Authentication

Authentication

Goal: Bob wants Alice to “prove” her identity to him

Protocol ap1.0: Alice says “I am Alice”

Failure scenario??“I am Alice”

Authentication

Goal: Bob wants Alice to “prove” her identity to him

Protocol ap1.0: Alice says “I am Alice”

in a network,Bob can not “see”

Alice, so Trudy simply declares

herself to be Alice“I am Alice”

Authentication: another tryProtocol ap2.0: Alice says “I am Alice” in an IP packet

containing her source IP address

Failure scenario??

“I am Alice”Alice’s

IP address

Authentication: another tryProtocol ap2.0: Alice says “I am Alice” in an IP packet

containing her source IP address

Trudy can createa packet

“spoofing”Alice’s address“I am Alice”

Alice’s IP address

Authentication: another tryProtocol ap3.0: Alice says “I am Alice” and sends her

secret password to “prove” it.

Failure scenario??

“I’m Alice”Alice’s IP addr

Alice’s password

OKAlice’s IP addr

Authentication: another tryProtocol ap3.0: Alice says “I am Alice” and sends her

secret password to “prove” it.

playback attack: Trudy records Alice’s

packetand later

plays it back to Bob

“I’m Alice”Alice’s IP addr

Alice’s password

OKAlice’s IP addr

“I’m Alice”Alice’s IP addr

Alice’s password

Authentication: yet another tryProtocol ap3.1: Alice says “I am Alice” and sends her

encrypted secret password to “prove” it.

Failure scenario??

“I’m Alice”Alice’s IP addr

encrypted password

OKAlice’s IP addr

Authentication: another tryProtocol ap3.1: Alice says “I am Alice” and sends her

encrypted secret password to “prove” it.

recordand

playbackstill works!

“I’m Alice”Alice’s IP addr

encryptedpassword

OKAlice’s IP addr

“I’m Alice”Alice’s IP addr

encryptedpassword

Authentication: yet another tryGoal: avoid playback attack

Failures, drawbacks?

Nonce: number (R) used only once –in-a-lifetime

ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice

must return R, encrypted with shared secret key“I am Alice”

R

K (R)A-B

Alice is live, and only Alice knows key to encrypt

nonce, so it must be Alice!

Authentication: ap5.0ap4.0 requires shared symmetric key • can we authenticate using public key techniques?

ap5.0: use nonce, public key cryptography

“I am Alice”

RBob computes

K (R)A-

“send me your public key”

K A+

(K (R)) = RA

-K A

+

and knows only Alice could have the

private key, that encrypted R such that

(K (R)) = RA-

K A+

ap5.0: security holeMan (woman) in the middle attack: Trudy poses as

Alice (to Bob) and as Bob (to Alice)

I am Alice I am Alice

R

TK (R)

-

Send me your public key

TK

+A

K (R)-

Send me your public key

AK

+

TK (m)+

Tm = K (K (m))+

T-

Trudy gets

sends m to Alice encrypted

with Alice’s public key

AK (m)+

Am = K (K (m))+

A-

R

ap5.0: security holeMan (woman) in the middle attack: Trudy poses as

Alice (to Bob) and as Bob (to Alice)

Difficult to detect: Bob receives everything that Alice sends, and vice versa. (so Bob and Alice can meet one week later and recall conversation) problem is that Trudy receives all messages as well!

Chapter3Message Authentication Message Authentication

seriouslyseriously

OUTLINE

• Approaches to Message Authentication

• Secure Hash Functions and HMAC

Authentication

• Requirements - must be able to verify that:1. Message came from apparent

source or author,2. Contents have not been altered,3. Sometimes, it was sent at a certain time or sequence.

• Protection against active attack (falsification of data and transactions)

Approaches to Message Authentication

• Authentication Using Conventional Encryption– Only the sender and receiver should share a key

• Message Authentication without Message Encryption– An authentication tag is generated and appended to

each message

• Message Authentication Code– Calculate the MAC as a function of the message and

the key. MAC = F(K, M)

One-way HASH One-way HASH functionfunction

One-way HASH function

• Secret value is added before the hash and removed before transmission.

Secure HASH Functions• Purpose of the HASH function is to produce a

”fingerprint.• Properties of a HASH function H :

1. H can be applied to a block of data of any size

2. H produces a fixed length output

3. H(x) is easy to compute for any given x.

4. For any given block x, it is computationally infeasible to find x such that H(x) = h

5. For any given block x, it is computationally infeasible to find with H(y) = H(x).

6. It is computationally infeasible to find any pair (x, y) such that H(x) = H(y)

xy

Simple Hash Function

• One-bit circular shift on the hash value after each block is processed would improve

Internet checksum: poor crypto hash function

Internet checksum has some properties of hash function:produces fixed length digest (16-bit sum) of message is many-to-oneBut given message with given hash value, it is easy to

find another message with same hash value:

I O U 10 0 . 99 B O B

49 4F 55 3130 30 2E 3939 42 D2 42

message ASCII format

B2 C1 D2 AC

I O U 90 0 . 19 B O B

49 4F 55 3930 30 2E 3139 42 D2 42

message ASCII format

B2 C1 D2 ACdifferent messagesbut identical checksums!

Message Digest Generation Using SHA-1

SHA-1 Processing of single 512-Bit Block

Other Secure HASH functionsSHA-1 MD5 RIPEMD-

160

Digest length 160 bits 128 bits 160 bits

Basic unit of processing

512 bits 512 bits 512 bits

Number of steps 80 (4 rounds of 20)

64 (4 rounds of 16)

160 (5 paired rounds of 16)

Maximum message size

264-1 bits

HMAC

• Use a MAC derived from a cryptographic hash code, such as SHA-1.

• Motivations:– Cryptographic hash functions executes faster in

software than encryptoin algorithms such as DES

– Library code for cryptographic hash functions is widely available

– No export restrictions from the US

HMAC Design Objectives

• To use available hash functions• To allow for easy replaceability of the embedded

hash function in case faster or more secure are found or required

• To preserve the original performance of the hash function

• To use and handle keys in a simple way• To have a well-understood cryptographic analysis

of the strength of the authentication mechanism

HMAC Structure