,205,11,161,224,19,110,34,175,192,
  101,117,169,86,66,56,241,128,13,156,165,125,139,110,138,50,108,129,251,137,
  26,186,110,117,113,207,179,59,213,18,175,14,203,192,2,97,131,125,167,227,
  182,87,72,123,54,156,60,195,88,224,96,46,126,245,251,247,147,110,147,173,
  82,106,93,210,55,71,127,133,41,37,181,17,106,16,158,220,136,43,75,133,96,
  240,151,116,40,44,254,2,32,74,226,193,172,48,211,71,109,163,143,30,92,28,
  30,183,25,16,176,207,77,93,139,242,114,91,218,126,123,234,18,9,245,53,46,
  172,215,62,92,249,191,17,27,0,58,151,33,23,169,93,177,253,152,147,198,196,
  226,42,202,166,99,250,127,40,221,196,121,195,198,235,30,159,159,95,182,107,
  175,137,177,49,72,63,131,162,198,186,22,255,230,237,195,56,147,177,101,52,
  227,125,32,180,242,47,92,212,6,148,218,107,125,137,123,15,51,107,159,228,
  238,212,60,54,184,48,110,248,252,208,46,23,149,78,169,201,68,242,193,251,
  156,227,42,90,109,102,172,61,207,124,96,98,79,37,218,16,212,139,162,0,183,
  235,171,75,18,84,160,120,173,156,187,99,24,58,88,213,148,24,193,111,75,169,
  10,158,207,148,84,249,156,248,19,221,2,175,1,8,74,221,212,244,123,34,223,
  175,54,166,101,51,175,141,80,87,9,146,72,223,46,251,199,192,2,22,125,16,15,
  99,26,159,165,133,172,169,26,236,44,86,182,162,81,143,249,15,207,12,232,15,
  205,199,78,133,199,19,232,183,33,183,72,117,72,27,43,254,13,17,252,1,143,
  137,154,10,4,77,85,24,85,143,200,81,76,171,43,124,42,191,150,70,10,90,178,
  198,40,233,233,225,146,231,209,254,2,90,216,5,97,105,204,82,88,81,99,92,
  159,116,192,223,148,252,12,24,197,211,187,212,98,252,201,154,184,65,54,47,
  13,106,151,168,208,112,212,74,204,36,233,98,104,58,103,1,194,13,26,109,101,
  60,42,3,215,20,25,99,176,63,28,112,102,121,190,96,198,228,196,78,38,82,37,
  248,42,150,115,6,10,22,101,42,237,175,69,232,212,231,40,193,70,211,245,106,
  231,175,150,88,105,170,139,238,196,64,218,250,47,165,22,36,196,161,30,79,
  175,14,133,88,129,182,56,140,147,168,134,91,68,172,110,195,134,156,68,78,
  249,215,68,250,11,23,70,59,156,99,75,249,159,84,16,206,93,16,130,34,66,210,
  82,252,53,251,84,59,226,212,154,15,20,163,58,228,109,53,214,151,237,10,169,
  107,180,123,174,159,182,8,240,115,115,220,131,128,79,80,61,133,58,24,98,
  193,225,56,36,159,254,199,49,44,160,28,81,140,163,24,143,114,31,237,235,
  250,83,72,215,44,232,182,45,39,182,193,248,65,174,186,52,219,30,198,48,1,
  134,151,81,114,38,124,7,213,205,138,28,22,216,76,46,224,241,88,156,7,62,
  23,104,34,54,25,156,93,212,133,182,61,93,255,195,68,244,234,53,132,151,140,
  72,146,127,113,227,34,243,218,222,47,218,113,18,173,203,158,133,90,156,214,
  77,20,113,1,231,164,52,55,69,132,24,68,131,212,7,153,34,179,113,156,81,
  127,83,57,29,195,90,64,211,115,202,188,5,42,188,142,203,109,231,53,206,72,
  220,90,23,12,1,178,122,60,221,68,6,14,154,108,203,171,142,159,249,13,55,52,
  110,214,33,147,164,181,50,79,164,200,83,251,40,105,223,50,0,115,240,146,23,
  122,80,204,169,38,198,154,31,29,23,236,39,35,131,147,242,163,138,158,236,
  117,7,108,33,132,98,50,111,46,146,251,82,34,85,5,130,237,67,40,170,235,124,
  92,66,71,239,12,97,136,251,1,206,13,51,232,92,46,35,95,5,123,24,183,99,243,
  124,75,155,89,66,54,72,17,255,99,137,199,232,204,9,248,78,35,218,136,117,
  239,102,240,187,40,89,244,140,109,229,120,116,54,207,171,11,248,190,199,81,
  53,109,8,188,51,93,165,34,255,165,191,198,130,220,41,192,166,194,69,104,
  124,158,122,236,176,24,60,87,240,42,158,143,37,143,208,155,249,230,21,4,
  230,56,194,62,235,132,14,50,180,216,134,28,25,159,64,199,161,236,60,233,
  160,172,68,169,2,5,252,190,20,54,115,248,63,93,107,156,8,96,85,32,189,118,
  66,114,126,64,203,97,235,13,18,102,192,51,59,5,122,171,96,129,40,32,154,4,
  191,234,75,184,112,201,244,110,50,216,44,88,139,175,58,112,7,52,25,64,112,
  40,148,187,39,234,96,151,16,158,114,113,109,164,47,108,94,148,35,232,221,
  33,110,126,170,25,234,45,165,180,210,193,120,247,155,127];

