The most naive mode of operation is called Electronic Cookboook (ECB) mode. It divides the message into blocks
Decryption is just the opposite - it divides the ciphertext into blocks
The ECB Mode is very simple so it comes as no surprise that it is not very secure.
The ECB mode should *never* be used.
In particular, it is not CPA-secure, since it is entirely deterministic. Moreover, it is not even semantically secure because if a block is repeated in the plaintext, then the corresponding ciphertext block will also be repeated in the ciphertext which reveals a lot of information about the underlying message.
A famous example of ECB's egregious insecurity is called the ECB penguin. Here is the original image of Linux's mascot [Tux](https://en.wikipedia.org/wiki/Tux_(mascot)), created by Larry Ewing:
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And here is the same image encrypted with AES-128 using ECB mode:
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Not particularly secure, is it?