使用Rijndael加密和解密字符串

Question

Before I start, I know there are a lot of similar questions on stackoverflow, but the majority of the answers consist of a large section of code with no explanation or help to aid the user.

What I need to do is rather simple, take a string as input from the user, then the password, then encrypt the string with the password and write the cipher text to a file. Then at a later date when the user wants, read the cipher text from the file and decrypt it and present the user with the original plain text. I can handle writing to and from the file, I just need help with the encryption of the string. (Note: I do not want to just encrypt/decrypt the whole file - as it has to contain some unencrypted stuff - just a string in the program)

Also, the 'Initialisation Vector' that is required - can it be the same as the password? Or can it be simply hard coded into the program? Is it even essential for security? If not then does the user have to remember the password and the IV?

Another thing, when the user comes to decrypt and and enters an incorrect password, do the methods in the Rijndael class just raise an error or what?

I am proficient when it comes to C# but I am new to cryptography so if you could step me through any code or answer you post I'd be most grateful.

Thanks

Answer 1

IV's are required for most Rijndael modes, typically you just prepend it to your ciphertext, no need for users to remember.

Since you are using a user typed password to decrypt, you want to use Authenticated Encryption so that your program can safely re-prompt if the wrong password is entered, otherwise your program might not give any indication there was an issue and spit out random text.

I have an example of using AES-CBC with HMAC-SHA256 authentication that I try and keep up to date and reviewed, it in fact has a helper method that handles encrypting with just plaintext and password, and is also well commented:

Modern Examples of Symmetric Authenticated Encryption of a string. C#

Answer 2

You need to take the following steps:

1. Calculate a key from the password. You need to generate a random salt of 8 bytes and an iteration count as input for your password based key derivation function (PBKDF) such as PBKDF2;
2. Encode your string using any encoding such as UTF-8, giving you the plaintext for your cipher;
3. Create a cipher, such as AES-128 in CBC mode, using PKCS#7 padding;
4. Create a random IV, this should be the blocksize in bytes (so for AES this would be 16 bytes);
5. Encrypt your plaintext using the given cipher, giving you the ciphertext;
6. Store salt, IV and ciphertext - if you require a string, you may use an encoding such as Base64.

PBKDF2 is implemented in Rfc2898DeriveBytes .

Note that this answer only shows how to achieve confidentiality, not integrity or authenticity.