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### October 2, 2003

Introduction to

Computer Security

Lecture 6

Cryptography

INFSCI 2935: Introduction of Computer Security

Overview of the DES

- A block cipher:
- encrypts blocks of 64 bits using a 64 bit key
- outputs 64 bits of ciphertext
- A product cipher
- performs both substitution and transposition (permutation) on the bits
- basic unit is the bit
- Consists of 16 rounds (iterations) each with a round key generated from the user-supplied key

INFSCI 2935: Introduction to Computer Security

DES

- Round keys are 48 bits each
- Extracted from 64 bits
- Permutation applied
- Deciphering involves using round keys in reverse

INFSCI 2935: Introduction to Computer Security

IP

L

R

0

0

Å

f

K

1

L

= R

R

= L

Å

(R

, K

)

f

1

0

1

0

0

1

L

= R

R

= L

Å

(R

, K

)

f

16

15

16

15

15

16

-1

IP

output

DES EncryptionINFSCI 2935: Introduction to Computer Security

(32 bits)

K

(48 bits)

i-1

i

Expansion

Permutation

Å

R

(48 bits)

6 bits into each

i

-1

S7

S1

S2

S3

S4

S5

S6

S8

4 bits out of each

P

32 bits

The f functionINFSCI 2935: Introduction to Computer Security

Controversy

- Considered too weak
- Diffie, Hellman said in a few years technology would allow DES to be broken in days
- Design using 1999 technology published
- Design decisions not public
- S-boxes may have backdoors

INFSCI 2935: Introduction to Computer Security

Undesirable Properties

- 4 weak keys
- They are their own inverses
- 12 semi-weak keys
- Each has another semi-weak key as inverse
- Complementation property
- DESk(m) = c DESk´(m´) = c´
- S-boxes exhibit irregular properties
- Distribution of odd, even numbers non-random
- Outputs of fourth box depends on input to third box
- Reasons for structure were suspicious

INFSCI 2935: Introduction to Computer Security

Differential Cryptanalysis

- A form of chosen plaintext attack
- Involves encrypting many texts that are only slightly different from one another and comparing results
- Requires 247 plaintext, ciphertext pairs
- Revealed several properties
- Small changes in S-boxes reduce the number of pairs needed
- Making every bit of the round keys independent does not impede attack
- Linear cryptanalysis improves result
- Requires 243 plaintext, ciphertext pairs

INFSCI 2935: Introduction to Computer Security

m1

m2

…

DES

DES

…

c1

c2

…

sent

sent

DES Modes- Electronic Code Book Mode (ECB):
- Encipher each block independently
- Cipher Block Chaining Mode (CBC)
- XOR each block with previous ciphertext block
- Uses an initialization vector for the first one

INFSCI 2935: Introduction to Computer Security

c1

c2

…

DES

DES

…

m1

m2

…

CBC Mode Decryption- CBC has self healing property
- If one block of ciphertext is altered, the error propagates for at most two blocks

INFSCI 2935: Introduction to Computer Security

Self-Healing Property

- Initial message
- 3231343336353837 3231343336353837 3231343336353837 3231343336353837
- Received as (underlined 4c should be 4b)
- ef7c4cb2b4ce6f3b f6266e3a97af0e2c 746ab9a6308f4256 33e60b451b09603d
- Which decrypts to
- efca61e19f4836f1 3231333336353837 3231343336353837 3231343336353837
- Incorrect bytes underlined; plaintext “heals” after 2 blocks

INFSCI 2935: Introduction to Computer Security

Current Status of DES

- Design for computer system, associated software that could break any DES-enciphered message in a few days published in 1998
- Several challenges to break DES messages solved using distributed computing
- NIST selected Rijndael as Advanced Encryption Standard, successor to DES
- Designed to withstand attacks that were successful on DES

INFSCI 2935: Introduction to Computer Security

Public Key Cryptography

- Two keys
- Private key known only to individual
- Public key available to anyone
- Idea
- Confidentiality:
- encipher using public key,
- decipher using private key
- Integrity/authentication:
- encipher using private key,
- decipher using public one

INFSCI 2935: Introduction to Computer Security

Requirements

- Given the appropriate key, it must be computationally easy to encipher or decipher a message
- It must be computationally infeasible to derive the private key from the public key
- It must be computationally infeasible to determine the private key from a chosen plaintext attack

