smucse 5349/7349 network and system security introduction

SMU CSE 5349/7349

Network and System Security

Introduction

SMU CSE 5349/7349

Information Security

• Computer security– Protect system resources– Protect data (dynamic/stored)

• Network security– Protect network resources– Protect exchanged dataSecurity = confidentiality + integrity

+ availability + authenticity

SMU CSE 5349/7349

Attacks

• Passive attacks– Affects data confidentiality– More difficult to detect, less harmful

• Active attacks– Affects data confidentiality, integrity,

and/or availability– Easier detection, more harmful

SMU CSE 5349/7349

Attacks (cont’d)

• Interruption• Interception• Modification• Fabrication

SMU CSE 5349/7349

Interruption

– An asset of the system is destroyed of becomes unavailable or unusable

– Attack on availability– Destruction of hardware– Cutting of a communication line– Disabling the file management

system

SMU CSE 5349/7349

Interception

– An unauthorized party gains access to an asset

– Attack on confidentiality– Wiretapping to capture data in a

network– Illicit copying of files or programs

SMU CSE 5349/7349

Modification– An unauthorized party not only

gains access but tampers with an asset

– Attack on integrity– Change values in a data file– Alter a program to make it perform

differently– Modify content of messages

transmitted on a network

SMU CSE 5349/7349

Fabrication– Unauthorized party inserts counterfeit

objects into the system– Attack on authenticity– Insertion of spurious messages in a

network– Addition of records to a file

SMU CSE 5349/7349

Network Security Services• Authentication

– validate authenticity claims (identity/data)

• Authorization– Enforce selective access (information/resources)

• Confidentiality– Prevent disclosure of data (entire message/selected

fields/traffic characteristics)

• Integrity– Prevent tampering of data (entire message/selected

fields)

• Non-repudiation– Proof of origin, proof of delivery

CIA

SMU CSE 5349/7349

Security Mechanisms

• Specific mechanisms– Access control list - resource protection

– Encryption – data confidentiality

– Digital signature – origin verification, non-repudiation

– Notarization – trusted third party assurance

– Traffic padding – traffic characteristic confidentiality

– Routing control – flow protection

SMU CSE 5349/7349

Mechanisms (cont’d)

• Pervasive mechanisms– Event detection– Security audit trail– Security recovery

SMU CSE 5349/7349

Password Security

SMU CSE 5349/7349

System Security

• Two issues– Attack by unauthorized users– Attack by authorized users

• Solution– Unauthorized – authenticate– Authorized – access control

SMU CSE 5349/7349

Authentication

• UsualUser ID + passwd

– ID public, passwd private• When to authenticate

– Beginning of session– Periodically within session– After a pause– Beginning of sensitive operation

SMU CSE 5349/7349

Selection of Passwd

• Psychological issues– Disclosing passwd – social engineering– Difficulty in entering– Difficulty in remembering

• System issues– Targeted and general attacks– Multi-lateral security– User training

SMU CSE 5349/7349

Attacks on Passwd

• Attack on passwd entry– Interface design– Eavesdropping– Trusted path (ex. Ctrl-Alt-Del)– Passwd retry counters

• Timing attack• Attack on smart cards

• Attack on passwd storage– Audit trail– Need for one-way encryption

SMU CSE 5349/7349

Passwd Cracking

• Brute force attack– Use all combination of characters

• Knowledge of victim– Names, license plate numbers…

• Dictionary attack

SMU CSE 5349/7349

Defense

• Use different passwds for users/systems• No default passwds• Against brute force

– Longer passwds– More than letters and numbers– Case sensitive

• Against dictionary– No words– No “obvious” passwds

SMU CSE 5349/7349

System Defenses

• Run passwd filters to weed out weak ones

• Generate random passwds for users

• Enforce passwd life-time• Limit failed logins• Notifying history to users

SMU CSE 5349/7349

Salting the Passwd

• What happens if two users choose the same passwd?– Will result in same hash

• Add some random information (salt)to the passwd before hashing

• Makes it harder to crack

SMU CSE 5349/7349

Access Control

SMU CSE 5349/7349

Authentication vs. Access Control

• Authentication about who you are• Access control – who may do what

to what?• Major portion of security breaches

organized by authorized users

SMU CSE 5349/7349

Control Structures

• Formalism– Subjects, Objects, Access permissions– Access control matrix

• Example

SMU CSE 5349/7349

Capabilities and Access Control Lists (ACL)

