@Override
  public EncryptedKeyset readEncrypted() throws IOException {
    throw new UnsupportedOperationException();
  }
Adds a PEM.

<p>A single PEM can contain multiple keys, but all must have the same {@code keyType}.
Invalid or unparsable keys are ignored.

<p>The first key in the first added PEM is the primary key.
java_src/src/main/java/com/google/crypto/tink/signature/SignaturePemKeysetReader.java