INFSCI 2935: Introduction to Computer Security

Diffie-Hellman

- Compute a common, shared key
- Called a symmetric key exchange protocol
- Based on discrete logarithm problem
- Given integers n and g and prime number p, compute k such that n = gk mod p
- Solutions known for small p
- Solutions computationally infeasible as p grows large – hence, choose large p

INFSCI 2935: Introduction to Computer Security

Algorithm

- Constants known to participants
- prime p; integer g other than 0, 1 or p–1
- Alice: (private = kA, public = KA)
- Bob: (private = kB, public = KB)
- KA = gkA mod p
- KB = gkB mod p
- To communicate with Bob,
- Anne computes SA, B= KBkA mod p
- To communicate with Alice,
- Bob computes SB, A = KAkB mod p
- SA, B = SB, A ?

INFSCI 2935: Introduction to Computer Security

Example

- Assume p = 53 and g = 17
- Alice chooses kA = 5
- Then KA = 175 mod 53 = 40
- Bob chooses kB= 7
- Then KB= 177 mod 53 = 6
- Shared key:
- KBkA mod p = 65 mod 53 = 38
- KAkB mod p = 407 mod 53 = 38

Let p = 5, g = 3

kA = 4, kB = 3

KA = ?, KB = ?,

S = ?,

INFSCI 2935: Introduction to Computer Security

RSA

- Relies on the difficulty of determining the number of numbers relatively prime to a large integer n
- Totient function (n)
- Number of + integers less than n and relatively prime to n
- Relatively prime means with no factors in common with n
- Example: (10) = 4
- 1, 3, 7, 9 are relatively prime to 10
- (77) ?
- (p) ?
- When p is a prime number
- (pq) ?
- When p and q are prime numbers

INFSCI 2935: Introduction to Computer Security

Algorithm

- Choose two large prime numbers p, q
- Let n = pq; then (n) = (p–1)(q–1)
- Choose e < n relatively prime to (n).
- Compute d such that ed mod (n) = 1
- Public key: (e, n); private key: d (or (d, n))
- Encipher: c = me mod n
- Decipher: m = cd mod n

INFSCI 2935: Introduction to Computer Security

Confidentiality using RSA

Y

X

Encryption

Message

Source

X

Decryption

Message

Source

Bob

Alice

kB

KB

Key Source

INFSCI 2935: Introduction to Computer Security

Example: Confidentiality

- Take p = 7, q = 11, so n = 77 and (n) = 60
- Say Bob chooses (KB) e = 17, making (kB) d = 53
- 17 x 53 mod 60 = ?
- Alice wants to send Bob secret message HELLO [07 04 11 11 14]
- 0717 mod 77 = 28
- 0417 mod 77 = 16
- 1117 mod 77 = 44
- 1117 mod 77 = 44
- 1417 mod 77 = 42
- Alice sends ciphertext [28 16 44 44 42]

INFSCI 2935: Introduction to Computer Security

Example

- Bob receives [28 16 44 44 42]
- Bob uses private key (kB), d = 53, to decrypt the message:
- 2853 mod 77 = 07 H
- 1653 mod 77 = 04 E
- 4453 mod 77 = 11 L
- 4453 mod 77 = 11 L
- 4253 mod 77 = 14 O
- No one else could read it, as only Bob knows his private key and that is needed for decryption

INFSCI 2935: Introduction to Computer Security

Authentication using RSA

Y

X

Encryption

Message

Source

X

Decryption

Message

Source

Bob

Alice

KA

kA

Key Source

INFSCI 2935: Introduction to Computer Security

Example: Origin Integrity/Authentication

- Take p = 7, q = 11, so n = 77 and (n) = 60
- Alice chooses (KA) e = 17, making (kA) d = 53
- Alice wants to send Bob message HELLO [07 04 11 11 14] so Bob knows it is what Alice sent and there was no changes in transit
- 0753 mod 77 = 35
- 0453 mod 77 = 09
- 1153 mod 77 = 44
- 1153 mod 77 = 44
- 1453 mod 77 = 49
- Alice sends [35 09 44 44 49]

