linuxmemes

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 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 a "plain English" library in Javascript to demystify all the magic of Let's Encrypt, ACME, and all those certificates. (Also to spin up my own certs in NodeJS/Chrome).

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

Edit: To be specific, PKCS8 is usually a PKCS1 (RSA) key with some wrapping to identify it (the OID). The integers (BigInts) you pick for RSA would have to line up in some way, but I would think it's doable. At worst there is maybe a character or two of garbage at the breakpoints for the RSA integers. And if you account for which ones are absent in the public key, then anybody reading it could get a kick out of reading your public certificate.