iRule to use specific HTTP profile

You don't need to switch between HTTP profiles if you're just encrypting cookies. An iRule will do the job. Here's a variation of the HTTP cookie encryption article:

https://devcentral.f5.com/wiki/iRules.EncryptingCookies.ashx

when CLIENT_ACCEPTED {
    set cookielist [list "USERID" "METRICS"]
    set encryption_passphrase "abcd1234"
}
when HTTP_REQUEST {
    foreach x [HTTP::cookie names] {
        if { [lsearch $cookielist $x] ne "-1" } {
            set decrypted [HTTP::cookie decrypt $x $encryption_passphrase]
            if { $decrypted eq "" } {
                HTTP::cookie remove $x
            }
        }
    }
}
when HTTP_RESPONSE {    
    foreach x [HTTP::cookie names] {
        if { [lsearch $cookielist $x] ne "-1" } {
            HTTP::cookie encrypt $x $encryption_passphrase
        }
    }
}