How can I encrypt a user's password in Silverlight?

I have a Silverlight 3 app which connects to a server to perform various act开发者_高级运维ions. My users log in using Forms Authentication but the actions they request are run on the server using the AppPool account so when they go in the audit logs they're recorded against the AppPool account. PCI DSS regulations now require that the user's own ID is in the audit logs which means the action must be taken using the user's creds. Now, I can save the user's creds when they log on and submit them with each request and the actions being taken by the server can use those creds. But the PCI regs say that if creds are saved they must be encrypted (to avoid someone taking a memory dump of the PC and getting the password).

The only way I can see of doing this is to get a public key from the server and encrypt the password with it, then submit the encrypted password and decrypt it on the server using the private key. But Silverlight doesn't have asymmetric cryptography.

I guess I'm too close to the problem and there must be another solution but I can't see what it is. Can anyone help?

CLARIFICATIONS

It's an internal application. Up until now, I've been using IIS Forms AuthN over SSL to Active Directory - I'm not worried about protecting the password in transit, just whilst it's held in memory on the client. As I understand it, because I'm using Forms Authentication, impersonation is not possible on the server unless I use LogonUser, which means I need the password on the server, so I need to transmit it each time, so I need to hold it in the client, in memory, until the app closes.

Are you saying you need to store the password for re-use in the silverlight app? If you are concerned about the password appearing in memory un-encrypted then Silverlight then I think you're in trouble.

The .NET framework does have a SecureString class for exact purpose you outline.

Unfortunately the Silverlight version of the framework does not have this class. Hence even if you were to keep the logical storage of the password encrypted at some point your code would need to decrypt it before using it. At the point there is memory allocated containing the string in unencrypted form.

I don't know much about Forms authentication but if you can map the User principle to a domain user (which you seem to indicate you need) then you will want to use impersonation when running your code on the server.

Alternatively stop using Forms authentication and use Windows integrated authentication where you definitely can use impersonation server-side.

Encryption should never be used for passwords. When you encrypt something then it follows there should be a way to decrypt it. One way hashes should always be used for passwords. md5 and sha1 have been proven to be far too weak for any secuirty system. Sha256 should be used, and in silverlight this library will take care of it: http://msdn.microsoft.com/en-us/library/system.security.cryptography.sha256%28VS.95%29.aspx

In fact storing passwords using "encryption" is recognized by the vulnerability family CWE-257. The use of a message digest is the ONLY way to safely store passwords. I didn't just make this up, this is coming from NIST. There are many other vulnerabilities that come up when storing passwords. Here is THE LIST that NIST has put together: