StaridLabs

Cryptography IIDomain 5 Pages 790-821Official CISSP CBK Third Edition

Jem JensenStaridLabs

A Quick Recap

Converting plaintext into ciphertext through transpositions or substitutions

Spartan Scytale Wrap message (written on a belt/strap) around a stick to decipher

Caesar cipher shift the whole alphabet

Nazi Enigma machines

Key used to encrypt/decrypt the plaintext

Substitution Ciphers

Replaces a letter for a letter

Like in the newspaper!

Examples:Caesar cipher

Decoder ring

Playfair Cipher

Used through WW2

Pre-shared keyword (Ex: CRYPTO)

Keyword is fed into a table, followed by the remaining alphabetMerge I&J

CRYPT

OABDE

FGHI/JK

LMNQS

UVWXZ

Playfair Cipher

Break apart plaintext into pairsSprinkle filler characters (Ex: K)

WE AR EK ST AR IK DL AK BS

Playfair Cipher

Draw a box between the 2 letters in the pairIf the box is larger than 1xn, swap letters with the other 2 corners (Ex: WE = BZ)

If the box is only 1 row thick, use the next letter in line (Ex: AR=RV)

CRYPT

OABDE

FGHI/JK

LMNQS

UVWXZ

Transposition Cipher

Change the order of letters

Like pig latin!Move first letter of a word to the end

Add an a sound at the end

Et-gay ti-ay?

Rail Fence

Like writing the message on a fence then tilting the posts to the side

Draw a box with 2+ columns thick

Enter the text in a zig-zag fashion

Read the text across the columns for encrypted version (Ex: gvecslirmaieahodeael)

Susceptible to frequency analysis

A quick side note

Frequency analysisE, T, A, & O are the most common letters in English words

Z, Q, & X are the least used

If you know a message is in English and you see a lot of E, T, A & Os or very few Z, Q, or X you know it's a transposition cipher and can probably start guessing some letters correctly

Rectangular Substitution Table

Sender and receiver agree on table dimensions and the order to read the message (Ex: 4x4 table, read top to bottom, left to right)

= WERAESIBATDSRAL

Also susceptible to frequency analysis

WEAR

ESTA

RIDL

ABS

Monoalphabetic Cipher

Like a Caesar cipher except we scramble the alphabet instead of shifting itCAESAR




MONO

Ex: In the above Caesar, FEED=IHHG
In the above Mono, FEED=IWWU

ABCDEF......YZ

MGPUWI......TK

ABCDEF......YZ

DEFGHI......BC

Polyalphabetic Cipher

Like a monoalphabetic except we add more rows of scrambled alphabets

POLY

Ex: In the Mono, FEED=IWWU
In the above Poly, FEED=IXWC

ABCDEF......YZ

MGPUWI......TK

NBVCXZ......FW

Blaise De Vigenre

Transposition table

Blaise De Vigenre

Sender/receiver have a pre-shared keyword

Locate the intersection of the keyword with the plaintext for the cipher equivalentExample
Keyword: CABBAGE
Plaintext: FEEDBOB
Ciphertext: HEFEBUF

Modular Mathematics

Modulus/Modulo OperationThe remainder after dividing a number by nExample
5 mod 2 : 5/2 : 3/2 = 1
4 mod 2 : 4/2 : 4/2 = 0

Running Key Cipher

Since there are 26 letters in the alphabet, we'll use mod 26Ciphertext = (plaintext + key) mod 26
C = P + K (mod 26)

Example:

Plaintext: E (or 4) C = (4 + 22) mod 26 = 0
Key: W (or 22) Ciphertext: A

ABCDEF......YZ

012345......2425

Running Key Cipher

To translate CHEEK using the key of FEED:

The longer the key, the better, since there's less repetition

PtxtCHEEK

KeyFEEDF

n(P)274410

n(K)54435

n(C)7118715

CtxtHLIHP

One-time Pads

Gilbert Vernam asserted that a cipher key as long as the plaintext (that doesn't repeat) should be unbreakable

Requires a sender and receiver to exchange the long key beforehand (ex: book ciphers)

Ideally the key is randomly chosen values like letters, numbers, Geiger counter readings

One-time Pads

Key of ksosdfshepwlqz

The key could be longer than the plaintext

PtxtDONOT

KeyKSOSD

n(P)314131419

n(K)101814183

n(C)133227622

CtxtNBGWS

A step back

So what have we learned?