• For large systems – Large number of subjects and objects– Control matrix very large and sparse

• Store the matrix rows and columns as linked lists– Rows – capabilities

• Columns - ACLs

SMU CSE 5349/7349

Capabilities

• What subject A is allowed to do• Whenever a new object is created, other

subjects may be given the capabilities to access it

SMU CSE 5349/7349

Intermediate Solutions

• Group permissions• Negative permissions• Protection rings• Permission trees

SMU CSE 5349/7349

Group Permissions

• Place number of users in a named group

• ACLs for groups

SMU CSE 5349/7349

Negative Permission

• To exclude specific permission from the group grant

SMU CSE 5349/7349

Protection Rings

• Assigning priority levels to objects and subjects– 0 kernel– 1 OS– 2 system utilities– 3 other programs

• Access decisions based on comparing the levels of Ss and Os.

SMU CSE 5349/7349

Hardware Protection

• Protection problem– Preventing one processor from

interfering with another• Confinement problem

– Preventing programs communicate outside the authorized channels

• Memory management – Reference monitor

SMU CSE 5349/7349

Security Processors

• Pentium 3 processor serial number• ARM (Acorn Risc Machine)• Smart cards• Protection of protection hardware

– Tamper resistance

SMU CSE 5349/7349

Attack (cont’d)

• Smashing the stack– Finger bug in UNIX– Does not check the length of the

input string– Trailing edge could be used to

overwrite the system stack– Want to be a root user?

SMU CSE 5349/7349

Cryptography

SMU CSE 5349/7349

Terminology

• Cryptography• Cryptanalysis• Cryptology• Plain text• Cipher text

SMU CSE 5349/7349

Historical Background

• Mono-alphabetic substitution– Caesar cipher

• Stream ciphersC= P + K mod 26

• One-time pad– Perfect secrecy (Shannon’s theorem)– No integrity

• Block codes– Playfair

• One-way functions

SMU CSE 5349/7349

Formal Classifications

• Type of operation– Substitution– Transposition

• Number of keys used– Symmetric (one key, private)– Asymmetric (two keys, public)

• Mode of plain text processing– Block vs. stream

SMU CSE 5349/7349

Cryptanalysis

• Attacks– Known plain text attack– Chosen plain text– Chosen cipher text

• Security– Unconditionally secure– Computationally secure

SMU CSE 5349/7349

Block Ciphers• Block of plain text converted into

block of cipher of equal length• Based on Fiestal block structure

– Unique transformation– Reversible

• Example• Small blocks vulnerable to attacks• Large blocks impractical to

implement

SMU CSE 5349/7349

Fiestal Cipher (FC)

• Based on product ciphers– Alternate substitutions and permutations

• Shannon’s proposal– Diffusion

• Dissipate plain text statistics into long-range cipher statistics

• Example: Averaging operation on characters• A permutation on binary bits followed by some

operations

– Confusion• To make the deduction of the key difficult

SMU CSE 5349/7349

FC Structure

• Multiple rounds– Round function (based on the round key)– Substitution (XOR)– Permutation (Exchange of halves)

• Parameters– Block size (64 bits)– Key size (128 bits)– Number of rounds (16)– Sub-key generation algorithm– Round function

SMU CSE 5349/7349

DES

Plaintext

Encrypt withsecret key

Ciphertext

Plaintext

Decrypt withsecret key

SMU CSE 5349/7349

DES• Currently the most widely used

block cipher in the world • IBM’s LUCIFER was the precursor• One of the largest users of the DES

is the banking industry, particularly with EFT

• Although the standard is public, the design criteria used are classified

SMU CSE 5349/7349

DES Security• Recent analysis has shown that DES is

well designed (diffusion & confusion)• Rapid advances in computing speed

though have rendered the 56 bit key susceptible to exhaustive key search – 1997 on a large network of computers in a

few months – 1998 on dedicated h/w in a few days – 1999 above combined in 22hrs!

