To answer your question nothing stops someone from getting a cert that is trusted by your ASA, however if your concern is if someone else will be able to build a tunnel to your ASA then as long you don't have that ASA's IP address configured as a peer then they will not be able to set up a tunnel with the ASA.
Regarding the trial certificate, many of the commercial certificate authorities will provide free certificates, at least for e-mail encryption use. However, most of these 'free' certificates are short trial certificates, good for 30 to 60 days at most. However, there are a few certificate authorities which do issue more than basic trial certificates. One of these is Thawte. In addition to their various paid certificates, Thawte offers free personal certificates which can be used for encrypting e-mail and digitally signing messages. The base certificate simply incorporates your e-mail address. By participating in their web of trust, you can include additional information, such as your name. To do this, you must accumulate 50 trust points from Thawte notaries. If you don't happen to live near any Thawte notaries, there is a trusted third party (TTP) program, but it is associated with a $25 processing fee.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...