8070 specifies a pa-data type PA_AS_FRESHNESS, which clients should reflect within signed PKINIT data to prove recent access to the client certificate private key. The contents of a freshness token are left to the KDC implementation. The MIT krb5 KDC uses the following format for freshness tokens (starting in release 1.17):
3961checksum, with no ASN.1 wrapper
The checksum is computed using the first key in the local krbtgt principal entry for the realm (e.g.
krbtgt/KRBTEST.COM@KRBTEST.COM if the request is to the
KRBTEST.COM realm) of the indicated key version. The checksum type must be the mandatory checksum type for the encryption type of the krbtgt key. The key usage value for the checksum is 514.