CryptoStream

[Fred Herring]

I have some basic understanding questions about encryption. I am looking at
implementing the Rijndael (256) encryption scheme. Does this imply that the
key must be a string of 256 characters? Is the key something that I can just
type out or is this something the crypto class needs to generate for me. My
second questions is about the IV byte array. What function is served by this
byte array and what is the recommended manner to create this byte array?

Thanks,
Fred Herring

 

[Alek Davis]

Fred,

Check these samples (I think they will answer your questions):

http://www.obviex.com/Samples/Encryption.aspx (explains how to use Rijndael)
http://www.obviex.com/Articles/CiphertextSize.aspx (there is a section
explaining what IV does and how it works)

Alek

 

[Alek Davis]

Quick answers:

Rijndael key is typically a 256-bit (not character) value. It can also be
128 or 192-bit long, but 256 bits is your best bet (256 bits = 32 bytes).
You can define the actual bits (bytes) of the Rijndael key yourself, but a
more common approach is to derive the key from a password (passphrase),
which can be a string of any length (this is done using the API call
sequence you can follow in the example mentioned in the previous post). By
the way, you may need to figure out how to protect the key (or passphrase).

You need to use the initialization vector (IV) in certain (better)
encryption modes, such as CBC, for encrypting the first block of the
plaintext data (and decrypting the first block of the ciphertext). In other
modes, IV may not be needed, but these modes are typically not recommended,
so the rule of thumb is: use IV. The size of IV must match the encryption
block size, which in many cases is 16-byte (not bit) long, so you can use
any 16-character ASCII string for this.

Alek

 

[Fred Herring]

Thankyou very much. This is much clearer to me now. So 32 byte keys and
vectors can be represented by strings 32 characters and or numbers <=255.

dim MyIV() as byte={z,27,199,c,p,14... for 32 items}

Fred

 

[Alek Davis]

Yes, they can be defined in bytes (bytes are chars or numbers <= 255). I'm
not sure if IV can be 32-byte long (I know it can be 16-byte long), so try
it out.

Alek