• DES also theoretically broken using Differential or Linear Cryptanalysis

• In practice, unlikely to be a problem yet

SMU CSE 5349/7349

Overview of DES Encryption

• Basic process consists of: – An initial permutation (IP) – 16 rounds of a complex key

dependent calculation F– A final permutation, being the inverse

of IP

SMU CSE 5349/7349

• 64-bit key (56-bits + 8-bit parity)• 16 rounds

Initial permutation

Round 1

Round 2

Round 16

56-bitkey

Final permutation

…

+

F

Li – 1 Ri – 1

Ri

Ki

Li

• Each Round

SMU CSE 5349/7349

Initial Permutation

• No improvement in security, cipher gets more complex

58, 50, 42, 34, 26, 18, 10, 2,60, 52, 44, 36, 28, 20, 12, 4,62, 54, 46, 38, 30, 22, 14, 6,57, 49, 41, 33, 25, 17, 9, 1,59, 51, 43, 35, 27, 19, 11, 3, 61, 53, 45, 37, 29, 21, 13, 5,63, 55, 47, 39, 31, 23, 15, 7

SMU CSE 5349/7349

DES Key Generation

• Sub-keys used in each round are formed by – An initial permutation of the key (PC1)

which selects 56-bits in two 28-bit halves – 16 stages consisting of:

• selecting 24-bits from each half • permuting them by PC2 for use in function f, • rotating each half separately either 1 or 2 places

depending on the key rotation schedule KS

• Can be described functionally as: SKi = PC2(KS(PC1(Key),i))

SMU CSE 5349/7349

Expansion Function E

• Expands RH side data input from 32 to 48 bits by duplicating some bits

• Specifically split input into 8 groups of 4 bits

• Duplicate bits from either side to form groups of 6 bits

SMU CSE 5349/7349

Substitution (S) Boxes

• 8 S-boxes, each of which maps 6 bits to 4 bits

• Each S-box is actually 4 little 4 bit boxes

• Outer bits 1 & 6 (row bits) select one of the 4 rows

• Inner bits 2-5 (col bits) are substituted for 4 others

SMU CSE 5349/7349

DES Decryption

• To decrypt a block of data must unwind all steps of data computation

• With Feistel design, same as just doing encryption steps again

• Except that must use subkeys in reverse order (SK16 first, then SK15 etc.)

SMU CSE 5349/7349

Avalanche Effect

• Desirable that a small change either in the plain-text or key produces a large change in the cipher

• DES shows strong avalanche

SMU CSE 5349/7349

Advanced Encryption Standard

AES

SMU CSE 5349/7349

Origins of AES

• In 1999, NIST issued a new standard that said 3DES should be used– 168-bit key length– Algorithm is the same as DES

• 3DES had drawbacks– Algorithm is sluggish in software– Only uses 64-bit block size

SMU CSE 5349/7349

Origins of AES (Cont’d)

• In 1997, NIST issued a CFP for AES– security strength >= 3DES– improved efficiency– must be a symmetric block cipher

(128-bit)– key lengths of 128, 192, and 256 bits

SMU CSE 5349/7349

Origins of AES (cont’d)

• First round of evaluation– 15 proposed algorithms accepted

• Second round– 5 proposed algorithms accepted

• Rijndael, Serpent, 2fish, RC6, and MARS

• Final Standard - November 2001– Rijndael selected as AES algorithm

SMU CSE 5349/7349

The AES Cipher• Block length is 128 bits• Key length is 128, 192, or 256 bits• NOT a Feistel structure

• Processes entire block in parallel during each round using substitutions and permutations

• The key that is provided as input is expanded

• Array of forty-four 32-bit words (w[i])• Four distinct words serve as round key (128 bits)

SMU CSE 5349/7349

Key/Block Size

Key Size (words/bytes/bits)

4/16/128 6/24/192 8/32/256

Plaintext block size (words/bytes/bits)