/*
The unwrapped signing key.
*/
let signingKey;

const signButton = document.querySelector(".pkcs8 .sign-button");

/*
Convert an array of byte values to an ArrayBuffer.
*/
function bytesToArrayBuffer(bytes) {
  const bytesAsArrayBuffer = new ArrayBuffer(bytes.length);
  const bytesUint8 = new Uint8Array(bytesAsArrayBuffer);
  bytesUint8.set(bytes);
  return bytesAsArrayBuffer;
}

/*
Get some key material to use as input to the deriveKey method.
The key material is a password supplied by the user.
*/
function getKeyMaterial() {
  let password = window.prompt("Enter your password");
  let enc = new TextEncoder();
  return window.crypto.subtle.importKey(
    "raw",
    enc.encode(password),
    {name: "PBKDF2"},
    false,
    ["deriveBits", "deriveKey"]
  );
}

/*
Derive an AES-GCM key using PBKDF2.
*/
async function getUnwrappingKey() {
  // 1. get the key material (user-supplied password)
  const keyMaterial = await getKeyMaterial();
  // 2 initialize the salt parameter.
  // The salt must match the salt originally used to derive the key.
  // In this example it's supplied as a constant "saltBytes".
  const saltBuffer = bytesToArrayBuffer(saltBytes);
  // 3 derive the key from key material and salt
  return window.crypto.subtle.deriveKey(
    {
      "name": "PBKDF2",
      salt: saltBuffer,
      "iterations": 100000,
      "hash": "SHA-256"
    },
    keyMaterial,
    { "name": "AES-GCM", "length": 256},
    true,
    [ "wrapKey", "unwrapKey" ]
  );
}

/*
Unwrap an RSA-PSS private signing key from an ArrayBuffer containing
the raw bytes.
Takes an array containing the bytes, and returns a Promise
that will resolve to a CryptoKey representing the private key.
*/
async function unwrapPrivateKey(wrappedKey) {
  // 1. get the unwrapping key
  const unwrappingKey = await getUnwrappingKey();
  // 2. initialize the wrapped key
  const wrappedKeyBuffer = bytesToArrayBuffer(wrappedKey);
  // 3. initialize the iv
  const ivBuffer = bytesToArrayBuffer(ivBytes);
  // 4. unwrap the key
  return window.crypto.subtle.unwrapKey(
    "pkcs8",               // import format
    wrappedKeyBuffer,      // ArrayBuffer representing key to unwrap
    unwrappingKey,         // CryptoKey representing key encryption key
    {                      // algorithm params for key encryption key
      name: "AES-GCM",
      iv: ivBuffer
    },
    {                      // algorithm params for key to unwrap
      name: "RSA-PSS",
      hash: "SHA-256"
    },
    true,                  // extractability of key to unwrap
    ["sign"]               // key usages for key to unwrap
  );
}