Privacy versus Authentication

• Confidentiality (Privacy)– Interceptors cannot read messages

• Authentication: proving the sender’s identity– The Problem of Impostors– Uses encryption– So encryption is not only for privacy and

confidentiality!

Authentication• Authentication methods: Passwords

– Most users pick short passwords that are easy to guess with exhaustive search

– Users often pick passwords that are common words or repetitive letter combinations; Even easier to guess

– Automated password cracking is very effective

Authentication• Authentication methods: Passwords

– Often, weak passwords protect more important systems

– Users must be forced to pick long passwords containing case changes and numerals, such as Tri6Vial

Authentication• Authentication methods

– Biometrics• Fingerprint analysis, iris analysis, etc.• New and not standardized

– Authentication Card• Push into slot of a machine• Also must give password usually

– Public Key Authentication• Prove that sender holds their private key, which only

they should know

Authentication

• Verifier is the party who wishes the other party to authenticate themselves

• Applicant is the other party, which wishes to prove its identity

ApplicantApplicant VerifierVerifier

Prove Your

Identity

Challenge-Response Authentication

• Verifier sends the applicant a challenge message

– This challenge message is a string of bits

ApplicantApplicant VerifierVerifier

ChallengeMessage

Challenge-Response Authentication

• Applicant sends back a response message– This is the challenge message encrypted with

the applicant’s private key

ApplicantApplicant VerifierVerifier

ResponseMessage

Challenge-Response Authentication• Verifier decrypts the response message with

the true party’s public key– If matches the challenge message, was

encrypted with the true party’s private key, which only the true party should know

– Applicant is authenticated

ApplicantApplicant VerifierVerifier

ResponseMessage

ChallengeMessage

Frequency of Authentication• Challenge-Response Authentication

– Only done initially– Or done at most a few times during a session

• Digital Signature Authentication (next)– Provides authentication for every message– Called message-by-message authentication– Also provides message integrity—proof that the

message has not been changed en route

Public Key Authentication

• Ultimate goal is to send an original plaintext message from the applicant to the verifier

– If security was not an issue, the applicant simply would send it

ApplicantApplicant VerifierVerifier

OriginalPlaintext

Public Key Authentication• Ultimate goal is to send an original

plaintext message from the applicant to the verifier

– If only confidentiality was an issue, would merely encrypt the original plaintext with a symmetric session key

ApplicantApplicant VerifierVerifier

CiphertextUsing

Symmetric Key

Public Key Authentication

• For authentication, also send a digital signature with each packet

• First create a message digest (MD)– A small binary string calculated on the basis of

all of the bits in the message

MessageMessageDigest

Calculation

Public Key Authentication• First create a message digest (MD)

– Normally, use a process called hashing– For a message of arbitrary size, hashing produces

a small number of predictable size– MD5: 128 bits– SHA-1: 160 bits

MessageMessageDigest

Hash

Public Key Authentication• First create a message digest (MD)

– Hashing is not reversible– Cannot get back original message if you know its

hash– Just done to produce something small enough

(message digest) to encrypt with public key encryption

MessageMessageDigest

Hash

Public Key Authentication

• Next create a digital signature– Encrypt the message digest with sender’s

private key, which only the sender should be able to do

– Also called signing the message digest with the sender’s private key

DigitalSignature

Encrypt withSender’s

Private Key

MessageDigest

Public Key Authentication• Next create a digital signature

– Encrypt message digest with sender’s private key, which only the sender should be able to do; creates the digital signature

– Message digest is short, so public key encryption is not too burdensome

DigitalSignature

Encrypt withSender’s

Private Key

MessageDigest

Public Key Authentication• Note

– Message digest is a hash of the original message

– MD is not encrypted– Digital signature is what you get when you

encrypt the MD with public key encryption– Do not confuse the two

DigitalSignature

Encrypt withSender’s

Private Key

MessageDigest

Public Key Authentication• Encrypt combined message and digital

signature with the symmetric session key and send to the receiver– This gives confidentiality (privacy) during

transmission– Easy to forget the encryption with the

symmetric session key

DigitalSignature

Message

Encrypt with symmetric session key

Public Key Authentication

– Receiver decrypts ciphertext with symmetric session key

– Then decrypts digital signature with sender’s public key to get the original message digest

– This is the transmitted message digest

DigitalSignature

Decrypt withSender’s

Public Key

TransmittedMessageDigest

Public Key Authentication

– Receiver then hashes the original plaintext, just as the sender did

– This is the computed message digest

OriginalPlaintext

ComputedMessageDigest

Hashed

Public Key Authentication

– If the transmitted and computed message digests match, the sender is authenticated as being the true party

• Because the digital signature was signed with the true party’ private key, as shown by decryption with the true party’s public key

Message Digestfrom Digital Signature

Message DigestComputed from

Original Plaintext

Public Key Authentication• Digital Signature also Provides Message

Integrity

– Proof that the message has not been altered en route

– If message has been changed by error or by an attacker, message digests will not match

Message Digestfrom Digital Signature

Message DigestComputed from

Original Plaintext