4/16/128 4/16/128 4/16/128

Number of rounds 10 12 14

Round key size (words/bytes/bits)

4/16/128 4/16/128 4/16/128

Expanded key size (words/bytes)

44/176 52/208 60/240

SMU CSE 5349/7349

Add Round Key

Inverse mix cols

Add round key

Inverse sub bytes

Inverse shift rows

Add round key

Mix Columns

Shift Rows

Add Round Key

Inverse sub bytes

Inverse shift rows

Inverse mix cols

Add round key

Inverse sub bytes

Inverse shift rows

Add round key

Substitute Bytes

Add round key

Shift Rows

Substitute Bytes

Add round key

Substitute Bytes

Shift Rows

Mix Columns

Expand Key

.

.

.

.

.

.

w[0,3]

w[4,7]

w[36,39]

w[40,43]

Plaintext Plaintext

Ciphertext Ciphertext

Rou

nd 1

Rou

nd 9

Rou

nd 1

0R

ound

1

Rou

nd 9

Rou

nd 1

0

SMU CSE 5349/7349

Encryption Rounds– Four different stages (1 permutation,

3 substitution)• Substitute bytes- Uses S-box to perform

byte by byte substitution of the block• Shift rows- A simple permutation• Mix columns- Substitution over GF(28)• Add round key- bitwise XOR of current

block and portion of expanded key

– For both encryption and decryption:• Start with add round key followed by nine

rounds of four stages, plus tenth round of three stages

SMU CSE 5349/7349

Round Features– Only Add round key makes use of the key

• All other stages reversible without knowledge of key

– Add round key alone is not formidable• The other three stages add diffusion, confusion,

and nonlinearity

– Each stage is reversible • SB, SR and MC use inverse function• ARK uses XOR

– Decryption uses expanded keys in reverse order

SMU CSE 5349/7349

Substitute Bytes

• Forward Substitute Byte Transformation• Uses table lookup (S-box) to perform

permutation– Has all 256 8-bit values

• Each byte of state mapped to new byte:– Leftmost 4 bits = row– Rightmost 4 bits = column

SMU CSE 5349/7349

SB Transformation

x

y

s3,1

s2,1

s0,0

s1,2

s3,3

s2,3

s1,3

s0,2 s0,3

s2,2

s3,0 s3,2

s2,0

s1,0 s1,1

s0,1

s0,0 s0,1

s1,0

s2,0

s3,0

s1,1

s2,1

s3,1

s0,2

s1,2

s2,2

s3,2

s0,3

s1,3

s2,3

s3,3

S-Box

SMU CSE 5349/7349

SB (cont’d)

• Inverse– The inverse substitute byte

transformation– Makes use of an inverse S-box

• Rationale– S-box designed to be resistant to

cryptanalytic attacks– Output cannot be described as a

simple mathematical function of input

SMU CSE 5349/7349

Shift Row Transformation

• First row- not altered• Second row- 1 byte circular left

shift• Third row- 2 byte circular left shift• Fourth row- 3 byte circular left shift

SMU CSE 5349/7349

SR Transformation

s0,0

s1,3

s3,2

s2,0

s1,0

s0,2 s0,3

s2,0

s3,3 s3,1

s2,2

s1,1 s1,2

s0,1s0,0

s1,2

s3,3

s2,3

s1,3

s0,2 s0,3

s2,2

s3,0 s3,2

s2,0

s1,0 s1,1

s0,1

s2,1 s2,3

s3,1 s3,0

SMU CSE 5349/7349

SR (cont’d)

• Inverse– Performs the circular shifts in the

opposite direction

• Rationale– Transformation ensures that the 4

bytes of one column are spread out to four different columns

SMU CSE 5349/7349

Mix Column Transformation

• Operates on each column individually– Each byte of a column is mapped to a new

value that is a function of all four bytes– Each element in product matrix is sum of

products of elements of one row and one column

– Additions and multiplications performed in GF(28)

