1. Course code: CSC445
Course title :
IntroductiontoCryptographyand informationsecurity
PART: 1
Prof. Taymoor Mohamed Nazmy
Dept. of computer science, faculty of computer science, Ain Shams uni.
Ex-vice dean of post graduate studies and research Cairo, Egypt
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2. Course Description
• 1-Basic concepts of cryptography and secure data:
• 2- Overview of Cryptography and information security,
• 3-Shannon and cryptography, Substitution Ciphers,
• 4-5-Transposition, and Polyalphabetic Ciphers, , Rotor Machine
• 6-7-Block Ciphers: symmetric key systems, DES, AES, Public Key Systems,
stream cipher.
• 8- RSA System, Key Management, , Linear Shift Registers,
• 9-Digital Signatures and Authentication ,
• 10-11-Watermarking and Steganography, Applications.
• 12- Review.
• Text book
• Stinson D. R., Cryptography: Theory and Practice, CRC Press
• Stallings_Cryptography_and_Network_Security-book
•
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3. About the lecturer
- Prof. of computer science since 2006,
- Director of Ain Shams information network,
- Vice dean of post graduate studies and research,
- Vice dean of environmental and social affairs,
- Member in editorial board of many Int. journals,
- Member in Scientific committee of many int. conferences
- Executive chair of int. conf. on information and intelligent systems,
- Published more than 60 scientific papers in int. journals and
conference,
Supervised more than 20 master and Ph. D thesis.
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4. About the course
• The materials of this course were collected from many resources, include the
reference book, other books, online courses, presentations, and web sites.
• There are many details will be given to simplify topics in this course, however at the
end of the course the most important topics will be highlighted.
• The delivered materials through this presentation is your main resource,
• This presentation will be delivered to your.
• There will be term exams 60%
• The final exam 40%
• The exam will include subjective and objective questions,
• Through this course try to extends your knowledge by improving your self learning
capabilities.
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5. • This course is to some extend a hard course,
but we will try to simplify it as much as we
can. So try to get in step by step.
7. Course Objectives
To know the terminologies of cryptography
To understand the impact of cryptography on information
security
To distinguish between symmetric and asymmetric
encryption
To recognize different types of encryption algorithms
To know the roles and the differences between
steganography and watermarking 7
9. What Is Information Security?
• The U.S. Government’s National Information Assurance Glossary
defines INFOSEC as:
“Protection of information systems against unauthorized
access to or modification of information, whether in storage,
processing or transit, and against the denial of service to
authorized users or the provision of service to unauthorized
users, including those measures necessary to detect,
document, and counter such threats.”
10. 10
Layers of security
A successful organization should have multiple
layers of security in place:
Physical security
Personal security
Operations security
Communications security
Network security
Information security
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11. Where is Security needed?
• Military communications-
• Electronic Commerce
• E-banking
• Secure Storage
• Internet Applications: e-mail etc
• Wireless networks: GSM, Wi fi, WiMAX, Blue Tooth
• All popular Web browsers use built-in encryption features
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12. Security Attacks
• Passive attacks
– Obtain message contents
– Monitoring traffic flows
• Active attacks
– Masquerade (reuse) of one entity as some other
– Replay previous messages
– Modify messages in transmit
– Add, delete messages
– Denial of service
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13. Passive Attacks
• Reading contents of messages
• Also called eavesdropping
• Difficult to detect passive attacks
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14. 14
Active Attacks
• Modification or creation of messages (by attackers)
• Four categories: modification of messages, replay, masquerade,
denial of service
• Easy to detect but difficult to prevent
• Defense: detect attacks and recover from damages
20. Watermarking, Steganography &
Digital signature
Goal of steganography
– Intruder cannot detect a message
– Primarily 1:1 communication
Goal of watermarking
– Intruder cannot remove or replace the message
– Primarily 1:many communication
Goal of Digital signature
– Simulate the security of a signature in digital, rather than written, form.
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21. What is Cryptography
Cryptography is the study of mathematical techniques related
to aspects of information security such as confidentiality, data
integrity, authentication, and non-repudiation. The modern
cryptography use the computer to implement encryption
algorithms.
The aim is to hide the meaning of the message rather than its
presence. This can be done by scrambling the letters around of
the plain text and turn into ciphertext.
To decrypt that ciphertext into plaintext, you need an
encryption key,( a series of bits that decode the text).