INFSCI 2935: Introduction to Computer Security

Example

- Bob receives 35 09 44 44 49
- Bob uses Alice’s public key (KA), e = 17, n = 77, to decrypt message:
- 3517 mod 77 = 07 H
- 0917 mod 77 = 04 E
- 4417 mod 77 = 11 L
- 4417 mod 77 = 11 L
- 4917 mod 77 = 14 O
- Alice sent it as only she knows her private key, so no one else could have enciphered it
- If (enciphered) message’s blocks (letters) altered in transit, would not decrypt properly

INFSCI 2935: Introduction to Computer Security

X

Confidentiality + AuthenticationEncryption

Message

Source

X

Z

Decryption

Y

Decryption

X

Message

Source

Bob

Alice

kB

kA

KA

KB

Key Source

Key Source

INFSCI 2935: Introduction to Computer Security

Example: Confidentiality + Authentication

- Alice wants to send Bob message HELLO both enciphered and authenticated (integrity-checked)
- Alice’s keys: public (17, 77); private: 53
- Bob’s keys: public: (37, 77); private: 13
- Alice enciphers HELLO [07 04 11 11 14]:
- (0753 mod 77)37 mod 77 = 07
- (0453 mod 77)37 mod 77 = 37
- (1153 mod 77)37 mod 77 = 44
- (1153 mod 77)37 mod 77 = 44
- (1453 mod 77)37 mod 77 = 14
- Alice sends [07 37 44 44 14]

INFSCI 2935: Introduction to Computer Security

Example: Confidentiality + Authentication

- Alice’s keys: public (17, 77); private: 53
- Bob’s keys: public: (37,77); private: 13
- Bob deciphers (07 37 44 44 14):
- (0713 mod 77)17 mod 77 = 07 H
- (3713 mod 77)17 mod 77 = 04 E
- (4413 mod 77)17 mod 77 = 11 L
- (4413 mod 77)17 mod 77 = 11 L
- (1413 mod 77)17 mod 77 = 14 O

INFSCI 2935: Introduction to Computer Security

Security Services

- Confidentiality
- Only the owner of the private key knows it, so text enciphered with public key cannot be read by anyone except the owner of the private key
- Authentication
- Only the owner of the private key knows it, so text enciphered with private key must have been generated by the owner

INFSCI 2935: Introduction to Computer Security

More Security Services

- Integrity
- Enciphered letters cannot be changed undetectably without knowing private key
- Non-Repudiation
- Message enciphered with private key came from someone who knew it

INFSCI 2935: Introduction to Computer Security

Warnings

- Encipher message in blocks considerably larger than the examples here
- If 1 character per block, RSA can be broken using statistical attacks (just like classical cryptosystems)
- Attacker cannot alter letters, but can rearrange them and alter message meaning
- Example: reverse enciphered message of text ON to get NO

INFSCI 2935: Introduction to Computer Security

Cryptographic Checksums

- Mathematical function to generate a set of k bits from a set of n bits (where k ≤ n).
- k is smaller then n except in unusual circumstances
- Keyed CC: requires a cryptographic key

h = CK(M)

- Keyless CC: requires no cryptographic key
- Message Digest or One-way Hash Functions

h = H(M)

- Can be used for message authentication
- Hence, also called Message Authentication Code (MAC)

INFSCI 2935: Introduction to Computer Security

Mathematical characteristics

- Every bit of the message digest function potentially influenced by every bit of the function’s input
- If any given bit of the function’s input is changed, every output bit has a 50 percent chance of changing
- Given an input file and its corresponding message digest, it should be computationally infeasible to find another file with the same message digest value

INFSCI 2935: Introduction to Computer Security

Definition

- Cryptographic checksum function h: AB:
- For any xA, h(x) is easy to compute
- Makes hardware/software implementation easy
- For any yB, it is computationally infeasible to find xA such that h(x) = y
- One-way proerpty
- It is computationally infeasible to find x, x´ A such that x ≠ x´ and h(x) = h(x´)

3’. Alternate form (Stronger): Given any xA, it is computationally infeasible to find a different x´ A such that h(x) = h(x´).