SMU CSE 5349/7349

Mix column transformation

s0,0

s1,2

s3,3

s2,3

s1,3

s0,2 s0,3

s2,2

s3,0 s3,2

s2,0

s1,0 s1,1

s0,1s0,0

s1,2

s3,3

s2,3

s1,3

s0,2 s0,3

s2,2

s3,0 s3,2

s2,0

s1,0 s1,1

s0,1

2113

3211

1321

1132

s2,1

s3,1

s2,1

s3,1

SMU CSE 5349/7349

Mix Columns (cont’d)• Inverse

– Uses the inverse transform matrix to reverse the mix column transformation

• Rationale– Ensures a good mixing among the

bytes of each column– Combined with shift rows ensures that

all output bits depend on all input bits

SMU CSE 5349/7349

Add Round Key

• 128 bits of state are bitwise XORed with the 128 bits of the round key

• Column-wise operation between the 4 bytes of state and one word of round key – Byte level operation

SMU CSE 5349/7349

Add Round Key Transformation

s0,0

s1,2

s3,3

s2,3

s1,3

s0,2 s0,3

s2,2

s3,0 s3,2

s2,0

s1,0 s1,1

s0,1 s0,0

s1,2

s3,3

s2,3

s1,3

s0,2 s0,3

s2,2

s3,0 s3,2

s2,0

s1,0 s1,1

s0,1

=XOR wi wi+1 wi+2 wi+3s2,1

s3,1

s2,1

s3,1

SMU CSE 5349/7349

Add Round Key

• Inverse– Identical to the forward add round

key transformation

• Rationale– Complexity of round key expansion

ensures security

SMU CSE 5349/7349

AES Key Expansion

• Key Expansion Algorithm– Takes as input 4-word (16-byte) key– Produces linear array of 44 words

(176 bytes)– Provides 4-word round key for 10

rounds of cipher + 1 additional Add round key

SMU CSE 5349/7349

AES Key Expansionk0

k15

k14

k13

k12k4

k11

k9

k8

k5k1

k2

k3

k6

k7

k10

gw0 w1 w2 w3

w4 w5 w6 w7

Function g:

1. One-byte circular left shift on a word so [b0, b1, b2, b3] is now [b1, b2, b3, b0]

2. Byte substitution using S-box

3. XOR 1 & 2 with a round constant

SMU CSE 5349/7349

Decryption

• Not identical to encryption• Equivalent structure exists• May need different

implementations if encryption and decryption are needed

• Quite often only encryption needed– Digest

SMU CSE 5349/7349

Modern Stream Ciphers

SMU CSE 5349/7349

Block vs. Stream• Block ciphers

– process plaintext in relatively large blocks(e.g. n64 bits)

– The same function is used to encrypt successive blocks memory less

• Stream ciphers– process plaintext in small blocks, and the

encryption function may vary as plaintext is processed have memory

– sometimes called state ciphers since encryption depends on not only the key and plaintext, but also on the current state.

SMU CSE 5349/7349

One-Time Pad

• Provably secure– Key generated independently and randomly

– no integrity checks– Key as long as the plain text– Must never ever reuse a stream key

• If two messages encrypted by XOR with same key can combine these to remove effect of key giving a book cipher

C1i = M1i ⊕ Ki

C2i = M2i ⊕ Ki

C1i ⊕ C2i = M1i ⊕ Ki ⊕ M2i ⊕ Ki = M1i ⊕ M2i

SMU CSE 5349/7349

Pseudo Random Generators

• Compromise to computationally secure– Instead of random, use pseudo random

sequence based on a short key– the generated stream must be:

• statistically random (knowing part of seq not enough)

– PRG may be controlled just by key influencing:– next-state function (output feedback mode)– output function (counter mode)

– PRG may be controlled both by data and key:– output function (cipher feedback mode)

SMU CSE 5349/7349

Synchronous Stream Ciphers

• Keystream generated independently of plaintext and of ciphertext

f

g

si

hk

si+1

zi

mi

ci

f

g

si

h-1k

si+1

zi

ci

SMU CSE 5349/7349

Synchronous Ciphers (cont’d)

– Both sender and receiver must be synchronized using same key and operating at the same position

• If sync. is lost due to inserted or deleted ciphertext, decryption fails

• No error propagation: A modified ciphertext during transmission does not affect the decryption of other ciphertext digits.

