Block cipher mode of operation

A block cipher is like a special secret code that can make your messages unreadable to anyone who doesn't have the key to decode it. But sometimes, you need to send really long messages that are too big for the cipher to handle all at once. That's when we use a block cipher mode of operation.

Basically, we split up the long message into small pieces called "blocks" that are the right size for the cipher to work with. Then, we use a mode of operation to decide how to encrypt each block.

One popular mode is called "Electronic Codebook" (ECB) mode. It's like having a bunch of secret codes that you use to encrypt each block separately. The problem with ECB is that if two blocks have the same content, they'll encrypt to the same code, which can make it easier for a clever attacker to figure out what the message says.

Another mode is called "Cipher Block Chaining" (CBC) mode. In CBC, each block is encrypted using the key and also the output of the encryption of the previous block. This way, even if two blocks have the same content, they'll encrypt differently because they'll have different inputs from the previous block. This makes it harder for an attacker to figure out what the message says.

There are lots of other modes of operation too, each with their own strengths and weaknesses. But the important thing to remember is that block cipher mode of operation lets us encrypt really long messages by dividing them up into smaller blocks and using a mode to decide how to encrypt each one.