a big and a small key

[Manuel Hendel]

Hello,

I heard of people using two different keys, a big 4096 RSA key and
small 1024 DSA and ElGamal key. The lifetime of the small key is just
one year. The lifetime of the big key is not limited. The big key is
just used to sign and get signed. The small get signed by the big key
and is replaced every year. The big key somewhere in a save place and
not for daily use.
How can this be done?

Thanks in advance,

Manuel

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