Full Answer Section

CBC encrypts each block of data using the previous ciphertext block as an input. This means that each block of ciphertext is dependent on all of the previous blocks. This makes CBC more secure than ECB, but it also makes it slower.

Here is a table comparing the two modes of operation:

Here are some reasons why one should not use ECB:

• ECB is vulnerable to known-plaintext attacks. In a known-plaintext attack, the attacker has access to both the plaintext and the ciphertext of a message. This allows the attacker to decrypt the message by analyzing the relationship between the plaintext and ciphertext blocks.
• ECB is vulnerable to chosen-plaintext attacks. In a chosen-plaintext attack, the attacker can choose the plaintext of a message. This allows the attacker to encrypt the message and then analyze the ciphertext to learn about the block cipher’s behavior.
• ECB is not suitable for encrypting messages that have repeating patterns. For example, if a message contains a lot of the same letter, such as “AAAAA”, then the ciphertext will also contain a lot of the same letter. This makes it easier for an attacker to guess the plaintext.

Here is how plaintext messages whose length is not a multiple of the block length is encrypted by using CBC:

• The plaintext message is padded with a random number of bytes so that its length is a multiple of the block length.
• The padded message is encrypted using CBC.
• The padding is removed from the ciphertext before it is sent to the recipient.

The padding is necessary to ensure that the ciphertext is always a multiple of the block length. This is because the block cipher can only encrypt data that is a multiple of the block length.