You could look at the source code and see that the Encrypt and Decrypt methods are using RSACryptoProvider which uses RSA. It is a decent encryption algorithm but not necessarily the strongest possible. .NET 3.5 introduced a newer AesCryptoProvider which is probably stronger and what I would have used except I implemented this method long before .NET 3.5 came out. Of course the stronger encryption you use requires a larger field in the database.
I don't recommend using the EncryptRijndaelManaged method because it has a hard coded key in source code. It was something I was experimenting with a long time ago for encrypting cookies but is only used in one place currently in mojoportal and not really storing anything sensitive in the cookie where I am using it.
Encryption is a complex topic and I don't claim to be a cryptography guru.
What I can tell you is I would never ever ever persist credit card information in the database. Keeping the credit card data around for easy future purchases is exposing the customer to risks for which you may be liable if the data is stolen. Maybe the Amazons of the world can mitigate the risks or handle the liability of doing such things in the name of customer convenience but I would never recommend it unless you have full time security engineer(s) designing and managing your application. For normal people storing credit card data is asking for trouble.
Encrypted data can be decrypted and it is very difficult to truly secure the keys, so just because you encrypt something should not lead to a sense of security. It is not required to store credit card data at all, it can be posted to the card processor using SSL and the auth code returned is all you need to keep.
What you need to understand about the CryptoHelper.Encrypt method is that the RSA key is located on disk in the root of the web in the mojoEncryption.config file. That means unless you put a different key there that you generate yourself anyone using mojoPortal can get the key. If you do generate your own key you should put it there before you use CryptpoHelper on any data because if you change the key it won't be able to decrypt data encrypted with the previous key.
You should probably read up on the Visa guidelines that discuss prohibited data retention, there are a number of pdf downloads here:
Hope it helps,