What we would achieve is by default to:
a) obfuscate all non public classes and methods
(options: {Members=DefaultImproved}{Naming=AlphaNumericAndNonDisplayable})
Don't forget to add antiILDASM and StringEncryption features to your obfuscation options.
Then in an explicit way in the code:
b) obfuscate selected strings of private methods containing selected strings
Please use NineRays.Obfuscator.DontEncryptStringsAttribute attribute provided by NineRays.ObfuscationAttributes.dll (you can find this assemblin in SDK\Obfuscation Attributes\ folder and add reference to this assembly to your project. It's not necessary to distribute this assembly with your assemblies, because attributes provided by that assembly are for marking purposes, they don't affect on code behavior. Or you can add source code of these attributes to your project and use attributes internally. Source code you can find in the same folder mentioned above).
Just mark by this attribute methods where you don't expect to use string encryption.
c) anonymize code for selected methods for which we would like to hide algorithm
In that case use NineRays.Obfuscator.DontAnonymizeAttribute attribute and mark by this attribute methods selected to avoid anonymization.