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22.
23. About the keys
• Keys can be created or generated in many ways, but computers commonly
generate them. Ideally, each key is truly random.
• A random number generator (RNG) or a pseudo-random number generator
(PRNG) , is frequently used for this purpose.
• The difference between an RNG and a PRNG is that the RNG
autonomously generates random numbers, whereas a PRNG is computer-
based and creates a somewhat random number based on seed values that
are readily available within the computer.
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24. Terminologies
• encipher (encrypt) - converting plaintext to ciphertext
• decipher (decrypt) - recovering plaintext from ciphertext
• cryptography - study of encryption principles/methods
• cryptanalysis (codebreaking) - the study of principles/
• methods of deciphering ciphertext without knowing key
• cryptology - the field of both cryptography and cryptanalysis
• decryption Algorithm, It is a mathematical process, that produces a
unique plaintext for any given ciphertext and decryption key. It is a
cryptographic algorithm that takes a ciphertext and a decryption key
as input, and outputs a plaintext. The decryption algorithm
essentially reverses the encryption algorithm and is thus closely
related to it.
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25. • Keyspace – Total number of possible values of keys in a crypto
algorithm
• Stream cipher = a cipher which acts on the plaintext one symbol at
a time.
• Block cipher = a cipher which acts on the plaintext in blocks of
symbols.
• Substitution cipher = a stream cipher which acts on the plaintext by
making a substitution of the characters with elements of a new
alphabet.
• Transposition cipher = a block cipher which acts on the plaintext
by permuting the positions of the characters in the plaintext.
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26. • Monoalphabetic substitution: uses only one
alphabet.
• Polyalphabetic substitution: more advanced;
uses two or more alphabets.
• Vigenère cipher: advanced cipher type that
uses simple polyalphabetic code; made up of
26 distinct cipher alphabets.
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27. Cryptanalysis: the science of breaking cryptographic
algorithms.
Cryptanalyst: a person who breaks cryptographic
codes; also referred to as “the attacker”.
• An interceptor (an attacker-intruder) is an
unauthorized entity who attempts to determine the
plaintext. He can see the ciphertext and may know the
decryption algorithm. He, however, must never know
the decryption key.
• Cryptosystem – The combination of
• Crypto algorithm+ key+ and key management
functions used to perform cryptographic operations.
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28. What Is Key Management and Why Is It Important?
• Key management is another important aspect of
encryption. Keys are how all of that encrypted
data becomes readable, so how you handle them
is just as sensitive as the data itself.
• Many businesses worry about this aspect of
encryption—after all, if you lose an encryption
key, you lose access to your data, too. That’s why
key management dictates how keys are stored
(and shared) .
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29. What is a cryptographic system composed of?
Plaintext: original message or data (also called cleartext)
Encryption: transforming the plaintext, under the control of the key
Ciphertext: encrypted plaintext
Decryption: transforming the ciphertext back to the original plaintext
Cryptographic key: used with an algorithm to determine the
transformation from plaintext to ciphertext, and v.v.
The key is a random string of 40 to 4,000 bits (ones and zeros) . Longer
keys are harder to guess and provide stronger confidentiality. For a given
cipher, different keys will generate different cipher texts from the same
plaintext 29
(encryption)
(encryption key)
C
PP (decryption)
Sender Receiver
(decryption key)
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30. Cryptography
• can characterize by:
– type of encryption operations used
• substitution / transposition / product
– number of keys used
• single-key symmetric or private / asymmetric or two-key or public
– way in which plaintext is processed
• block / stream
31. • In cryptography, we start with the unencrypted data, referred to
as plaintext. Plaintext is encrypted into ciphertext, which will
in turn (usually) be decrypted into usable plaintext.
• The encryption and decryption is based upon the type of
cryptography scheme being employed and some form of key.
For those who like formulas, this process is sometimes written
as:
• C = Ek(P) or E(P,K)
P = Dk(C) or D(C,k)
• where P = plaintext, C = ciphertext, E = the encryption
method, D = the decryption method, and k = the key.
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32. Page 32
Private Key Algorithms (one key)
Encryption
Decryption
Key1
Key1
Cyphertext
Ek(M) = C
Dk(C) = M
Original Plaintext
Plaintext
33. Cryptographic techniques allow a sender to hide data so that an intruder
can gain no information from the intercepted data. The receiver, of
course must be able to recover the original data from the disguised data.
