← QCE Digital Solutions Unit 4

DES

Data Encryption Standard: an early symmetric block cipher using 64-bit blocks and a 56-bit key. Now insecure because modern computers can brute-force its small key.

Part of: QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 25

Understanding DES in QCE Digital Solutions Unit 4

What it does: DES is a block cipher. It splits the plaintext into 64-bit blocks and encrypts each block using the same key. A symmetric cipher uses one shared key. The key that encrypted the data with DES is the same key that decrypts it. DES works on 64-bit blocks. The 56 bits is the size of its key, not the block. DES has a 56-bit key. Do not mix this up with its 64-bit block size. With only 56 bits there are about 72 quadrillion possible keys, and modern computers can try them all in a short time. Brute force means trying every possible key. Counting letter frequencies is frequency analysis, which is a different attack.

Common mistakes: DES encrypts data in 64-bit blocks, not 128-bit blocks. The line should be BLOCK_BITS = 64. DES uses a 56-bit key. A 128-bit key would be far too large to brute-force. The line should be KEY_BITS = 56.

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This module's key terms are also in a printable crossword (Modules 17–88) and word search (Modules 17–125).

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Learning

Duo

DES is a symmetric block cipher.

FalseTrue

Matches

DES is secure because it has a 56-bit key, which is too large to ever brute-force

↓

A 56-bit key is small, not large. Modern computers can try every possible key, so DES is no longer secure.

Applying

Meanings

Term

DES

Meaning

Data Encryption Standard: an early symmetric block cipher using 64-bit blocks and a 56-bit key. Now insecure because modern computers can brute-force its small key.

Practice

block_bits = 64

Testing

Gaps

blockBits =

Check Answer

Spots

1BLOCK_BITS = 128

2KEY_BITS = 56

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Quick Questions

1. DES is a symmetric block cipher.

A. True ✓

B. False

DES uses the same key to encrypt and decrypt, and it works on fixed-size blocks of data.

2. How big is each block of data that DES encrypts?

A. 56 bits

B. 64 bits ✓

DES works on 64-bit blocks. The 56 bits is the size of its key, not the block.

3. How long is the effective key in DES?

A. 64 bits

B. 56 bits ✓

DES has a 56-bit key. Do not mix this up with its 64-bit block size.

4. Why is DES now considered insecure?

A. Its small key can be brute-forced by modern computers ✓

B. Its algorithm cannot decrypt messages any more

With only 56 bits there are about 72 quadrillion possible keys, and modern computers can try them all in a short time.

5. DES uses one public key to encrypt and a different private key to decrypt.

A. False ✓

B. True

That describes asymmetric encryption such as RSA. DES is symmetric, so one shared secret key does both jobs.

Match the Pairs

DES → Data Encryption Standard, an early symmetric block cipher that is now considered insecure

key_bits = 56 → The size of the DES key, which is small enough for a modern computer to brute-force

DES encrypts data in fixed-size blocks of 64 bits at a time → block_size = 64

DES is a symmetric cipher, so the same secret key encrypts and decrypts the data → plain = des_decrypt(cipher, key)

DES is secure because it has a 56-bit key, which is too large to ever brute-force → A 56-bit key is small, not large. Modern computers can try every possible key, so DES is no longer secure.

DES is an asymmetric cipher with a public key and a private key → DES is symmetric. It uses one shared secret key, not a public and private key pair.

Key Term

DES

Data Encryption Standard: an early symmetric block cipher using 64-bit blocks and a 56-bit key. Now insecure because modern computers can brute-force its small key.

Fill in the Blanks

1. blockBits = 64

Hint: Type the number of bits in each block that DES encrypts at a time.

2. keyBits = 56

Hint: Type the number of bits in the DES key.

3. totalKeys = 2 ** 56

Hint: Type the Python operator that raises a number to a power, to count every possible key.

4. for key in range(2 ** 56): decrypt(cipher, key)

Hint: Type the function that loops through every possible key in a brute-force attack.

5. keyBits = 56 if keyBits < 128: print("DES is weak")

Hint: Type the variable that holds the size of the DES key being checked.

Spot the Error

Question 1

A BLOCK_BITS = 128

B KEY_BITS = 56

C keys = 2 ** KEY_BITS

D print(keys)

Line A — DES encrypts data in 64-bit blocks, not 128-bit blocks. The line should be BLOCK_BITS = 64.

Question 2

A KEY_BITS = 128

B keys = 2 ** KEY_BITS

C print('Keys:', keys)

Line A — DES uses a 56-bit key. A 128-bit key would be far too large to brute-force. The line should be KEY_BITS = 56.

Question 3

A cipher = 'DES'

B block_size = 56

C key_size = 56

Line B — 56 is the DES key size, not its block size. DES works on 64-bit blocks, so the line should be block_size = 64.

Question 4

A def key_count(bits):

B return 2 * bits

C print(key_count(56))

Line B — Each extra key bit doubles the number of possible keys, so the count is 2 ** bits (2 to the power of bits). Multiplying by 2 gives a tiny number.

Question 5

A keys = 2 ** 56

B speed = 10 ** 12

C secs = keys * speed

D print(secs)

Line C — The time for a brute-force search is the number of keys divided by the number of guesses per second. It should be secs = keys / speed.

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