The presentation describes basics of cryptography and information security. It covers goals of cryptography, history of cipher symmetric and public key cryptography

1. Cryptography & Information Security
A presentation in ‘knowledge session’
for 10+2 students
25 Oct 16
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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2. Cryptography
In this information era
• Information is related to money & need to be
protected. Ex bank password, transaction
• Information is encoded into bit stream (data) and
transmitted on wireless networks/ Intranet /Internet
• During transmission information is vulnerable to attack
Solution : Cryptography
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
2

3. Cryptography
Solution : Encryption/ Decryption
Cryptography = Crypto + Graphy
Greek word Crypto Hidden secret
Greek word Graphy writing
Cryptography  art of hidden secret writing
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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4. Basic Terms in Cryptography
• Encryption/Decryption: scrambling a message or data using a
cryptographic algorithm or vice versa.
• Plaintext: the message or data
• Cipher text: the encrypted (scrambled) version of the message.
• Key: info used in cipher known only to sender/ receiver
• Cipher: the algorithm that does the encryption.
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
4

5. Goals of Cryptography (Cipher)
• Authentication
• Data Confidentiality
• Data Integrity
• Non-Repudiation
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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6. Confidentiality
• Confidentiality means that only authorized parties are able to
understand the data
• It is okay if unauthorized parties know that there is data, or even
if they copy the data, so long as they cannot understand it.
Solution : Encryption
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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A B
C could view the secret message by
eavesdropping on the communication
Loss of privacy/confidentiality
C
M

7. Authentication
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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Two solutions are:
–Passwords
–Digital signatures
A B
C
M
C could send a massage to B pretending to be A. If B cannot verify
the source of the information then we lack authentication

8. Data Integrity
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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A B
C could alter/corrupt the message, or the message could
change while in transit. If B does not detect this, then we have
Loss of Integrity
C
M
Technical solutions include:
•Encryption
•Hashing algorithms

9. Non-repudiation
Ensuring that
• That the intended recipient actually got the message.
• That the alleged sender actually sent the message.
This is a difficult problem.
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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A BM
A might repudiate having sent m to B
solution:
–Digital signatures

10. “Attacks” on information
• passive attacks –
eavesdropping on, or monitoring of, transmissions to:
– obtain message contents, or
– monitor traffic flows
• active attacks –
modification of data stream to:
– masquerade of one entity as some other
– replay previous messages
– modify messages in transit
– denial of service
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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11. Caesar Cipher
• Earliest known substitution cipher by Julius Caesar used in
military affairs
• replaces each letter by 3rd letter onwards
• example: MEET ME AFTER THE TEA PARTY
PHHW PH DIWHU WKH WHD SDUWB
• only have 26 possible ciphers A maps to A,B,..Z
• could simply try each in turn (brute force search)
eg. break cipher text "GCUA VQ DTGCM“
plaintext “EASY TO BREAK”
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
11

12. Scytale: by the Spartans in ~700 B.C.
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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- transposes the characters of the message
- key  diameter of the baton
- practical size of the key space is small

13. One-Time Pad
• Invented by Major Joseph Mauborgne & Benam in 1917
• if a truly random key as long as the message is used, the cipher
will be secure
• called a One-Time pad
• is unbreakable since ciphertext bears no statistical relationship to
the plaintext
• since for any plaintext & any ciphertext there exists a key
mapping one to other
• can only use the key once though
• have problem of safe distribution of key
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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14. Enigma : first electro-mechanical
ciphering machine
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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- patented by Arthur Scherbius in 1918
- introduced in the German Army in 1926
Three main parts:
– keyboard – for typing in plaintexts and ciphertexts
– display panel – for displaying plaintexts and ciphertexts
– mixing unit – to produce ciphertext from plaintext and vice versa
(the soul of Enigma is the rotor)

15. Confusion and Diffusion
According to Shannon (1949), confusion and diffusion are two
properties of the operation of a secure cipher
• diffusion – dissipates statistical structure of plaintext over bulk of
ciphertext
• confusion – makes relationship between ciphertext and key as
complex as possible
• In other words cipher needs to completely change statistical
properties of original message
a one-time pad does this perfectly
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
15

16. Symmetric Cipher Model
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
16
Ciphertext C
“Eorz xs wkh
Eulgjh ehiruh
sodwrrq uhdfkhv”
Plaintext M
“Blow up the
Bridge before
platoon reaches”
Key K
Encryption
Algorithm
Key K
Decryption
Algorithm
Plaintext
“Blow up the
Bridge before
platoon reaches”
•Also known as conventional/ private-key/ single-key Cipher
•was only type prior to invention of public-key in 1976

17. Symmetric Cipher Model
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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•A simple example Key K = Alphabet + x
•Increased level of security
Alternate letters Alphabet + x
Alphabet – x
•Key based system encrypts blocks of data of same length as key
Example : 4 bit Key 1011 (to be X-OR ed with 4 bit data)
Message M (numbers) 6 2 9 7
M (binary form) 6 2 9 7 0110 0010 1001 0111
Key 1011 1011 1011 1011
C 13 9 2 12 1101 1001 0010 1100
Decrypted Message 0110 0010 1001 0111

18. Symmetric Cipher Model
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
18
•Strength does not lie in the Algorithm as it is public
• Strength lies in the Key
• Key has to be random & periodically changed
• Key may be A value, An Image, Typing speed …………
• Key needs to be transmitted to receiver (either manually or on a
secured channel).
A severe problem on shared networks, wireless networks, internet

19. Asymmetric (Public) key Cipher Model
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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Ciphertext C”
Plaintext M
Sender A
Public Key of B
Encryption
Algorithm
Receiver B
Private Key of B
Decryption
Algorithm
Plaintext”
•Also known as conventional/ private-key/ single-key Cipher
•was only type prior to invention of public-key in 1976

20. Asymmetric (Public Key) Cryptography Ex.
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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Private Key d
e*d=1 mod[(p-1)*(q-1)]
7d=1mod(12*18)
7d=1mod(216)
d=31
Public Key
N=247
e=7
Encryption
Plaintext : 4
Encryption C=Me mod[N]
C=47mod(247)
C = 16384 mod(247)
Ciphertext =82
Let N=p x q, N=247, p=13, q=19 a prime number
Pick another prime number e=7
Decryption
Ciphertext: 82
M=Cd mod[N]
M=8231mod(247)
M=4

21. Security of Cipher : Size of Key
• Security of Cipher lies in the key.
• The large size key, more secure is the Cipher
• Most security experts believe that 256-bit keys are good for
the lifetime of the universe (many billions of years).
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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Those who claim to have unbreakable cipher simply
because they can not break , are either genius or fools;
unfortunately there are more of later

22. Size of Key
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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Time required to break cipher by brute force approach (assuming all
key has to be tried, assuming trying each key takes 1 machine cycle in
decryption

23. Size of Key
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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24. Thank You
25 October 2016
Dr. Naim R Kidwai, Professor & Dean,
JIT Jahangirabad
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