CS 5950/6030 Network Security Class 10 (F, 9/23/05) Leszek Lilien Department of Computer Science Western Michigan University [Using some slides courtesy.

CS 5950/6030 Network SecurityClass 10 (F, 9/23/05)

Leszek LilienDepartment of Computer Science

Western Michigan University

[Using some slides courtesy of:Prof. Aaron Striegel — at U. of Notre Dame

Prof. Barbara Endicott-Popovsky and Prof. Deborah Frincke — at U. WashingtonProf. Jussipekka Leiwo — at Vrije Universiteit (Free U.), Amsterdam, The

Netherlands]

2

2. Cryptology ...

2F. The AES (Advanced Encryption Standard) Algorithm2F.1. The AES Contest

2F.2. Overview of Rijndael2F.3. Strength of AES2F.4. Comparison of DES and AES

2G. Public Key Encryption2G.1. Motivation for PKE2G.2. Characteristics of PKE2G.3. RSA Encryption

2H. The Uses of Encryption 2H.1. Cryptographic Hash Functions – PART 1

Class 9

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2H.1. Cryptographic Hash Functions (1)

Integrity: How can you be sure that a recived msg/doc was

not modified by an attacker or malfunction? Answer: use cryptography to ensure integrity

Idea: Wax seals on letters in Middle Ages

— easy to see if broken Cryptographic „seal” on doc/msg

— so that any change to it will be readily detected

4

Cryptographic Hash Functions (2)

A technique:compute a hash fcn / checksum / msg digest

More formally: Problem: How to send n-bit msg so that R can

easily verify that it is intact

Solution: Send a msg of n+k bits n bits — original msg k bits — checksum / msg digest

Generated based on the n bits

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Cryptographic Hash Fcns (3)

Simple Parity for Error Detection (1) Simple (non-cryptographic) technique: parity

Add a single parity bit to detect if a message is correct Example 1: odd parity

Force the block of data to have an odd # of 1’s Data = 1011 — n = 4 Sent block = 10110 — n+k = 4+1

— looked at ‘1011’, added 0 to have odd # of 1’s Data = 0110 Sent block = 01101

— looked at ‘0110’, added 1 to have odd # of 1’s

Example 2: ASCII parity bit ASCII has 7 bits for data, 8th bit is single parity bit

Either odd or even parity used

[cf. A. Striegel, U. Notre Dame]

6


Simple Parity for Error Detection (2)

How parity enhances msg integrity? Can detect error in 1 bit (or in odd # of bits)

e,.g, if R gets 01001, R knows it’s wrong(S sent 01101)

Cannot detect error in 2 bits (or in even # of bits) Bec. parity stays OK -> undetectable integrity

violation e.g, if R gets 01011, R knows it’s wrong

(S sent 01101)

Cannot repair errors either E.g., R doesn’t know which bit in 01001 is wrong[cf. A. Striegel, U. Notre

Dame]

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Better Checksums against Errors & Attacks

There are better checksums than simple odd/even parity Can detect multiple errors Can even repair multiple errors

These checksums are to fix errors, not deal with attacks

For attacks needcryptographic checksums / strong hash functions

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Strong Hash Function Formal definition:

strong hash function (cryptographic checksum) is h: A -> B such that:

1) For any x A, h(x) is easy to compute2) For any y B, it is computationally infeasible to

findinverse of y, i.e., x A such that h(x) = y

3) It is computationally infeasible to find a pair of colliding input values, i.e. x, x’ A such that x ≠ x’ and h(x) = h(x’)

Alternate (stronger) form for (3):Given any x A, it is computationally infeasibleto find x’ A such that x ≠ x’ and h(x) = h(x’)

Due to (1) and (2), hash fcn is a one-way function[cf. A. Striegel, U. Notre Dame, Barbara Endicott-Popovsky, U. Washington]

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Collisions & Attacks on Msg Integrity (1) Note:

n bits of msg (x) mapped into k bits of its checksum (y)

k < n => collisions must exist But it is computationally infeasible to find collisions for

good hash fcns

Goal of a successful attack on msg integrity: Change msg1 in such a way that checksum remains

unchanged (so R doesn’t detect the forgery) I.e., find msg2 that collides with the original msg1

w.r.t. checksum value Finding msg2 is computationally infeasible (for good

hash)

=> forging msg1 undetectably is computationally infeasible

[cf. A. Striegel, U. Notre Dame]

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File Checksum File checksum

Calculated, a fcn defined on all bits of the file Result encrypted and stored with the file Each time file used by legitimate users,

checksum recalculated, encrypted, stored with the file

File sent to R When file received by R:

R decrypts checksum c1 received in the file R independently calculates file checksum c2 If c1 = c2 => file integrity is OK Otherwise – file integrity violated

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Class 9 ended here

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2. Cryptology ...

