Doc ID024209 Rev 2

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AN4240

AES-128 encryption and decryption overview

2

AES-128 encryption and decryption overview

The CSE module supports two cipher modes: Electronic Code Book (ECB) and Cipher

Block Chaining (CBC) and CMAC authentication mode.

Block ciphers like AES algorithm works with a granularity of 64 or 128 bits. This means that

if someone wants to encode data, it is necessary to split the message with an equivalent

granularity.

In the ECB cipher mode the output cipher message depends not only on the chosen

cryptographic key and the input data, but same input is encrypted in the same manner.

This could provide an opportunity to hack the message using an example statistical

analysis.

For this reason, the CBC cipher mode introduces more entropy to the encryption process

using a chaining structure which is described in the following sections.

2.1

Electronic Code Book (ECB)

This cipher mode is the easiest one, because the input message (plaintext) is passed to the

cryptographic block with a key and the output message (ciphertext) is obtained directly, see

Figure 2.

This means that input message with a low variance could be encrypted in an equivalent

manner and it could have a low security threshold.

Figure 2. ECB block diagram