What kind of XML file is the RSA key in?
.Net's RSACryptoServiceProvider
class can read public keys from XML using the FromXmlString
method in the following format:
<RSAKeyValue>
<Modulus>3EgNS5XumwoQYU4uvr2OTtlZ4YJWUcGqTAVLQPtzejB7JSiETGdveuH7jGRFi2lNqruRL+SGpr6KJvvijG7wOQheIsJC48lDnS692pZH3rDcWgGuqjwssFKhJ5GSu3Tetrf4DOKVOeTaG5cU0pATV6aDU0Npy0a+5vkU5e3+5jE=</Modulus>
<Exponent>AQAB</Exponent>
</RSAKeyValue>
EDIT
As I understand your procedure, you're using the RSA public key as an AES symmetric key. DO NOT DO THIS! It adds the illusion of security without doing anything to protect your data. As an analogy, it's like sending a safe along with its key, but putting the key in a pink box first. If you do it this way, anyone who gets the public RSA key will be able to decrypt your data; the private RSA key wouldn't be used at all.
If a third party is forcing you to do it this way, show them this answer, or ask any half-decent cryptographer.
DO NOT ALLOW THEM TO DO IT THIS WAY
What you should be doing is creating a random AES key, encrypting it with the RSA public key, and then sending the encrypted key along with the encrypted data. This way, the data will only be readable by people who have the private RSA key, as anyone else wouldn't be able to decrypt the symmetric AES key.