Intro to modern cryptography

Zahid Mian
Part of the Brown-bag Series

 History andTerms
 WhyWe Need Encryption
 How is Encryption Used In Systems
 Difference in Ciphers
 Various Implementations
 HashValues
 Digital Signatures
 Why Attacks Are Successful

 Hieroglyphs
 4000 years ago
 Use pictures to represent words/messages
 Caesar ShiftCipher
 Replace characters by an offset (or shifting)
 Offset = 2, then “hello” -> “jgnnq”
 Improvements over time
 Steganography,Vigenere Coding

 Modern really means digital
 Much more mathematically oriented
 Operating on bit values
 Use of public key and secret key
 Even though algorithm are public, it’s nearly
impossible to obtain original information

 Plaintext –The original information
 Cipher – A secret way of writing (an algorithm)
 Encryption Key – A value that is used to encrypt plaintext
(aka, public key)
 Decryption Key – A value that is used to decrypt
Ciphertext (aka, private key)
 Ciphertext -The encrypted information
 Algorithm – (Often) Public algorithm that uses keys to
encrypt or decrypt information
 Interceptor/Attacker – unauthorized entity that tries to
determine the cipher text (aka, hacker)
 Symmetric – single key used for encryption and
decryption
 Asymmetric – uses public key and private key

http://www.infosectoday.com/Articles/Intro_to_Cryptography/CryptoFig05a.jpg
This is how things generally work today, but there are a lot of
pieces that make this happen.

 Passive – Simply gaining unauthorized access to
information.
 Most common password is 123456
 Active – A hacker attempts to make changes to
data on the target or data en route
 Masquerading: hacker pretends to be someone
 Denial of Service (DoS): deny access to legitimate
users by overwhelming the system (common)
 Using technical information to steal user id (e.g.,
using session id)

 Known Plaintext Attack (KPA)
 Attacker knows plaintext of some part of the ciphertext; attempts to
decrypt the rest
 Chosen Plaintext Attack (CPA)
 Attacker has ciphertext and plaintext; attempts to determine key
 Brute Force Attack (BFA)
 Attacker tries to determine key by attempting all possible values
 Man in the Middle (MIM)
 Simple, but technically challenging method of duping both the sender
and the receiver into thinking keys came from correct source
 Side Channel Attacks (SCA)
 Data Remanence (reading data that should have been deleted)
 Row Hammer (low-level memory hack)
 Fault Analysis (forcing errors and reading outputs for clues)

 Traditionally Ciphers were single character
 Caesar Cipher: shift a single character
 Two-character Cipher (Playfair)
 Use two characters as key
 String Cipher (Vigenere)
 Use a string (word) as key
 One-Time Pad
 Key length equals length of plaintext
 Block/Stream (DES,AES)
 Plaintext is processed in blocks/streams of bits at a
time

 Underlying model for many block ciphers
 Same algorithm for encryption/decryption
 Steps
 Input is split into two halves
 RHS input is transformed by function f which receives
a subkey
 LHS combined with transformed input from RHS
using XOR operation
 RHS and LHS are switched to obtain the input for next
round
 Repeat

32 bit 32 bit
f
32 bit 32 bit
32 bit 32 bit
Ki
LHSi-1 RHSi-1
LHSi-1 = RHSi-1 RHSi-1 = LHSi-1

 Block size is 64 bits
 Key length is 64 bits (though only 56 are used)
 Round-key generator creates 48-bit Key
• Very Strong Cipher
• Fallen out of favor
because small key value
• Can be hacked with
exhaustive search

 Don’t abandon original DES; change usage
 Effectively use 3 Keys (3X56 = 168)
 Problem of short key solved
 Kind of slow
 Process
 Encrypt plaintext using K1
 Decrypt output of Step1 using K2
 Encrypt output of Step2 using K3
 Output of Step3 is ciphertext
 Decrypt in reverse order

 The more popular algorithm today
 Much faster thanTriple DES
 128-bit data; 128/192/256-bit keys
 Key size depends on the number of rounds
 A “bit” more complex algorithm (pun
intended)
 For details see online resources
 To-Date no attacks againstAES have been
successful

 Asymmetric Cipher (diff keys)
 Necessary due to growth of
Internet
 Used for smaller pieces of data
 ThreeTypes
 RSA (widely used)
 ElGamal
 Elliptic Curve Cryptography (ECC)

http://etutorials.org/Programming/Programming+.net+security/Part+III+.NET+Cryptography/Chapter+15.+Asymmetric+Encryption/15.1+
Asymmetric+Encryption+Explained/

 Function that converts arbitrarily long numeric
input into a fixed numeric output called a hash
 Very Efficient
 Hard to reverse value
 Hard to produce same hash for diff inputs
 Some well known Hash Functions
 Message Digest (MD), Secure Hash Function (SHA),
RIPEMD
 Most Common Use: Password Storage
 Secure Apps will not save your password in plaintext

 How can we be sure a message is authentic?
 In the old days, a signature on a letter proved
authenticity
 Digital Signatures do the same thing-they
ensure that the message is from the original
sender with the original message
 Adds trust when exchanging data
 Using Encryption with Digital Signatures is
important
 Certifying Authority (CA) responsible for
management of certificates
 Generating, issuing, publishing, verifying, revoking

Encryption of Email
Decryption of Email

 User Carelessness / Stolen Credentials
 Stolen equipment (laptops, phones, etc.)
 Incorrect Implementation / Backdoor
 Broken Processes / InsiderThreats
 PhishingAttacks
 Sending sensitive data over plaintext
 Zero-day threats / Maintenance
 Application vulnerabilities

Intro to modern cryptography

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