– Active attacks: the insertion, deletion, or replay of ciphertext digits by an active adversary causes immediate loss of synchronization

SMU CSE 5349/7349

Self-synchronizing Ciphers

• Key-stream generated as a function of the key and a fixed number of previous ciphertext digits

g hkzi

mi

ci

…

SMU CSE 5349/7349

Linear Feedback Shift Registers

• LFSRS– Well-suited to hardware

implementation– Can produce sequences of large

period– Can produce sequences with good

statistical properties c1 c2 Cl

SMU CSE 5349/7349

LFSRs (cont’d)

• Balanced, non-linear, correlation immune

NonlinearCombiner Function F

LFSR-L1

LFSR-L2

LFSR-Ln

output

SMU CSE 5349/7349

General SC Criteria• Long period with no repetitions • Statistically random • Large linear complexity (based on size of

equiv LFSR) • Correlation immunity (have tradeoff with

linear complexity) • Confusion (output bits depend on all key

bits) • Diffusion • Use of highly non-linear Boolean functions

SMU CSE 5349/7349

RC4

• Proprietary cipher owned by RSA • Variable key size, byte-oriented stream

cipher • Widely used (web SSL/TLS, wireless WEP) • Key forms random permutation of all 8-

bit values • Uses that permutation to scramble input

info processed one byte at a time

SMU CSE 5349/7349

RC4 Key Schedule

• Starts with an array S of numbers: 0..255

• S forms internal state of the cipher • given a key k of length l bytes

for i = 0 to 255 doS[i] = i

j = 0

for i = 0 to 255 do j = (j + S[i] + k[i mod l]) (mod 256)

swap (S[i], S[j])

SMU CSE 5349/7349

Stream Generation

• Encryption continues shuffling array valuesi = j = 0

for each message byte Mi

i = (i + 1) (mod 256)j = (j + S[i]) (mod 256)swap(S[i], S[j])t = (S[i] + S[j]) (mod 256)

Ci = Mi XOR S[t]

SMU CSE 5349/7349

RC4 Security Issues

• First group of outputs correlate with key

• Should discard first 256 outputs • Should never reuse a key • After a few GB, see some values

occurring slightly too often

SMU CSE 5349/7349

WEP Encryption

• Mobile station shares key with Access Point

• Transmitting a message M– Compute checksum of M and append it to M

Generate keystream using RC4(IV,Key) Xor <M,C(M)> with keystream Transmit IV and cipher text

• Upon receiving, reverse steps

SMU CSE 5349/7349

WEP(cont’d)

• Key length = 40. IV length = 24

Message CRC

Keystream = RC(IV,k)

Cipher TextIV

XOR

SMU CSE 5349/7349

A5 Algorithm• Stream cipher used to encrypt GSM

phones • Has several variants (A5, A5/1, A5/2) • A5/1 uses 3 LFSRs of 19, 22 and 23 bits

using sparse feedback polys – all have been broken (A5/2 in Aug99, A5/1 in

Apr 2000) - basic attack has complexity 240 – guess state of LFSRs 1 & 2, try to determine

3 from keystream – real problem is registers are too small &

feedback polys are sparse

SMU CSE 5349/7349

Public-Key Cryptography

Asymmetric Encryption

SMU CSE 5349/7349

Private-Key Encryption

• Traditional private/secret/single key cryptography uses one key shared by both sender and receiver

• If this key is disclosed, communications are compromised

• Symmetric, parties are equal • Does not protect sender from receiver

forging a message & claiming is sent by sender

SMU CSE 5349/7349

Public-Key Cryptography• Involves the use of two keys:

– A public-key, which may be known by anybody, and can be used to encrypt messages, and verify signatures

– A private-key, known only to the recipient, used to decrypt messages, and sign (create) signatures

• Asymmetric, parties are not equal • Clever application of number theory • Single most significant advance in the

