E-CommerceArchitecturesand Technologies

Rob Oshana

Southern MethodistUniversity

Cryptography

Security Service Layers

Authentication

Access Control

Data Confidentiality

Data Integrity

Non-repudiation

Authentication• Typically the first step to gain access to a system

– user name and password

• Process of proving your identity

• Kerberos is an example

• Data Origin Authentication Service – provides confirmation that the source of data received

is as claimed.

• Peer-Entity Authentication Service – provides confirmation that a peer entity in an

association is the one claimed

Access Control

• Provides protection against the unauthorized use of accessible resources using network protocols– permissions for files, directories, and

processes

• Specifies what resources a user or service may access on the network

• A prerequisite for access control is proper authentication

Data Confidentiality

• Protection of data from unauthorized disclosure– connection confidentiality– connectionless confidentiality– traffic flow confidentiality

• Protection of data from passive threats

Data Integrity• Provides protection from active threats

– Connection Integrity with Recovery Service – Connection Integrity without Recovery Service – Selective Field Connection Integrity Service – Connectionless Integrity Service – Selective Field Connectionless Integrity

Service

Non-repudiation

• The denial by one of the entities involved in a communication of having participated in all or part of the communication

• Prevents one of the entities involved in a communication to later deny having participated in all or part of the communication – Non-Repudiation of Origin Service – Non-Repudiation of Receipt Service

Security Transport Protocols

Network layer Transport layer Application layerAbove the

application layer

IP

TCP

IP

TCP

IP

TCP

IP

TCP

SSL

Tel

net

FT

P

HT

TP

Tel

net

ove

r S

SL

FT

P o

ver

SS

L

HT

TP

ove

r S

SL

Sec

ure

Tel

net

Sec

ure

FT

P

Sec

ure

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TP

Tel

net

FT

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HT

TP

Secure Messaging(S-MIME, PGP)

Encryption and Authentication Algorithms

and Technology

Historical Ciphers

• Nonstandard hieroglyphics, 1900BC• Atbash cipher (Old Testament, reversed

Hebrew alphabet, 600BC)• Caesar cipher;

– letter = letter + 3– ‘fish’ -> ‘ilvk’

• rot13: Add 13/swap alphabet halves– usenet convention used to hide possibly offensive

jokes– applying it twice restores original text

Substitution Ciphers

• Simple substitution cipher;– a=p, b=m, c=f…

• Break via letter frequency analysis• Polyalphabetic substitution cipher

– 1. A=p, b=m, c=f…– 2. A=l, b=t, c=a…– 3. A=f, b=x, c=p,…

• Break by decomposing into individual alphabets, then solve as simple substitution

One-time Pad (1917)• OTP is unbreakable provided

– pad is never reused

– unpredictable random numbers are used (physical sources, eg radioactive decay)

Message s e c r e t18 5 3 17 5 19

OTP +15 8 1 12 19 5--------------------------------------------------

17 13 4 3 24 24 g m d c x x

One time Pad• Used by

– Russian spies– Washington-Moscow “hot-line”– CIA covert operations

• Many snake oil algorithms claim unbreakability by claiming to be a OTP– pseudo-OTPs provide pseudo-security

• Cipher machines attempted to create approximations to OTPs, first mechanically, then electronically

Cipher Machines (1920)

• Basic component is a wired rotor– simple substitution

• Step the rotor after each letter– polyalphabetic substitution, period = 26

‘A’ ->

-> ‘M’

Cipher Machines

• Chain multiple rotors• Each steps the next one when a full turn

is complete

‘A’ ->

-> ‘P’

Cipher Machines

• Two rotors, period = 26 X 26 = 676• Three rotors, period = 26 X 26 X 26 =

17,576• Rotor sizes are chosen to be relatively

prime to give maximum-length sequence

• Key is rotor wiring and rotor start position

Cipher Machines

• Famous rotor machines– Japan, Red, Purple– Germany, Enigma

• Secure if used properly– use of predictable openings (“nothing

to report”, “Mein Fuehrer”)– use of same key over an extended

period

Stream Ciphers• Binary pad (keystream), use XOR

instead of addition

• Plaintext = original, unencrypted data

• Ciphertext = encrypted data

• Two XORs with the same data always cancel out

Plaintext 1 0 0 1 0 1 1Keystream 0 1 0 1 1 0 1Ciphertext 1 1 0 0 1 1 0Keystream 0 1 0 1 1 0 0Plaintext 1 0 0 1 0 1 1

