docs: private PGPSecretKey getSecretKey(InputStream input, String keyId) throws IOException, PGPException {
          PGPSecretKeyRingCollection keyrings = new PGPSecretKeyRingCollection(PGPUtil.getDecoderStream(input), new JcaKeyFingerprintCalculator());
  
          Iterator rIt = keyrings.getKeyRings();
  
          while (rIt.hasNext()) {
              PGPSecretKeyRing kRing = (PGPSecretKeyRing) rIt.next();
              Iterator kIt = kRing.getSecretKeys();
  
              while (kIt.hasNext()) {
                  PGPSecretKey key = (PGPSecretKey) kIt.next();
  
                  if (key.isSigningKey() && String.format("%08x", key.getKeyID() & 0xFFFFFFFFL).equals(keyId.toLowerCase())) {
                      return key;
                  }
              }
          }
  
          return null;
      }