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