Figuring out the CAS policy when developing custom web parts can be tedious. Here are some quick starts for functionality that you might be using in your web parts.
Accessing the Registry:
<IPermission class="AspNetHostingPermission" version="1" Level="Minimal" />
<IPermission class="SecurityPermission" version="1" Flags="Execution" />
<IPermission version="1" Unrestricted="True" class="Microsoft.SharePoint.Security.SharePointPermission, Microsoft.SharePoint.Security, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
<IPermission version="1" Unrestricted="True" class="System.Security.Permissions.RegistryPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" [...]
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