Yeah, except for the first few bytes. PKCS8 has some initial header information, but most of it is the OCTET_STRING of the private key itself.

The PEM (human “readable”) version is Base64, so you can craft up a string and make that your key. DER is the that converted to binary again:

/**
 * @see https://datatracker.ietf.org/doc/html/rfc5208#section-5
 * @see https://datatracker.ietf.org/doc/html/rfc2313#section-11
 * Unwraps PKCS8 Container for internal key (RSA or EC)
 * @param {string|Uint8Array} pkcs8
 * @param {string} [checkOID]
 * @return {Uint8Array} DER
 */
export function privateKeyFromPrivateKeyInformation(pkcs8, checkOID) {
  const der = derFromPrivateKeyInformation(pkcs8);
  const [
    [privateKeyInfoType, [
      [versionType, version],
      algorithmIdentifierTuple,
      privateKeyTuple,
    ]],
  ] = decodeDER(der);
  if (privateKeyInfoType !== 'SEQUENCE') throw new Error('Invalid PKCS8');
  if (versionType !== 'INTEGER') throw new Error('Invalid PKCS8');
  if (version !== 0) throw new Error('Unsupported PKCS8 Version');
  const [algorithmIdentifierType, algorithmIdentifierValues] = algorithmIdentifierTuple;
  if (algorithmIdentifierType !== 'SEQUENCE') throw new Error('Invalid PKCS8');
  const [privateKeyType, privateKey] = privateKeyTuple;
  if (privateKeyType !== 'OCTET_STRING') throw new Error('Invalid PKCS8');
  if (checkOID) {
    for (const [type, value] of algorithmIdentifierValues) {
      if (type === 'OBJECT_IDENTIFIER' && value === checkOID) {
        return privateKey;
      }
    }
    return null; // Not an error, just doesn't match
  }

  return privateKey;
}

I wrote “plain English” library in Javascript to demystifying all the magic of Let’s Encrypt, ACME, and all those certificates.

https://github.com/clshortfuse/acmejs/blob/96fcbe089f0f949f9eb6830ed2d7bc257ea8dc32/utils/certificate/privateKeyInformation.js#L40