However, there is always a chance for an intruder to discover the
transmitted data base on the method used for encryption.
Intruder in crypto system
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34. Types of encryption
• 1- Secret-key algorithms: Also known as symmetric algorithms, or
private-key, this algorithm uses the same key for encryption and
decryption. This is a touch more vulnerable because anyone who gets a
hold of that one key can read anything you encrypt. Also, passing that
secret key over internet or network connections makes it more vulnerable
to theft.
• 2- Public-key algorithms: These are also known as asymmetric
algorithms. With public-key encryption, there are two different, related
encryption keys—one for encryption, and one for decryption. The public
key is how the information is sent to you, and the private key decodes it.
The benefit here is the key isn’t subject to being sent over insecure
networks, but it does require more computer processing power so it’s a bit
slower.
• 3- Hash function: A mathematical transformation that takes a message of
arbitrary length and computes it a fixed-length (short) number. 34
36. • 4- Block ciphers: Like the Data Encryption
Standard (DES), or Triple Data Encryption
Standard 3DES, these encrypt data a block at a
time. Triple DES uses three keys and is a pretty
great encryption option for financial institutions
that need to protect sensitive information.
• 5- Stream ciphers: A symmetric algorithm, it
uses a keystream, a series of randomized
numbers, to encrypt plaintext one character at a
time. Rabbit, W7, and RC4 are popular stream
ciphers.
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37. • 6- Elliptic curve cryptography: A new form of public-key
encryption, it can be practically unbreakable for normal
computers, or “hard.”
• 7- Blockchain cryptography: Blockchain technology is
essentially a type of distributed database, best known as the
basis for Bitcoin, that uses cryptography to safely store data
about financial transactions.
• There are 2.9 to 5.8 million unique users using a
cryptocurrency wallet, most of them using bitcoin.
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38. Some popular Encryption algorithms
• 1- Advanced Encryption Standard (AES): A block cipher, that
offers 128-, 192-, and 256-bit encryption, the last two reserved for
instances that require extra-strength protection.
• 2- RSA: This asymmetric algorithm uses paired keys and is pretty
standard for encrypting information sent over the internet.
• 3- IDEA (International Data Encryption Algorithm): This block
cipher with a 128-bit key has a great track record for not being
broken.
• 4- Signal Protocol: This open-source encryption protocol is used for
asynchronous messaging, like email.
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39. • 5- Blowfish and Twofish: Both of these block
ciphers are popular among e-commerce
platforms for protecting payment information.
Twofish is the successor and offers longer
encryption keys.
• 6- Ring Learning With Errors or Ring-
LWE: This protocol adding in a new type of
encryption that might be unbreakable by
quantum computers.
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42. “Human ingenuity cannot invent a cipher
which human ingenuity cannot resolve.”
Edgar Allan Poe, 1841
History of Crypto: 3000BC-2XXX
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43. Encryption before and after computer
• Before using computer in encryption the
massages were encrypted based on changing in
the characters sequence, or order, and the keys
were a character, a set of characters, or number.
• After using the computers the massages were
encrypted base on changing on the bits that
represent the characters, and the keys were a set
of bits of specific length.
45. Rosetta Stone
the Rosetta Stone helped scholars at long last crack the code of hieroglyphics,
the ancient Egyptian writing system
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46. • There are three eras in the history of Cryptography:
– The Manual era
– The Mechanical era
– The Modern era
• Manual era refers to Pen and Paper Cryptography and dates back to 2000 B.C.eg .
• first known use of a modern cipher was by Julius Caesar (100 BC- 44 BC)
• Mechanical era refers to the invention of cipher machines. E.g.: Japanese Red and Purple
Machines , German Enigma.
• The modern era of cryptography refers to computers.
• There are infinite permutations of cryptography available using computers. E.g.: Lucifer,
Rijndael , RSA , ElGamal.
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47. Or the eras
• Pre-WWII
– Cryptography as a craft
– Widely used, but few provable techniques
• 1940s-1970
– Secret key encryption introduced
– Information theory used to characterize security
• 1970-present
– Public key systems introduced
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49. Cryptographic Attacks
• The basic intention of an attacker is to break a cryptosystem
and to find the plaintext from the ciphertext. To obtain the
plaintext, the attacker only needs to find out the secret
decryption key, as the algorithm is already in public
domain.