Stream Ciphers

• Using the keystream and ciphertext we can recover the plaintext

• But..using the plaintext and ciphertext we can recover the keystream

• Using two ciphertexts from the same keystream we can recover the XOR of the plaintexts

• Any two will recover the third (don’t reuse keys of stream cipher)

RC4

• Stream cipher optimized for fast software implementation

• 2048 bit key, 8 bit output• Extremely fast• Used in SSL (Netscape, MSIE), Lotus

Notes, Windows, Adobe Acrobat, Oracle Server

• Easy to get wrong

Block Ciphers

• Originated in early 70’s– banking security systems

F()

F()

L R

encrypt

key

F()

F()

L R

key

decrypt

Block Ciphers

• F() function is a simple transformation, does not have to be reversible

• Each step is called a round, the more rounds, the greater the security

• DES is an example of block cipher– 16 rounds– 56 bit key– 64 bit block size (L,R = 32 bits)

Attacking Block Ciphers

• Differential cryptanalysis– looks for correlations in f() function input

and output

• Linear cryptanalysis– looks for correlations between key and

cipher input and output

• Related-key cryptanalysis– looks for correlations between key changes

and cipher input/output

Data Encryption Standard (DES)

• Widely-used method of encryption using a private (secret) key

• Restricted for exportation to other countries• 72 quadrillion or more possible encryption

keys that can be used• For each given message, the key is chosen

at random from among this number of keys• Sender and receiver must know and use the

same private key

Strength of DES

• Key size = 56 bits

• Brute force = 2**55 attempts

• Differential cryptanalysis = 2**47

• Linear cryptanalysis = 2**43

• Can be done relatively easily with FPGA or ASIC (8 cents/key)

• 1998: German court ruled DES unsafe for financial applications

Other Block Ciphers

• Triple DES (3DES)– encrypt+decrypt+encrypt with 2 (112 bits)

or 3(168 bits) DES keys– 1998 - banking auditors were requiring the

use of 3DES rather than DES

• RC2– companion to RC4, 1024 bit key– RC2 and RC4 have special status for US

exportability

Other Block Ciphers

• AES– Advanced Encryption Standard,

replacement for DES– 128 bit block size, 128/192/256 bit key

Relative PerformanceFast

Slow

RC4

3DES

AES

DES

RC2

Public Key Encryption• How can you use two different keys?

– One is the inverse of the other:– key1 = 3, key2 = 1/3, message M = 4– Encryption: Ciphertext C = M X Key1– = 4 X 3– = 12– Decryption: Plaintext M = C X key2– = 12 X 1/3– = 4

• One key is published, one is kept private -> public-key cryptography (PKC)

Example: RSA• N, e=public key, n=product of two primes q and p• d=private key• Encryption: C = M**e mod n• Decryption: M = C**d mod n• p,q = 5,7• n = p X Q• =35• e=3• d= e**-1 mod ((p-1)(q-1))• = 16

Example: RSA

• Message M = 4

• Encryption: C = 4**3 mod 35 = 29

• Decryption: M 29**16 mod 35 = 4

RSA

• An Internet encryption and authentication system that uses an algorithm developed by Ron Rivest, Adi Shamir, and Leonard Adleman

• Most commonly used encryption and authentication algorithm

• Included as part of the Web browsers from Netscape and Microsoft

RSA

• Other applications;– Lotus Notes– Intuit's Quicken

• Owned by RSA Security– licenses the algorithm technologies– sells development kits– technologies are part of existing or

proposed Web, Internet, and computing standards

How RSA Works• Algorithm involves multiplying two large

prime numbers (a prime number is a number divisible only by that number and 1) and additional operations to derive a set of two numbers that constitutes the public key and another set that is the private key

• Once the keys have been developed, the original prime numbers are no longer important and can be discarded

How RSA Works

• Both the public and the private keys are needed for encryption /decryption but only the owner of a private key ever needs to know it

• Using the RSA system, the private key never needs to be sent across the Internet

• The private key is used to decrypt text that has been encrypted with the public key

How RSA Works

• If I send you a message, I can find out your public key (but not your private key) from a central administrator and encrypt a message to you using your public key

• When you receive it, you decrypt it with your private key

How RSA Works

• You can also authenticate yourself to me (so I know that it is really you who sent the message) by using your private key to encrypt a digital certificate– When I receive it, I can use your public

key to decrypt it.

