Introduction to Cryptography Techniques

Introduction to Introduction to Cryptography Cryptography

TechniquesTechniquesHow secure is that banking network How secure is that banking network

traffic?traffic?

Social and ComputingSocial and ComputingImplications of Implications of CryptographyCryptography The internet is a collection of The internet is a collection of

networks designed to deliver data networks designed to deliver data packets.packets.

Packets are easy to sniff.Packets are easy to sniff. The internet is not secure, but is The internet is not secure, but is

used to connect banks, the power used to connect banks, the power grid, pipelines, transportation grid, pipelines, transportation systems, etc.systems, etc.

TermsTerms Plaintext – the readable messagePlaintext – the readable message Ciphertext – the coded messageCiphertext – the coded message

Encryption

Decryption

plaintext ciphertext plaintextkey key

Types of AttacksTypes of Attacks Ciphertext OnlyCiphertext Only – adversary uses just

the ciphertext to gain either the key or the plaintext (really bad encryption)

Known PlaintextKnown Plaintext – adversary gets the key using some ciphertext and its plaintext

Chosen PlaintextChosen Plaintext – adversary introduces some plaintext to generate some ciphertext

Symmetric Key Symmetric Key EncryptionEncryption Both parties share a secret key The single key is used for both

encryption and decryption Encryption and decryption are

equal efforts

Shift CiphersShift Ciphers

key = amount to shift each character

Example: Rotate13‘A’ + 13 = 1 + 13 = 14 = ‘N’

So, the message “aardvark” becomes “nneqinex”.

Shift CiphersShift Ciphers

Advantage of Rot13:Easy to implement. Rot13('A') = 'N' (1 + 13)%26 = 14

Rot13('N') = 'A' (14 + 13)%26 = 1So, one function does both encoding and decoding.

Disadvantage of Any Rotation:Very easy to break – just try all 26 possibilities.aka - Brute Force Brute Force attack.

Substitution CipherSubstitution Cipher

Key = list of character substitutionsKey = list of character substitutions

Example: Key = “Chair”A B C D E F G H I J K L M N O P Q R S T U V W X Y ZY Z c h a i r B D E F G J K L M N O P Q S T U V W X

Disadvantage:Susceptible to Character Frequency

Analysis

Character FrequenciesCharacter Frequencies

Character FrequenciesCharacter Frequencies

Start of Word Letter Frequencies Letter t a i s o c m f p w Freq 0.1594 0.155 0.0823 0.0775 0.0712 0.0597 0.0426 0.0408 0.040 0.0382

End of Word Letter Frequencies Letter e s d t n y r o l f Freq 0.1917 0.1435 0.0923 0.0864 0.0786 0.0730 0.0693 0.0467 0.0456 0.0408

Polyalphbetic CiphersPolyalphbetic Ciphers

Key is repeated and used to shift Key is repeated and used to shift characters.characters.

Example plaintextnow is the time for all + keyaar dv ark aard var kaaCiphertextopo mo uzp ujei bpj lmm

Polyalphbetic CiphersPolyalphbetic Ciphers

Advantage: Thwarts character frequency analysis. For example, an “e” will encrypt to several different letters.

Disadvantage: Statistics can still be used to break the code.

Polyalphbetic CiphersPolyalphbetic Ciphers

How to Break Them:1 - Look for repeated strings.

For example, if the characters “thi” appear together frequently, then it could be because the key is hitting a common word.

Text = and we need to test and retest Key = ste ve stev es teve ste vestev Sum = thi sj gyjz yh njoy thi njmyxp

Polyalphbetic CiphersPolyalphbetic Ciphers

How to Break Them:2 – Determine Probable Key Length

The start of strings “thi” are frequently separated by distances that are multiples of 5. So, key length is probably five.

3A – Try keys of that length.3B – Use CharFreqAnal on characters

separated by that length.

One-Time PadOne-Time Pad Key is used to shift the plaintext. Key is used only once. Key has same length as the

message.

Advantage: Unbreakable! Disadvantage: Requires lots of keys.

DESDES History History DData EEncryption SStandard Solicited in 1973 by the National

Bureau of Standards (National Institute of Standards and Technology)

Developed by IBM and the NSA Adopted in 1977

DES Design PrinciplesDES Design Principles Confusion Confusion – complicate the

relationship between key and ciphertext

Diffusion Diffusion – spread structure of plaintext around the ciphertext

DES Design OverviewDES Design Overview http://www.itl.nist.gov/fipspubs/fip46-

2.htm

Key = 56 bits plus 8 parity bits 70,000,000,000,000,000 possible

keys of 56 bits Key generates 16 subkeys 16 rounds of functions

Breaking DESBreaking DES 1993 1993 – design of $1M machine to search

entire key space in one day 1997 1997 – design of $1M machine to search

entire key space in one hour 1999 1999 - “DES Challenge” prize claimed in 22

hours by distributed.net 2006 - University of Bochum and Kiel, University of Bochum and Kiel,

Germany, uses $10,000 hardware cost to Germany, uses $10,000 hardware cost to get average time of 6.4 days.get average time of 6.4 days.

triple DES is much less breakable

Unix CryptUnix Crypt

““man 3 crypt”man 3 crypt” #include <unistd.h> char *crypt(const char *key, const char *salt);

crypt is the password encryption function. It is based on the Data Encryption Standard algorithm with variations intended (among other things) to discourage use of hardware implementations of a key search.

Password SaltPassword Salt Based on time when password createdBased on time when password created First two letters in the passwd fieldFirst two letters in the passwd field Used to discourage a brute force Used to discourage a brute force

attackattackEncrypting every dictionary word then comparing that list to passwd entries will not work since every dictionary word can yield 4096 different possibilities.

Even if my password is the same for Even if my password is the same for two systems, they have different salts two systems, they have different salts so they look differentso they look different

Public Key EncryptionPublic Key Encryption Two Keys : encryption and

decryption Encryption key is public Decryption key is private Once sender encrypts a message,

even they can’t decrypt it

Public Key EncryptionPublic Key Encryption

1.1. Receiver sends their public key to Receiver sends their public key to the senderthe sender

2.2. Sender encrypts message using Sender encrypts message using that public keythat public key

3.3. Sender sends encrypted messageSender sends encrypted message4.4. Receiver decrypts message using Receiver decrypts message using

their private keytheir private key

SummarySummary

Nothing on a public Nothing on a public network is completely network is completely

safe.safe.