# Question 264 of 530 from exam 400-251: CCIE Security written exam

### Question

How does 3DES use the DES algorithm to encrypt a message?

### Answers

### Explanations

A.

3DES (Triple Data Encryption Standard) is a symmetric-key encryption algorithm that uses the DES (Data Encryption Standard) encryption algorithm three times to increase the security of the data. The encryption process involves the use of three keys, denoted K1, K2, and K3.

The correct answer is A. 3DES encrypts a message with K1, decrypts the output with K2, then encrypts it with K3.

Here is a detailed explanation of the 3DES encryption process:

Key generation: Three different 56-bit keys (K1, K2, and K3) are generated randomly or derived from a single 168-bit key.

Encryption process: a. The plaintext message is broken up into 64-bit blocks. b. The first 64-bit block of plaintext is encrypted using the DES algorithm and the first key (K1). c. The output of step 2b is then decrypted using the DES algorithm and the second key (K2). d. The output of step 2c is then encrypted again using the DES algorithm and the third key (K3). e. Steps 2b-2d are repeated for each subsequent 64-bit block of plaintext.

Decryption process: a. The first 64-bit block of ciphertext is decrypted using the DES algorithm and the third key (K3). b. The output of step 3a is then encrypted using the DES algorithm and the second key (K2). c. The output of step 3b is then decrypted using the DES algorithm and the first key (K1). d. Steps 3a-3c are repeated for each subsequent 64-bit block of ciphertext.

The three keys are used in a specific order to encrypt and decrypt the data. The first key (K1) is used to encrypt the plaintext message. The second key (K2) is then used to decrypt the output of the first encryption. The third key (K3) is used to encrypt the output of the second decryption. This process is repeated for each block of plaintext to create the ciphertext.

When decrypting the ciphertext, the process is reversed. The third key (K3) is used to decrypt the first block of ciphertext. The output of this decryption is then encrypted using the second key (K2). Finally, the output of the second encryption is decrypted using the first key (K1). This process is repeated for each block of ciphertext to recover the original plaintext message.

In summary, 3DES uses the DES algorithm three times in succession to encrypt and decrypt data. Each encryption and decryption step involves a different key (K1, K2, K3) and is used in a specific order to increase the security of the encryption.