Summary of RSAOperation Use Whose Kind of keySend anencryptedmessage

Use thereceiver’s

Public

Send anencryptedsignature

Use the sender’s Private

Receive anencryptedmessage

Use thereceiver’s

Private

Receive anencryptedsignature

Use the sender’s Public

Public Key Algorithms

• RSA (Rivest-Shamir-Adleman)– digital signatures and encryption in one

algorithm– private key = sign and decrypt– public key = signature check and

encrypt

• DH (Diffie-Hellman)– key exchange algorithm

Public Key Algorithms

• DSA (Digital Signature Algorithm)

• All have roughly the same strength– 512 bit key is marginal– 1024 bit key is recommended minimal

size– 2048 bit key is better for long term

security

Symmetric key

• Same key used to encrypt and decrypt• Sender and receiver must hold same

secret or key confidentiality• Data Encryption Standard (DES)

algorithm• Merchants must administer secret

keys to all customers and provide them through secure channel (hard!)

Symmetric/secret-key cryptography

Net

Information

Encrypt

Encryptedinformation

Decrypt

Asymmetric key• Two distinct keys

– public key– private key

• Data encrypted using public key can only be decrypted using the corresponding private key

• Multiple senders can encrypt information using the public key– receiver uses the private key to decrypt

• Receiver must protect the private key

Asymmetric/public-key cryptography

Net

Public key Privatekey

What the Sender Does

1011001

Messageto send

Hashalgorithm

Messagedigest

Private key

EncryptionDigital

signature

Sender

Receiver

Randomkey

Receiverpublic key

Encryptedmessage

Randomkey

Encrypteddigital

signatureDigital envelope

What the Receiver Does

ReceiverPrivate key

SenderRandom

keyEncryptedmessage Original

Message

Encrypteddigital

signature

1011001

Messagedigest

1011001

Messagedigest

Digital envelope

Hashfunction

Senderpublic key

Hash Algorithms

• Reduce variable length input to fixed length (128 or 160 bit) output

• Requirements– can’t deduce input from output– can’t generate a given output (CRC fails

this requirement)– can’t find two inputs which produce the

same output (CRC fails this too)

Hash Algorithms

• Used to– produce fixed length fingerprint of arbitrary

length data– produce data checksums to enable

detection of modifications– distill passwords down to fixed length

encryption keys

• Also called message digests or fingerprints

Public-key cryptography• Easier for customer to download public key from a

merchant• Public-key can be used with secret-key without too

much difficulty– customer generates a random number used to encrypt

payment info using DES– DES key is then encrypted using the public key of the

merchant– info and encrypted key sent tp merchant– merchant first decrypts the key; then uses key to decrypt

payment information

Secret-key/Public-key combination

Net

DES key encryptedinfo

publickey

encryptedDES key

Net

privatekey

Secret key and Public KeyFeatures Secret Key Public KeyNumber of keys Single key Pair of keysType of keys Key is secret One key is

private, one keyis public

Keymanagement

Simple but difficultto manage

Need digitalcertificates andtrusted thirdparties

Relative speeds Very fast SlowerUsage Used for bulk data

encryptionUsed for lessdemandingapplicationssuch asencryptingsmalldocuments orto signmessages

Key Sizes and Algorithms

• Conventional key is used once per message

• Public key is used for hundreds or thousands of messages

• Public key compromise is much more serious than a conventional key compromise– Compromised logon password, attacker can

delete your files

Key Sizes and Algorithms

• Compromised private key, attacker can– drain credit card– clean out bank account– sign contracts/documents– identify theft

• 512 public key versus 40 bit conventional key is good balance for weak security