2F. The AES (Advanced Encryption Standard) Algorithm2F.1. The AES Contest

2F.2. Overview of Rijndael2F.3. Strength of AES2F.4. Comparison of DES and AES

2G. Public Key Encryption2G.1. Motivation for PKE2G.2. Characteristics of PKE2G.3. RSA Encryption

2H. The Uses of Encryption 2H.1. Cryptographic Hash Functions – PART 1 2H.1. Cryptographic Hash Functions – PART 2 2H.2. Key Exchange 2H.3. Digital Signatures 2H.4. Certificates – PART 1

Class 9

Class 10

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Keyed vs. Keyless Crypto Checksum (1)

Keyed crypto checksum Key needed to compute checksum

Keyed hash fcns DES, AES Use it in chaining mode:

link next msg block to value of the previous msg block

Example chaining: E(current block) XOR E(previous block)

=> connects block to all previous blocks

If file sent, file’s checksum could be the last block

If chaining used, file checksum (=last block) depends on all previous blocks => depends on all bits of the file

14


Keyed vs. Keyless Crypto Checksum (2)

Keyed crypto checksum – CONT. Used for integrity + authentication

Integrity: checksum makes msg modification difficult

Authentication: only S and R know symmetric keyR: if msg integrity is OK, it must have been sent by S

15


Keyed vs. Keyless Crypto Checksum (3) Keyless crypto checksum

No key required to compute checksum

Keyless hash functions MD5/MD4: any msg 128-bit digest (hash,

checksum) SHA/SHS: any msg 160-bit digest Other: MD2, HAVAL, Snefru, ...

Used for integrity (not authentication) Integrity: checksum makes msg modification difficult

(with truly public key anybody can send msg, but nobody but S can easily modify this msg)

No authentication: n (or all) people know public key –R can’t prove which one of them sent a given msg

[cf. A. Striegel, U. Notre Dame, Barbara Endicott-Popovsky, U. Washington]

16

2H.2. Key Exchange (1) Motivation:

X and Y don’t know each other X needs to send protected msg to Y

E.g., shopping on a web site

can do it if can securely exchange KE

This is the problem of key exchange Important Hard

Circular (chicken-’n-egg) problem?„To establish secure session need secure channel”

Circle can be broken – by public key cryptography

Can send public key even on insecure channel

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Key Exchange (2)

Deriving Symmetric Key via PKE (1)

Given S and R / kPRIV-S, kPUB-S -- kPRIV-R, kPUB-R

Solution 1: S determines secret key K S encrypts K with kPRIV-S : C = E(kPRIV-S, K) S sends C to R R decrypts C to get K: D(kPUB-S, C) = K S & R communicate using secret (symmetric) key

K

BUT: Solution 1 is not good!!! Question: Why?

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Key Exchange (3)


Given S and R / kPRIV-S, kPUB-S -- kPRIV-R, kPUB-R

Solution 1: S determines secret key K S encrypts K with kPRIV-S: C = E(kPRIV-S, K) S sends C to R R decrypts C to get K: D(kPUB-S, C) = K S & R communicate using secret (symmetric) key K

BUT: Solution 1 is not good !!! Answer:

Attacker who has kPUB-S can also perform decryption! The easier the more people know kPUB-S

Trivial if kPUB-S is truly public

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Key Exchange (4)


Solution 2: S determines secret key K S encrypts K with kPUB-R: C = E(kPUB-R, K) S sends C to R R decrypts C to get K: D(kPRIV-R, C) = K S & R communicate using secret (symmetric) key K

Solution 2 is better Only R can decode K (only R knows kPRIV-R)

...but Solution 2 still is not quite good Question: Why?

Hint: what about msg authentication?

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Key Exchange (5)

Deriving Symmetric Key via PKE (4) Solution 2:

S determines secret key K S encrypts K with kPUB-R: C = E(kPUB-R , K) S send C to R R decrypts C to get K: D(kPRIV-R , C) = K S & R communicate using secret (symmetric) key K

Solution 2 is better Only R can decode K (only R knows kPRIV-R)

...but Solution 2 still is not quite good Answer:

No msg authentication(R has no assurance that msg was sent by S

– anybody could have encoded with kPUB-R)

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Key Exchange (6)


Solution 3: S determines secret key K S encrypts K with both kPRIV-S & kPUB-R :

C = E(kPUB-R , E(kPRIV-S, K)) S sends C to R R decrypts C to get K:

D( kPUB-S , D(kPRIV-R, C) ) -- order important ! make sure you see this

(see Fig. 2-11 p.78)

Solution 3 is good! Only R can decode K (only R knows kPRIV-R)

Authentication: R is assured that S sent C Only S could have encoded K with kPRIV-S

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2H.3. Digital Signatures (1)

Outline: a. Problem Definition b. Properties of Electronic Signatures c. Using PKE for Digital Signatures d. Using Hash Fcns for Digital Signatures

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Digital Signatures (2)

a. Problem Definition (1) Motivation:

Need to sign and transmit electronic doc’s or msgs, incl. checks

Analogous to signing & transmitting „paper” letters, doc’s, etc., incl. checks

Roles of signatures (for both paper a& electronic) Proves unforgeability of doc/letter/check Authenticates person S who signed doc/letter/check Provides non-repudiation: S cannot say sb else

signed it Facilitates proving integrity (e.g., 2 signed legal copies for 2

parties)