INFSCI 2935: Introduction to Computer Security

Collisions

- If x ≠ x´ and h(x) = h(x´), x and x´ are a collision
- Pigeonhole principle: if there are n containers for n+1 objects, then at least one container will have 2 objects in it.
- Application: suppose n = 5 and k = 3. Then there are 32 elements of A and 8 elements of B, so at least one element of B has at least 4 corresponding elements of A

INFSCI 2935: Introduction to Computer Security

Keys

- Keyed cryptographic checksum: requires cryptographic key
- DES in chaining mode: encipher message, use last n bits. Requires a key to encipher, so it is a keyed cryptographic checksum.
- Keyless cryptographic checksum: requires no cryptographic key
- MD5 and SHA-1 are best known; others include MD4, HAVAL, and Snefru

INFSCI 2935: Introduction to Computer Security

Message Digest

- MD2, MD4, MD5 (Ronald Rivest)
- Produces 128-bit digest;
- MD2 is probably the most secure, longest to compute (hence rarely used)
- MD4 is a fast alternative; MD5 is modification of MD4
- SHA, SHA-1 (Secure Hash Algorithm)
- Related to MD4; used by NIST’s Digital Signature
- Produces 160-bit digest
- SHA-1 may be better
- SHA-256, SHA-384, SHA-512
- 256-, 384-, 512 hash functions designed to be use with the Advanced Encryption Standards (AES)
- Example:
- MD5(There is $1500 in the blue bo) = f80b3fde8ecbac1b515960b9058de7a1
- MD5(There is $1500 in the blue box) = a4a5471a0e019a4a502134d38fb64729

INFSCI 2935: Introduction to Computer Security

Hash Message Authentication Code (HMAC)

- Make keyed cryptographic checksums from keyless cryptographic checksums
- h keyless cryptographic checksum function that takes data in blocks of b bytes and outputs blocks of l bytes. k´ is cryptographic key of length b bytes
- If short, pad with 0 bytes; if long, hash to length b
- ipad is 00110110 repeated b times
- opad is 01011100 repeated b times
- HMAC-h(k, m) = h(k´ opad || h(k´ ipad || m))
- exclusive or, || concatenation

INFSCI 2935: Introduction to Computer Security

Security Levels

- Unconditionally Secure
- Unlimited resources + unlimited time
- Still the plaintext CANNOT be recovered from the ciphertext
- Computationally Secure
- Cost of breaking a ciphertext exceeds the value of the hidden information
- The time taken to break the ciphertext exceeds the useful lifetime of the information

INFSCI 2935: Introduction to Computer Security

Key Points

- Two main types of cryptosystems: classical and public key
- Classical cryptosystems encipher and decipher using the same key
- Or one key is easily derived from the other
- Public key cryptosystems encipher and decipher using different keys
- Computationally infeasible to derive one from the other

INFSCI 2935: Introduction to Computer Security

Key Management

INFSCI 2935: Introduction of Computer Security

Issues

- Authentication and distribution of keys
- Session key
- Key exchange protocols
- Kerberos
- Mechanisms to bind an identity to a key
- Generation, maintenance and revoking of keys

INFSCI 2935: Introduction to Computer Security

Notation

- XY : { Z || W } kX,Y
- X sends Y the message produced by concatenating Z and W enciphered by key kX,Y, which is shared by users X and Y
- AT : { Z } kA || { W } kA,T
- A sends T a message consisting of the concatenation of Z enciphered using kA, A’s key, and W enciphered using kA,T, the key shared by A and T
- r1, r2 nonces (nonrepeating random numbers)

INFSCI 2935: Introduction to Computer Security

Session, Interchange Keys

- Alice wants to send a message m to Bob
- Assume public key encryption
- Alice generates a random cryptographic key ks and uses it to encipher m
- To be used for this message only
- Called a session key
- She enciphers ks with Bob’s public key kB
- kB enciphers all session keys Alice uses to communicate with Bob
- Called an interchange key
- Alice sends { m } ks{ ks} kB

INFSCI 2935: Introduction to Computer Security

Benefits

- Limits amount of traffic enciphered with single key
- Standard practice, to decrease the amount of traffic an attacker can obtain
- Makes replay attack less effective
- Prevents some attacks
- Example: Alice will send Bob message that is either “BUY” or “SELL”.
- Eve computes possible ciphertexts {“BUY”} kB and {“SELL”} kB.
- Eve intercepts enciphered message, compares, and gets plaintext at once

