← QCE Digital Solutions Unit 4

Triple DES

Applies DES three times (encrypt, decrypt, encrypt) with two or three keys to lengthen the effective key and strengthen DES. It is slower than AES.

Part of: QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 26

Understanding Triple DES in QCE Digital Solutions Unit 4

What it does: Three DES passes per block cost about three times as much work as one, which is one reason Triple DES is slower than AES. Triple DES uses encrypt-decrypt-encrypt (EDE). Hashing is a one-way function and is not part of it. Triple DES can use two keys or three keys. Three keys give the strongest option. Triple DES is a symmetric block cipher. The sender and receiver must both hold the same secret key or keys. The repeated passes with extra keys lengthen the effective key, so brute-force attacks take much longer.

Common mistakes: Triple DES uses encrypt-decrypt-encrypt. The middle step must decrypt with DES_dec, not encrypt again. Two-key Triple DES needs two different keys, with k3 reusing k1. Setting k2 = k1 makes all three keys the same, so it is no stronger than single DES. k2 should be new_key(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
How many times does Triple DES apply the DES algorithm to each block?
TwiceThree times
Matches
ciphertext = DES(plaintext, k1) # Triple DES
↓
This applies DES only once with one key, so it is just ordinary DES and not Triple DES. Triple DES needs three passes (encrypt, decrypt, encrypt) with two or three keys.
Applying
Meanings
Term
Triple DES
Meaning
Applies DES three times (encrypt, decrypt, encrypt) with two or three keys to lengthen the effective key and strengthen DES. It is slower than AES.
Practice
step1 <- DES_ENCRYPT(plaintext, key1)
Testing
Gaps
s2 = (s1, key2)
Check Answer
Spots
1c1 = DES_enc(plain, k1)
2c2 = DES_enc(c1, k2)

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Quick Questions

1. How many times does Triple DES apply the DES algorithm to each block?
A. Three times ✓
B. Twice
Triple DES runs DES three times on each block. That is where the name comes from.
2. Which sequence of steps does Triple DES use?
A. Encrypt, decrypt, encrypt ✓
B. Encrypt, hash, encrypt
Triple DES uses encrypt-decrypt-encrypt (EDE). Hashing is a one-way function and is not part of it.
3. Why was Triple DES created?
A. To replace DES with a faster cipher
B. To lengthen the effective key and strengthen DES ✓
DES has a short 56-bit key. Triple DES gets a longer effective key by repeating DES, which makes it slower, not faster.
4. How many keys can Triple DES use?
A. Always exactly one key
B. Two or three keys ✓
Triple DES can use two keys or three keys. Three keys give the strongest option.
5. Compared with AES, Triple DES is...
A. Slower ✓
B. Faster
Running DES three times costs extra processing, so Triple DES is slower than AES. This is one reason AES is preferred today.

Match the Pairs

ciphertext = E(k3, D(k2, E(k1, plaintext))) → Triple DES encryption: encrypt with key 1, decrypt with key 2, then encrypt with key 3.
keys = [k1, k2, k1] → Two-key Triple DES: the first and third DES passes reuse the same key.
Triple DES is slower than AES because it runs the DES algorithm three times on every block. → timeTripleDES = 3 * timeDES
Using three different keys lengthens the effective key and strengthens DES. → keys = [k1, k2, k3]
ciphertext = DES(plaintext, k1) # Triple DES → This applies DES only once with one key, so it is just ordinary DES and not Triple DES. Triple DES needs three passes (encrypt, decrypt, encrypt) with two or three keys.
Triple DES is faster than AES because it uses three keys → Triple DES is slower than AES, not faster. It runs DES three times, and extra keys do not make it quicker.

Key Term

Triple DES
Applies DES three times (encrypt, decrypt, encrypt) with two or three keys to lengthen the effective key and strengthen DES. It is slower than AES.

Fill in the Blanks

1. s1 = ENCRYPT(data, key1) s2 = DECRYPT(s1, key2) s3 = ENCRYPT(s2, key3)
Hint: Type the function name for the middle step, which does the opposite of the first and last steps.
2. keys = [key1, key2, key1]
Hint: In the two-key version of Triple DES, type the key that is reused for the third pass.
3. keyBits = 3 * 56
Hint: Type the number of separate DES keys in the three-key version.
4. IF time3DES > timeAES THEN OUTPUT "Slower than AES" ENDIF
Hint: Type the comparison operator showing Triple DES takes longer than AES.
5. FOR pass = 1 TO 3 data = DES(data, keys[pass]) NEXT pass
Hint: Type how many times DES is applied in Triple DES.

Spot the Error

Question 1
A c1 = DES_enc(plain, k1)
B c2 = DES_enc(c1, k2)
C cipher = DES_enc(c2, k3)
Line B — Triple DES uses encrypt-decrypt-encrypt. The middle step must decrypt with DES_dec, not encrypt again.
Question 2
A k1 = new_key(56)
B k2 = k1
C k3 = k1
D c = TDES(plain, k1, k2, k3)
Line B — Two-key Triple DES needs two different keys, with k3 reusing k1. Setting k2 = k1 makes all three keys the same, so it is no stronger than single DES. k2 should be new_key(56).
Question 3
A keys = 2
B bits = 56
C total = keys + bits
D OUTPUT total
Line C — Each extra DES key adds its own 56 bits, so the total key length is keys multiplied by bits, not added. It should be keys * bits.
Question 4
A p1 = DES_dec(cipher, k3)
B p2 = DES_dec(p1, k2)
C plain = DES_dec(p2, k1)
Line B — Decrypting Triple DES reverses the steps: decrypt with k3, encrypt with k2, decrypt with k1. The middle step must be DES_enc.
Question 5
A des_ops = 1
B tdes_ops = 3 * des_ops
C slowdown = des_ops / tdes_ops
D OUTPUT slowdown
Line C — Triple DES does three DES operations, so it is about 3 times slower. The slowdown is tdes_ops / des_ops. The reversed division gives 1/3, which would wrongly suggest it is faster.

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