• Hence, he applies maximum effort towards finding out the
secret key used in the cryptosystem. Once the attacker is
able to determine the key, the attacked system is considered
as broken or compromised.
• Based on the methodology used, attacks on cryptosystems
are categorized as follows −
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50. • Ciphertext Only Attacks (COA) − In this method, the attacker has
access to a set of ciphertext(s). He does not have access to
corresponding plaintext. COA is said to be successful when the
corresponding plaintext can be determined from a given set of
ciphertext. Occasionally, the encryption key can be determined from
this attack. Modern cryptosystems are guarded against ciphertext-
only attacks.
• Known Plaintext Attack (KPA) − In this method, the attacker
knows the plaintext for some parts of the ciphertext. The task is to
decrypt the rest of the ciphertext using this information. This may be
done by determining the key or via some other method.
• Chosen Plaintext Attack (CPA) − In this method, the attacker has
the text of his choice encrypted. So he has the ciphertext-plaintext
pair of his choice. This simplifies his task of determining the
encryption key.
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51. • Dictionary Attack − This attack has many variants, all of
which involve compiling a ‘dictionary’. In simplest method
of this attack, attacker builds a dictionary of ciphertexts and
corresponding plaintexts that he has learnt over a period of
time.
• In future, when an attacker gets the ciphertext, he refers the
dictionary to find the corresponding plaintext.
• Brute Force Attack (BFA) − In this method, the attacker
tries to determine the key by attempting all possible keys. If
the key is 8 bits long, then the number of possible keys is
28 = 256. The attacker knows the ciphertext and the
algorithm, now he attempts all the 256 keys one by one for
decryption.
• The time to complete the attack would be very high if the
key is long.
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52. Brute-Force Attack
• Try every key to decipher the ciphertext.
• On average, need to try half of all possible keys
• Time needed proportional to size of key space
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53. • Man in Middle Attack (MIM) − The targets of this attack
are mostly public key cryptosystems where key exchange is
involved before communication takes place.
• Host A wants to communicate to host B, hence requests
public key of B.
• An attacker intercepts this request and sends his public key
instead. Thus, whatever host A sends to host B, the attacker
is able to read.
• In order to maintain communication, the attacker re-
encrypts the data after reading with his public key and sends
to B.
• The attacker sends his public key as A’s public key so that B
takes it as if it is taking it from A.
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54.
55. The General Goals of Cryptography
• Confidentiality; assuring that only
authorized parties are able to understand the
data.
• Integrity; ensuring that when a message is
sent over a network, the message that
arrives is the same as the message that was
originally sent.
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56. Goals (cont.)
• Authentication; ensuring that whoever
supplies or accesses sensitive data is an
authorized party.
• Nonrepudiation; ensuring that the intended
recipient actually received the message &
ensuring that the sender actually sent the
message.
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57. More on Confidentiality
• Confidentiality means that only authorized parties
are able to understand the data (authorized from
the perspective of the party that encrypted the
data).
• It is okay if unauthorized parties know that there is
data. It is even okay if they copy the data, so long
as they cannot understand it.
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58. Authentication
• How can we know that a party that provides us
with sensitive data is an authorized party?
• How can we know that the party that is
accessing sensitive data is an authorized party?
• This is a difficult problem on the Internet.
• Two solutions are:
– Passwords
– Digital signatures
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59. Integrity
• This involves ensuring that when a message (or
any kind of data, including documents and
programs) is sent over a network, the data that
arrives is the same as the data that was
originally sent. It is important that the data has
not been tampered with.
• Technical solutions include:
– Encryption
– Hashing algorithms
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60. Nonrepudiation
• Ensuring that the intended recipient actually
got the message.
• Ensuring that the alleged sender actually sent
the message.
• This is a difficult problem. How do we prove
that a person's cryptographic credentials have
not been compromised?
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61. Symmetric-key
ciphers:
Block ciphers
Stream ciphers
Public-key
ciphers
Cryptographic Goals
Cryptographic goals
Confidentiality Data integrity Authentication Non-repudiation
Message authentication
Entity authenticationArbitrary length
hash functions
Message
Authentication
codes (MACs)
Digital signatures
Authentication
primitives
Digital signatures
MACs
Digital
signatures
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