INFSCI 2935: Introduction to Computer Security

Key Exchange Algorithms

- Goal: Alice, Bob use a shared key to communicate secretely
- Criteria
- Key cannot be sent in clear
- Attacker can listen in
- Key can be sent enciphered, or derived from exchanged data plus data not known to an eavesdropper
- Alice, Bob may trust third party
- All cryptosystems, protocols publicly known
- Only secret data is the keys, ancillary information known only to Alice and Bob needed to derive keys
- Anything transmitted is assumed known to attacker

INFSCI 2935: Introduction to Computer Security

Classical Key Exchange

- How do Alice, Bob begin?
- Alice can’t send it to Bob in the clear!
- Assume trusted third party, Cathy
- Alice and Cathy share secret key kA
- Bob and Cathy share secret key kB
- Use this to exchange shared key ks

INFSCI 2935: Introduction to Computer Security

Simple Key Exchange Protocol

{ request for session key to Bob } kA

Alice

Cathy

{ ks }kA , { ks }kB

Alice

Cathy

{ ks } kB

Alice

Bob

{m}ks

Alice

Bob

Eve

INFSCI 2935: Introduction to Computer Security

Problems

- How does Bob know he is talking to Alice?
- Replay attack: Eve records message from Alice to Bob, later replays it; Bob may think he’s talking to Alice, but he isn’t
- Session key reuse: Eve replays message from Alice to Bob, so Bob re-uses session key
- Protocols must provide authentication and defense against replay

INFSCI 2935: Introduction to Computer Security

Needham-Schroeder

Alice || Bob || r1

Alice

Cathy

{ Alice || Bob || r1 || ks , { Alice || ks } kB } kA

Alice

Cathy

{ Alice || ks } kB

Alice

Bob

{ r2 } ks

Alice

Bob

{ r2 – 1 } ks

Alice

Bob

INFSCI 2935: Introduction to Computer Security

Argument: Alice talking to Bob

- Second message
- Enciphered using key only she, Cathy know
- So Cathy enciphered it
- Response to first message
- As r1 in it matches r1 in first message
- Third message
- Alice knows only Bob can read it
- As only Bob can derive session key from message
- Any messages enciphered with that key are from Bob

INFSCI 2935: Introduction to Computer Security

Argument: Bob talking to Alice

- Third message
- Enciphered using key only he, Cathy know
- So Cathy enciphered it
- Names Alice, session key
- Cathy provided session key, says Alice is other party
- Fourth message
- Uses session key to determine if it is replay from Eve
- If not, Alice will respond correctly in fifth message
- If so, Eve can’t decipher r2 and so can’t respond, or responds incorrectly

INFSCI 2935: Introduction to Computer Security

Problem withNeedham-Schroeder

- Assumption: all keys are secret
- Question: suppose Eve can obtain session key. How does that affect protocol?
- In what follows, Eve knows ks

{ Alice || ks } kB [Replay]

Eve

Bob

{ r3 } ks [Eve intercepts]

Eve

Bob

{ r3 – 1 } ks

Eve

Bob

INFSCI 2935: Introduction to Computer Security

Solution: Denning-Sacco Modification

- In protocol above, Eve impersonates Alice
- Problem: replay in third step
- First in previous slide
- Solution: use time stamp T to detect replay
- Needs synchronized clocks
- Weakness: if clocks not synchronized, may either reject valid messages or accept replays
- Parties with either slow or fast clocks vulnerable to replay
- Resetting clock does not eliminate vulnerability

INFSCI 2935: Introduction to Computer Security

Needham-Schroeder with Denning-Sacco Modification

Alice || Bob || r1

Alice

Cathy

{ Alice || Bob || r1 || ks || { Alice || T || ks } kB } kA

Alice

Cathy

{ Alice || T || ks } kB

Alice

Bob

{ r2 } ks

Alice

Bob

{ r2 – 1 } ks

Alice

Bob

INFSCI 2935: Introduction to Computer Security

Otway-Rees Protocol

- Corrects problem
- That is, Eve replaying the third message in the protocol
- Does not use timestamps
- Not vulnerable to the problems that Denning-Sacco modification has
- Uses integer n to associate all messages with a particular exchange

