Message length limited when using RSA public-key cryptography.

RSA is used to encrypt messages that are shorter than the modulus of the public key. For 1024-bit keys, this means that the message must be 117 bytes or fewer (the modulus is 128-bytes, minus 11 for the padding of the message).

Attempting to encrypt a message that is larger than the modulus will result in the error:

 System error: Message too long. (700) 

If you need to use RSA on a larger message, the normal method is to use a hybrid scheme, similar to the following:

• Generate a key from a password.
• Use the key with a symmetric encryption algorithm (such as AES) to encrypt the large message.
• Encrypt the key using RSA.

You could do so using code similar to the following:

//generate a key and use the key to encrypt the large message
//Note: our components will generate the Key and IV from a given password
aes1.KeyPassword = "mypassword";
aes1.InputMessage = someLargeMessage;
aes1.Encrypt();

//save the key
string myKey = aes1.Key;

//now encrypt the key
rsa1.RecipientCert = new Certificate(cstPEMKeyFile, "C:\\PATH\\TO\\certFile.pem", "certPassword", "*");
rsa1.InputMessage = myKey;
rsa1.Encrypt();

//store the encrypted key
string encryptedKey = rsa1.OutputMessage;