# MIT Kerberos Documentation

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# PKINIT freshness tokens

RFC 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):

- a four-byte big-endian POSIX timestamp
- a four-byte big-endian key version number
- an RFC 3961 checksum, 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.