INFSCI 2935: Introduction to Computer Security

The Protocol

n || Alice || Bob || { r1 || n || Alice || Bob } kA

Alice

Bob

n || Alice || Bob || { r1 || n || Alice || Bob } kA ||

{ r2 || n || Alice || Bob } kB

Cathy

Bob

n || { r1 || ks } kA || { r2 || ks } kB

Cathy

Bob

n || { r1 || ks } kA

Alice

Bob

INFSCI 2935: Introduction to Computer Security

Argument: Alice talking to Bob

- Fourth message
- If n matches first message, Alice knows it is part of this protocol exchange
- Cathy generated ks because only she, Alice know kA
- Enciphered part belongs to exchange as r1 matches r1 in encrypted part of first message

INFSCI 2935: Introduction to Computer Security

Argument: Bob talking to Alice

- Third message
- If n matches second message, Bob knows it is part of this protocol exchange
- Cathy generated ks because only she, Bob know kB
- Enciphered part belongs to exchange as r2 matches r2 in encrypted part of second message

INFSCI 2935: Introduction to Computer Security

Replay Attack

- Eve acquires old ks, message in third step
- n || { r1 || ks } kA || { r2 || ks } kB
- Eve forwards appropriate part to Alice
- Alice has no ongoing key exchange with Bob: n matches nothing, so is rejected
- Alice has ongoing key exchange with Bob: n does not match, so is again rejected
- If replay is for the current key exchange, and Eve sent the relevant part before Bob did, Eve could simply listen to traffic; no replay involved

INFSCI 2935: Introduction to Computer Security

Kerberos

- Authentication system
- Based on Needham-Schroeder with Denning-Sacco modification
- Central server plays role of trusted third party (“Cathy”)
- Ticket (credential)
- Issuer vouches for identity of requester of service
- Authenticator
- Identifies sender
- Alice must
- Authenticate herself to the system
- Obtain ticket to use server S

INFSCI 2935: Introduction to Computer Security

Overview

- User u authenticates to Kerberos server
- Obtains ticket Tu,TGS for ticket granting service (TGS)
- User u wants to use service s:
- User sends authenticator Au, ticket Tu,TGS to TGS asking for ticket for service
- TGS sends ticket Tu,s to user
- User sends Au, Tu,s to server as request to use s
- Details follow

INFSCI 2935: Introduction to Computer Security

Ticket

- Credential saying issuer has identified ticket requester
- Example ticket issued to user u for service s

Tu,s = s || { u || u’s address || valid time || ku,s } ks

where:

- ku,s is session key for user and service
- Valid time is interval for which the ticket is valid
- u’s address may be IP address or something else
- Note: more fields, but not relevant here

INFSCI 2935: Introduction to Computer Security

Authenticator

- Credential containing identity of sender of ticket
- Used to confirm sender is entity to which ticket was issued
- Example: authenticator user u generates for service s

Au,s = { u || generation time || kt } ku,s

where:

- kt is alternate session key
- Generation time is when authenticator generated
- Note: more fields, not relevant here

INFSCI 2935: Introduction to Computer Security

Protocol

user || TGS

user

Cathy

{ ku,TGS } ku || Tu,TGS

Cathy

user

service || Au,TGS || Tu,TGS

user

TGS

user || { ku,s } ku,TGS || Tu,s

user

TGS

Au,s || Tu,s

user

service

{ t + 1 } ku,s

user

service

INFSCI 2935: Introduction to Computer Security

Analysis

- First two steps get user ticket to use TGS
- User u can obtain session key only if u knows key shared with Cathy
- Next four steps show how u gets and uses ticket for service s
- Service s validates request by checking sender (using Au,s) is same as entity ticket issued to
- Step 6 optional; used when u requests confirmation

INFSCI 2935: Introduction to Computer Security

Problems

- Relies on synchronized clocks
- If not synchronized and old tickets, authenticators not cached, replay is possible
- Tickets have some fixed fields
- Dictionary attacks possible
- Kerberos 4 session keys weak (had much less than 56 bits of randomness); researchers at Purdue found them from tickets in minutes

