1. Encryption Basics Types of ciphers Algorithms Modes Key Length
Internet Cryptography by Richard Smith (33 – 42)
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2. Encryption using Exclusive-OR
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3. Example - encryption Assume the plaintext is 1 1 0 0
And the key is
The encrypted message (ciphertext) using exclusive-or is:
(plaintext)
(key)
(Ciphertext)
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4. Example - decryption Assume the ciphertext is 0 1 1 0
And the key is (asymmetric key)
The decrypted message (plaintext) using exclusive-or is:
(ciphertext)
(key)
(original plaintext)
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5. Exercise – using exclusive-OR is called substitution
Given the plaintext is and the key is , determine the ciphertext using exclusive-or.
The answer is  =
Given the ciphertext is and the key is , determine the plaintext using exclusive-or.
The answer is  =
Think about the answer if the key is
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6. Transposition 0 0 1 0 1 1 0 1  bit 0 ciphertext
It is to re-position the bit position using an agreed method (algorithm)
 bit 0 plaintext
 bit 0 ciphertext
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7. Exercise
Given the plaintext is , determine the ciphertext using the previous algorithm.
The answer is :
To decrypt it, you have to reverse the process.
Determine the plaintext, if the ciphertext is
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8. Type of Ciphers There are two types: Stream and Block Stream cipher:
It is designed to accept a crypto key and a steam of plaintext to produce a stream of ciphertext
Block cipher
It is designed to take a block of a particular size, encrypt them with a key of a particular size and yield a block of cipertext block that is the same size of he plainetxt block.
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9. Stream of Ciphers
The simplest stream cipher is called the Vernam cipher. A bit from the key is added with the carry discarded. (exclusive-or)
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10. Properties of stream and block cipher algorithms
Stream cipher: Vernam cipher
Block ciphers that are in use (uses complex method, not just exclusive-or)
Secret Key Block
Data Block
Crypto Key Size
DES (data encryption standard) 64 56
International Data Encryption Algorithm
128
SKIPJACK 80
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11. Example of stream cipher
Give the key pattern of a stream cipher at a particular moment is:
Determine the ciphertext, given the stream of plaintext at that moment is
Recall that 1+1 = 0 (remove the carry), 1+0 = 1, 0 + 1= 1, = 0 (You can use exclusive or as well.)
The answer is:
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12. Problem of Block cipher
Assume that you want to encrypt a plaintext of “book by book” and the key is 32-bit length (4 bytes). The ciphertext can be “%6Yiui90%6Yi”.
You know that
The first ciphertext book is %6Yi
The ciphertext _by_ (_ means space) is ui90 and second ciphertext is %6Yi.
You find that it is vulnerable (easy) to decrypt.
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13. Diagram of Electronic Code Book
Drawback: same output for the same input, it is very easy to decrypt.
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14. NOTE- Electronic Code Book Mode
WE EXPECT, the same plaintext will not produce the same ciphertext pattern
The ciphertext depends on the initialization vector as well.
That is why the initalization vector should pass to the remote as well. If it is corrupted, it never reproduces the same plaintext. (here, the vector is a random 32-bit pattern)
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15. Block Ciphers
There are four common methods to resolve the above problem:
Electronic Code Book  not block (already mentioned)
Cipher Block Chaining
Cipher Feedback
Output Feedback Block
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16. Cipher Block Chaining 2019/5/11
Note: The output depends on the initialized vector, also same input will not produce the same output.

17. Cipher Feedback
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18. Cipher Feedback
It is similar to CBC except that it feeds the ciphertext book back through the block cipher.
This mode is called CTAK in military systems.
An advantage is that: it is not limited to the cipher’s block size.
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19. Output Feedback
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20. Output Feedback Block (OFB)
OFB is similar to Cipher feedback but is simpler.
The output from the encryption is connected to the input of next encryption stage.
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21. Brute Force - key lengths and the time to crack the system
Type of Key
Key length
No. of keys
Time to test one case
AVG time (divided by 2)
3-digit luggage lock
10 (2^10 = 1024)
1000
2 sec
17 min
(2 sec x 1000/2)/60 sec)
4-digit bank card 14
10000
60 sec
3.5 days
Netscape export crypto 40
50 micro sec
10 months
DES key 56
52780 years
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22. Summary
Simple encryption using exclusive-or (add without carry out) and transposition
Type of encryption: stream and block
Block uses fixed length plaintext and key
Block mode: Electronic code book (ECB), cipher block chaining, cipher feedback and output feedback
Drawback of ECB: It is easy to be cracked.
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