If your iChat and Back to My Mac certificates have expired, the following steps will help you renew them.
Back to My Mac:
1. Open System Preferences and select the Mobile Me section.
2. Click the Sign Out button.
3. Once you are signed out, click the button again to sign back in.
4. Then select the Back to My Mac tab.
5. Click the Stop button to disable.
6. Once the Stop is done, click the button again to enable it.
iChat:
1. Open iChat and log in using your Mobile Me account.
2. From the iChat menu, choose Log Out of … .
3. Once you are logged out, log back in.
4. Go to the Preferences section and select Accounts.
5. Under Accounts, select the Mobile Me account and then select the Security tab.
6. If iChat Encryption is enabled, disable it and then enable it again.
7. If iChat Encryption is disable, enable it.
The basis behind this is that the easiest way to renew the certificates if they are expired is to disable the service and then enable them again. This sends the Mobile Me servers a request for the certificates, and if they are expired, new ones are created.
Now, if you wait about 5 minutes and sign into you Mobile Me account on the http://www.me.com website and go to your Account Settings section and look under the Security Certificates area, you should see new certificates.
Also, if you open the Keychain Access app on your Mac, you should see updated certificates that are no longer expired.
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