INFSCI 2935: Introduction to Computer Security

Public Key Key Exchange

- Here interchange keys known
- eA, eB Alice and Bob’s public keys known to all
- dA, dB Alice and Bob’s private keys known only to owner
- Simple protocol
- ks is desired session key

{ ks } eB

Alice

Bob

INFSCI 2935: Introduction to Computer Security

Problem and Solution

- Vulnerable to forgery or replay
- Because eB known to anyone, Bob has no assurance that Alice sent message
- Simple fix uses Alice’s private key
- ks is desired session key

{ { ks } dA } eB

Alice

Bob

INFSCI 2935: Introduction to Computer Security

Notes

- Can include message enciphered with ks
- Assumes Bob has Alice’s public key, and vice versa
- If not, each must get it from public server
- If keys not bound to identity of owner, attacker Eve can launch a man-in-the-middle attack (next slide; Cathy is public server providing public keys)

INFSCI 2935: Introduction to Computer Security

Man-in-the-Middle Attack

send me Bob’s public key

Eve intercepts request

Alice

Cathy

send me Bob’s public key

Cathy

Eve

eB

Cathy

Eve

eE

Eve

Alice

{ ks } eE

Eve intercepts message

Bob

Alice

{ ks } eB

Bob

Eve

INFSCI 2935: Introduction to Computer Security

Key Generation

- Goal: generate difficult to guess keys
- Problem statement: given a set of K potential keys, choose one randomly
- Equivalent to selecting a random number between 0 and K–1 inclusive
- Why is this hard: generating random numbers
- Actually, numbers are usually pseudo-random, that is, generated by an algorithm

INFSCI 2935: Introduction to Computer Security

What is “Random”?

- Sequence of cryptographically ransom numbers: a sequence of numbers n1, n2, … such that for any integer k > 0, an observer cannot predict nk even if all of n1, …, nk–1 are known
- Best: physical source of randomness
- Electromagnetic phenomena
- Characteristics of computing environment such as disk latency
- Ambient background noise

INFSCI 2935: Introduction to Computer Security

What is “Pseudorandom”?

- Sequence of cryptographically pseudorandom numbers: sequence of numbers intended to simulate a sequence of cryptographically random numbers but generated by an algorithm
- Very difficult to do this well
- Linear congruential generators [nk = (ank–1 + b) mod n] broken (a, b and n are relatively prime)
- Polynomial congruential generators [nk = (ajnk–1j + … + a1nk–1 a0) mod n] broken too
- Here, “broken” means next number in sequence can be determined

INFSCI 2935: Introduction to Computer Security

Best Pseudorandom Numbers

- Strong mixing function: function of 2 or more inputs with each bit of output depending on some nonlinear function of all input bits
- Examples: DES, MD5, SHA-1
- Use on UNIX-based systems:

(date; ps gaux) | md5

where “ps gaux” lists all information about all processes on system

INFSCI 2935: Introduction to Computer Security

Digital Signature

- Construct that authenticates origin, contents of message in a manner provable to a disinterested third party (“judge”)
- Sender cannot deny having sent message (service is “nonrepudiation”)
- Limited to technical proofs
- Inability to deny one’s cryptographic key was used to sign
- One could claim the cryptographic key was stolen or compromised
- Legal proofs, etc., probably required;

INFSCI 2935: Introduction to Computer Security

Common Error

- Classical: Alice, Bob share key k
- Alice sends m || { m }k to Bob

This is a digital signature

WRONG

- This is not adigital signature
- Why? Third party cannot determine whether Alice or Bob generated message

INFSCI 2935: Introduction to Computer Security

Classical Digital Signatures

- Require trusted third party
- Alice, Bob each share keys with trusted party Cathy
- To resolve dispute, judge gets { m }kAlice, { m }kBob, and has Cathy decipher them; if messages matched, contract was signed

{ m }kAlice

Alice

Bob

{ m }kAlice

Bob

Cathy

{ m }kBob

Cathy

Bob

INFSCI 2935: Introduction to Computer Security

Public Key Digital Signatures

- Alice’s keys are dAlice, eAlice
- Alice sends Bob

m || { m }dAlice

- In case of dispute, judge computes

{ { m }dAlice }eAlice

- and if it is m, Alice signed message
- She’s the only one who knows dAlice!

INFSCI 2935: Introduction to Computer Security

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