Hi,
with Rebex Security this is possible. Just apply the Encrypt
method once again to the output of the previously encrypted file. This is a code sample that encrypts the file with TwoFish and then encrypts the TwoFish's output with AES:
// encrypt file with TwoFish
var encryption = new FileEncryption();
encryption.EncryptionAlgorithm = FileEncryptionAlgorithm.TwofishCbc;
encryption.SetPassword("secret password for twofish");
encryption.Encrypt("file.txt", "file.txt.enc");
// encrypt the encrypted file with AES - second layer
encryption.EncryptionAlgorithm = FileEncryptionAlgorithm.AesCbc;
encryption.SetPassword("secret password for aes");
encryption.Encrypt("file.txt.enc", "file.txt.enc.enc");
This is working just fine for me.