gentoo-overlay/dev-libs/opencryptoki/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
  <herd>crypto</herd>
  <use>
    <flag name='tpm'>
      Enable support for Trusted Platform Module (TPM) using <pkg>app-crypt/trousers</pkg>
    </flag>
  </use>
</pkgmetadata>