Crypto can provide integrity controlsIf the message was altered by a modification or error, it won't decrypt

Some degree of authenticationOnly the people who have the key should be able to use it. If we only share the key with one person, we can know it's them

Symmetric Cryptography

The sender and receiver use a single shared key for encryption/decryption

Key management is the biggest problemWe can't send the keys over the same channel we're trying to encrypt!

Send the key via out-of-band distribution (ex: fax, mailing a CD, calling on the phone)

Symmetric Cryptography

Fast, secure cheap

Doesn't provide non-reputiation

Limited message integrity and access controlWe can tell the message changed and it requires the key in order to be read

Physical example:10 people have a key to the server room. Who entered at 11pm? We only know it was one of the 10

Examples of Symmetric Algorithms

Caesar Cipher

Spartan Scytale

Enigma Machines

DES

Data Encryption StandardHarst Feistal

Take input block, divide it in half, and XOR several timesEach XOR is called a round

Became gov't standard in 1977

Repaced by AES

Block Cipher Modes

ECB Electronic Codebook ModeEssentially a digital representation of a codebook. Feed in plaintext and it outputs the ciphertext using a table

Useful for very short messages to reduce repeating (64-bit)

CBC Cipher Block Chaining ModeUses an IV and chaining function so that subsequent output will be different even if the input was identical

Initial input block is XORed with the randomly chosen IV

Output if then XORed with next input

Block Cipher Modes

CFB Cipher Feedback ModeInput broken into segments (usually 8 bit)

Segments are XORed with a random IV

Previous segment is XORed with the next segment

Drawback if a bit is corrupted, everything after will be too

OFB Output Feedback ModeLike CFB but uses encrypted keystream instead of ciphertext so that one corruption won't affect future encryption

Can also generate keystream ahead of time for future use

CTR Counter ModeLike OFB but increments a counter for keystream

Allows for out-of-order processing

Pros/Cons of DES

Fast

Breaking DES was unrealistic back in the day

Susceptible to brute-forcing since key is only 56 bits long

Double DES

Since brute forcing if our main method of attack... let's just double the key!

Done by running DES over DES with diff keys

Yo dawg! I heard you like DES so I DESed your DES so you can DES while you DES!

Double DES

Meet in the middle attackDouble DES had hoped that their key doubling would result in an attack taking exponentially longer

Can still brute forceJust takes twice as long

Step 1: Encrypt plaintext with all possible keys

Step 2: Decrypt using all possible keys

Triple DES (3DES)

Third time's the charm!

Eliminates meet in the middle attack

I don't know many more times can I DES the DES with a DES...

AES

Beat out 3DES in terms of:Security

Speed

Larger block size

Rijndael algorithm

CCMP

Counter mode cipher block chaining message authentication code protocolAES with 128 bit key

48 bit IVPrevents replay attacks

CTR counterProvides data privacy

MIC message integrity check code

Used in 802.11i standard

Algorithms

RijndaelSubstitute bytes (s-box substitution)

shift rows (transposition)

mix columns (substitution)

add round key (XOR with this round's key)

IDEA (1991)8 rounds of transposition and substitution

CAST (1996)48 rounds

Algorithms

SAFERPatent-free, 64 or 128 bit

Used in bluetooth

BlowfishExtremely fast and memory efficient

Slow to regenerate keys

Currently unreakable

Twofish

Algorithms

RC5RSA algorithm

Very adaptable 4 modes of operation

RC4 (1987)Stream cipher

Varying length keys

Pros/Cons of Symmetric Algorithms

Very fast, secure

Can be implemented in hardware and software

Key management can be difficultCan be challenges with distributing the keys

Manage additional out of bound channel

Can't provide non-reputiation

Next Week:Asymmetric Algorithms!