Note: signature might not identify the signing person if not legible

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Problem Definition (2)

Security requirements for digital signatures: Signature will not reveal signer’s private key Only owner of private key can produce a valid

signature Verification of a correct signature succeeds Modification of a signed message can be

detected

[cf. J. Leiwo]

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b. Properties of Electronic Signatures (1)

M – msg / Sg(S, M) – signature of S on MNote: M = C or M = P M = P – if authentication but no secrecy needed

Required properties for electronic signatures: Unforgeable:

Only S can produce the pair [M, Sg(S, M)]

Authenticable (can verify authenticity)/ non-repudiable:

R can verify that Sg(S,M) in [M, Sg(S, M)] comes from S

Only S could have produced M”+”Sg(S,M) Sg(S, M) is firmly attached to M

M

Sg(S, M)

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Properties of Electronic Signatures (2)

Desirable properties for electr. signatures: Not alterable (assures „integrity”) :

Once sent, M”+”Sg(S,M) cannot beundetectably altered by S, R, or interceptor

[I’d rather consider this a part of „unforgeability” above]

Not reusable:If M is received again, S detects that M is „old”

E.g., can’t deposit a copy of a check to „double-deposit”

Digital signature is a protocol that mimics effect of signature on paper

M

Sg(S, M)

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c. Using PKE for Digital Signatures (1)

Transmitting signed msgs with PKE Original message:

Privacy transformation: C = E(P, KPUB-R) Only R can decrypt it (with KPRIV-R)

Authenticity transformation = signing:Sg = Sg(S, C) = D(C, KPRIV-S)

Only S can produce Sg(S, C) (with KPRIV-S)

Sent message:

Note: Remember that for some PKE algorithms (incl RSA):D( E(M, _), _ ) = E( D(M, _), _ ) = M (commutativity of E-D)

C

Sg

P

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Using PKE for Digital Signatures (2)

Transmitting signed msgs with PKE - cont. Received msg:

[ C = E(P, KPUB-R) ]

[Sg = Sg(S, C) = D(C, KPRIV-S)]

R verifies Sg with S’s public key KPUB-S:

If E( Sg, KPUB-S) = C, then signature is valid bec. E( Sg, KPUB-S) = E( D(C, KPRIV-S), KPUB-S) = C

R decodes C with R’s private key KPRIV-R:

P = D(C, KPRIV-R)

C

Sg

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Using PKE for Digital Signatures (3) Properties:

[ C = E(P, KPUB-R) ]

[Sg = Sg(S, C) = D(C, KPRIV-S)]

Unforgeability:If C is forged,it will not „correspond” to Sg ( i.e., E( Sg, KPUB-S) ≠ C )

Authenticity:If Sg is valid, S is authenticated (only S can produce valid S’s signature)

Non-repudiation (undeniability):If Sg is valid, only S could have produced it, and have sent C”+”Sg

C

Sg

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d. Using Hash Fcns for Digital Signatures

Using hash fcn H in digital signatures— signature over H(m), not over m length H(m) << length (m)

Before: Now:

[Fig — cf. J. Leiwo]

s = SgDA(x) = D(x, KPRIV-A)

EA(x) = E(x, KPUB-A)

Note:Any alteration of m is detected by B’s „Verify” step even if m is not encoded with KPUB-B —due to use of H(m)

m

Sg(S, H(m))

m

Sg(S, m)

m = P or m = C

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2H.4. Certificates (1)

Outline a. Introduction b. Trust Through a Common Respected Individual c. Certificates for Identity Authentication d. Trust Without a Single Hierarchy

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Certificates (2)

a. Introduction (1) Need for trust in human interactions

Trust w.r.t.: Individuals Institutions (e.g., bank, hospital, car dealer) Artifacts (e.g., car, Internet browser, software house)

Trust in small village vs. big city Small village: implicit trust

Everybody knows everybody Mr. X „feels” how much to trust Ms. Y

Big city: need to consider trust explicitly Ask around to find trusted entities

Inquire friends, office mates, etc. about good car dealer, dentist, etc.

Check „reputation databases”E.g., BBB=Better Business Bureau

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Certificates (3)

Introduction (2) Selected trust characteristics

Trust comes in degrees of trust Vs. binary trust (with a single trust threshold)

Ubiquity of trust in social and artificial systems Many users/computer systems err by trusting

blindly (trust without evidence or verification!) E.g., OS trusts all application pgms – any allowed to run E.g., sers trust unknown web sites with personal data

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End of Class 10

CS 5950/6030 Network Security Class 10 (F, 9/23/05) Leszek Lilien Department of Computer Science Western Michigan University [Using some slides courtesy.

Documents

cryptographic hash fcns

cryptographic hash functions

odd parity

simple parity

ascii parity

single parity

cryptographic seal

msg of n