3000 year history of cryptography

SMU CSE 5349/7349

Basic Principle• Public-key is easily computed from the

private key and other information about the cipher (a polynomial time (P-time) problem)

• However, knowing the public-key and public description of the cipher, it is still computationally infeasible to compute the private key (an NP-time problem)

• Public-key may be distributed to anyone wishing to communicate securely with its owner – Secure distribution of the public-key is a

non-trivial problem

SMU CSE 5349/7349

Classes of Public-key Algorithms• Public-key distribution schemes (PKDS)• Public-key encryption (PKE)

– Used to encrypt any arbitrary message – Anyone can use the public-key to encrypt a

message – Owner uses the private-key to decrypt the

messages – any public-key encryption scheme can be

used as a PKDS by using the session key as the message

• Signature schemes– Encrypt with Prv key and verify with pub key

SMU CSE 5349/7349

Diffie-Hellman Key Exchange

• One of the first public key systems– Mainly used for key exchange– Do not do any encryption as such– Modified versions used in IPsec

(Will talk more about it when we discuss key distribution)

SMU CSE 5349/7349

RSA Scheme

• Best known and widely regarded as most practical public-key scheme

• Proposed by Rivest, Shamir & Adleman (RSA) in 1977

• Based on exponentiation in a finite (Galois) field over integers modulo a prime

• Security relies on the difficulty of calculating factors of large numbers

SMU CSE 5349/7349

RSA Setup• First,  each user generates their

public/private key pair by: – Selecting two large primes at random (~100

digit), p, q – Computing their system modulus N=p.q – Selecting at random the encryption key e,

where e<N, gcd(e,ø(N))=1 – Solving the following congruence to find the

decryption key d: e.d=1 mod ø(N) and 0<=d<=N

• Public encryption key ={e,N} • Private decryption key ={d,p,q}

SMU CSE 5349/7349

RSA Parameter Selection• Choose  large p, q • Choose the encryption exponent e

to be a small number – Typically e may be the same for all

users– Originally a value of 3 was suggested –  Now regarded as too small –  65535 is often used –  Note that the decryption exponent d

will then be large

SMU CSE 5349/7349

Sample RSA• Choose modulus N=11*47=517 • Compute ø(N) = (p-1)(q-1) = 10*46 = 460 • Choose encryption exponent 3 • Check GCD(3,ø(N)) = GCD(3,460) = 1 • Compute decryption exponent d by

solving:      e.d=1 mod ø(N) where 0<=d<=N      d=Inverse(3,460)=307

• Public key is: K=(3,517) • Private key is: K-1=(307,11,47)

SMU CSE 5349/7349

Example (cont’d)

• Given message M = 26 •  Encryption: C = 263 mod 517 =

515 •  Decryption: M = 515307 mod 517

= 26

SMU CSE 5349/7349

How Does it Work?

• Euler’s theorem• The multiplicative group for Zn, denoted

with Z*n, is the subset of elements of Zn relatively prime with n

• The totient function of n, denoted with Φ(n), is the size of Z*n

• For each element x of Z*n, we have x Φ(n) mod n = 1

SMU CSE 5349/7349

Totient for RSA “n”

For primes, p and q: n = pq, (n) = (pq) – Numbers which are not relatively prime

– p, 2p, 3p, …, (q – 1)p ; q-1 of them-q, 2q, 3q, …, (p – 1)q ; p-1 of them

(n) = pq – 1 – (q – 1) – (p – 1)

= pq – (p + q) + 1

= (p – 1) (q – 1) = (p) (q)

SMU CSE 5349/7349

Encryption/Decryption

• C = M e mod n (Encryption)• Decryption:

– C d mod n= M ed mod n= M kΦ(n)+1 mod n= M (It’s easy to see that when M does

not divide n)

SMU CSE 5349/7349

Security of RSA

• Brute force– Trying all possible private keys

• Mathematical attacks– Equivalent factoring product of two

primes• Timing attacks

– Based on the running time of the decryption algorithm

SMU CSE 5349/7349

Attacks on Protocol

• Chosen cipher-text attack• Common modulus attack• Low encryption/decryption

modulus attack