cs457 – introduction to information systems security cryptography 1a

CS457 – Introduction to Information Systems Security

Cryptography 1a

Elias Athanasopouloselathan@ics.forth.gr

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Cryptography Elements

Symmetric Encryption- Block Ciphers- Stream Ciphers

Asymmetric EncryptionCryptographic Hash FunctionsApplications

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Computer Security“The protection afforded to an automated information system in order to attain the applicable objectives of preserving the integrity, availability, and confidentiality of information system resources (includes hardware, software,

firmware, information/ data, and telecommunications). ”

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CIA Confidentiality

- Data confidentiality: Assures that private or confidential information is not made available or disclosed to unauthorized individuals.

- Privacy: Assures that individuals control or influence what information related to them may be collected and stored and by whom and to whom that information may be disclosed.

Integrity- Data integrity: Assures that information and programs are changed only in a

specified and authorized manner. - System integrity: Assures that a system performs its intended function in an

unimpaired manner, free from deliberate or inadvertent unauthorized manipulation of the system.

Availability- Assures that systems work promptly and service is not denied to authorized users.

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An Example

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Confidentiality: Bad guys cannot

see messages

Integrity: Bad guys cannot change

messages

Availability: The system is

operational

System/ServiceCommunication

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How to achieve CIA?

Crypto algorithms- “Hide” messages, sign messages, provide

guarantees about sent/received messages, etc.Secure systems

- Apply Crypto concepts in protocols, systems, etc.Software defenses

- Ensure that software is not manipulated

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The need to remain secret

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Cryptographic Jargon Plaintext

- This is the original intelligible message or data that is fed into the algorithm as input.

Encryption algorithm - The encryption algorithm performs various substitutions and transformations on

the plaintext. Secret key

- The key is a value independent of the plaintext and of the algorithm. The algorithm will produce a different output depending on the specific key being used at the time.

Ciphertext:- This is the scrambled message produced as output. It depends on the plaintext

and the secret key. Decryption algorithm

- This is essentially the encryption algorithm run in reverse.

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NOT Security via Obscurity

The encryption/decryption algorithm is assumed to be known

Security is based on- Secrecy of the key- Hard to infer the plaintext via the ciphertext by

just knowing the keyCryptanalysis

- Infer the plaintext from ciphertext without knowing the key

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Symmetric Encryption

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Caesar Cipher

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C = E(3, p) = (p + 3) mod 26

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Key: 3

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Or…

Plain: abcdefghijklmnopqrstuvwxyz Cipher: DEFGHIJKLMNOPQRSTUVWXYZABC

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Encrypting a message

Plain: meet me after the toga party Cipher: PHHW PH DIWHU WKH WRJD SDUWB

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Is this secure?

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Simple Brute Force attack: we need to try

25 different keys.

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Increase the key space

Permutations: {a, b, c}- abc, acb, bac, bca, cab, cba

Size: 6 (or 3! ~ 1*2*3)- In general the size is n! (n is the size of the set)

For the alphabet: 26!

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Is this secure?

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Frequency Analysis

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Example

UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ

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Count Frequencies

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Compare with English

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And try…

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Voila!

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One-Time PadCiphertext: ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS key: pxlmvmsydofuyrvzwc tnlebnecvgdupahfzzlmnyih plaintext: mr mustard with the candlestick in the hall

Ciphertext: ANKYODKYUREPFJBYOJDSPLREYIUNOFDOIUERFPLUYTS key: pxlmvmsydofuyrvzwc tnlebnecvgdupahfzzlmnyih plaintext: miss scarlet with the knife in the library

Key = Size of message

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Transposition

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Modern Symmetric Ciphers

DES, 3DES, and AES- AES is the dominant one, today

Based on- Substitutions and transpositions

Very complexType

- Block- Stream

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Block vs Stream

Block cipher - one in which a block of plaintext is treated as a

whole and used to produce a ciphertext block of equal length. Typically, a block size of 64 or 128 bits is used.

Stream cipher- one that encrypts a digital data stream one bit or

one byte at a time.

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Block Cipher

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Stream Cipher

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Block cipher

Plaintext of n bits produces a ciphertext of n bits- Block size: n bits

Space of different plaintext blocks: 2^n- Each block must be unique

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Encryption should be reversible

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General n-bit-n-bit Block (n = 4)

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Decryption/Encryption

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Problems

Vulnerable to statistical attacks - Small blocks can take limited transformations- Increase n

Key size: 4 bits * 16 rows- In general: n * 2^n- Approximate the ideal case

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Feistel Cipher

Goal- Approximate the ideal cipher- Reduce statistical properties between plaintext,

ciphertext, and key(s) Difussion

- Each plaintext digit affect the value of many ciphertext digits

Confusion- The statistics of the ciphertext and the value of the

encryption key is as complex as possible CS-457

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Feistel Cipher

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Substitution: right part is transformed by F(Ki) and

XORed with left part.

Permutation: right part swapped with left part.

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Properties

Block size: - Larger block sizes mean greater security but reduced

encryption/decryption speed for a given algorithm. A block size of 64 bits is reasonable tradeoff. AES uses a 128-bit block size.

Key size: - Larger key size means greater security but may decrease

encryption/ decryption speed. Key sizes of 64 bits or less are now widely considered to be inadequate, and 128 bits has become a common size.

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Properties

Number of rounds:- The essence of the Feistel cipher is that a single round

offers inadequate security but that multiple rounds offer increasing security. A typical size is 16 rounds.

Subkey generation algorithm: - Greater complexity in this algorithm should lead to

greater difficulty of cryptanalysis. Round function F:

- Again, greater complexity generally means greater resistance to cryptanalysis.

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Extra (desired) properties

Fast software encryption/decryption: - In many cases, encryption is embedded in applications

or utility functions in such a way as to preclude a hardware implementation.

Ease of analysis: - There is great benefit in making the algorithm easy to

analyze. It is easier to analyze that algorithm for cryptanalytic vulnerabilities and therefore develop a higher level of assurance as to its strength. DES, for example, does not have an easily analyzed functionality.

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Block modes

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Mode Description Typical Application

Electronic Codebook (ECB) Each block of 64 plaintext bits is encoded independently using the same key.

• Secure transmission of single values (e.g., an encryption key)

Cipher Block Chaining (CBC)

The input to the encryption algorithm is the XOR of the next 64 bits of plaintext and the preceding 64 bits of ciphertext.

• General-purpose block- oriented transmission • Authentication