PKINIT freshness tokens

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: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.