Different passwords for subkeys

[Phillip Susi]

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I keep a subkey pair for daily use that I keep a copy of on my work
machine, and reissue each yea and the master key only at home.  I
would like to protect the master key with a password that is different
from that used on the daily use subkey, but when I use --edit-key and
specify the master key id, and use the passwd command to change the
password, it applies it to all subkeys.  How can I set a different
password only for the master key?
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