Key Sizes and Algorithms

• Recommendations for public keys– use 512 bit keys for

micropayments/smart cards– use 1K bit keys for short term use (1 yr)– use 1.5K bit keys for longer term use– use2K bit keys for certification

authorities, long term contract signing

Basic ServicesPhysical world Digital world

authentication Digital Certificate

Non-repudiationDigital Signature

confidentiality Encryption

CertificateAuthority

Certificate request

Digital Certificate

NameAuthoritySerial #VersionExpiration DateKeyDigital Signature

X.509X.509

Digital Certificate

Conventional Encryption

Insecurechannel

secure channel

Problem of communicating a large message in secret isreduced to communicating a small key in secret

Key Agreement

Insecurechannel

Key agreement

Provides part of the required secure channel forexchanging a conventional encryption key

Certificate Authority

Mary’spublic key

Mary’sprivate key

John’spublic key

John’sprivate key

Mary’sprivate key

John’sprivate key

Certificate Authority• Trusted Third Party

– similar to a passport office

• Determines policies for PKI• Registers users, system• Validates users, privileges• Issues certificates• Supports life cycle (revoke, renew)• Publishes directories• Manages risk• Protects CA signing key

Certificate Authority

Mary’spublic key

Mary’sprivate key

John’spublic key

John’sprivate key

Mary’sprivate key

John’sprivate key

Mary’spublic key

John’sprivate key

Public KeyInfrastructure (PKI)Registration

AuthorityX.500 Certificate

Authority

Payment Integrity• Hashing algorithms used to prevent fraud

or other sources or error– generates value unique to the data being sent

• hash value or “message digest”

– one way public cipher• no secret key• no way to reproduce the original information• impossible to hash other data to the same value• hash value sent with data and used to compare to

hash value generated at the other end

Hashing• Hash algorithm is public

– anyone can alter data and recalculate new value

• Message digest encrypted using private key of the sender– this is called a “digital signature”– possible to identify sender– only the owner of the private key can encrypt

message digest– private key used to encrypt (sign) the information– public key used to verify signature

Digital SignaturesCreation

Paymentinfo

Hashing Messagedigest

Private key

Transmission

Verification

Reception

Hashing

Private key

Messagedigest

Messagedigest

Compare thetwo

digests

How safe is a digital signature?

• Algorithm used by SET generates a 160 bit message digest– changing a single bit in the message will on

average change half the bits in the message digest

• Odds of two messages having the same message digest are one in 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 (10**16)

Digital Signature Examples

1. Only Online Mart can decrypt the order.2. Verify that Susan sent the order.

Scenario 1: Susan ordering from Online Mart

Susan Online MartEncrypt with Online Mart’s public keyOrder Info

Encrypt with Susan’s private key

Digital signature

Order Info

Digital signature

Decrypt with Online Mart’s private key

Decrypt with Susan’s public key

Scenario 2: Online Mart sends confidential info to SusanSusanOnline Mart

Confidential Info

Cipheredtext

Confidential Info

Encrypt with Susan’s public key

Decrypt with Susan’s private key

Payment and Purchase Order Process

• 1. Account holder registration

• 2. Merchant registration

• 3. Account holder (customer) ordering

• 4. Payment authorization

Account holder registration

• Must register with a 3rd party (TP)– must have a copy of the TP

public/private key set• e-mail• web page download• disk• flashcard

– account holder can register account for Internet use with public key

Account holder registration• Registration includes

– name and address– account number– identifying personal information

• Account holder S/W will– create/attach account holder public key to form– generate message digest from the info– encrypt info and disgest (secret key)– encrypt secret key using TP public key– transmit everything to TP

Account Holder Registration

HashingMessage

digest

Messagedigest

Account holder

public key

1 2

3

4

5Secret

key

TP public

key

Encryptedmessage

Transmission

Third Party Registration

• 1. Decrypts the secret key

• 2. Decrypts the information, message digest, and account holders public key

• 3. Computes and compares message digests

• If information is verified TP digitally signs info with private key and sends back to account holder to save and use in future transactions

Third Party Receives Registration

Encryptedmessage

Reception

Encryptedmessage

Messagedigest

Hashing Messagedigest

TPprivate

key

Comparison

1

2

3

Merchant Registration

• Merchants must register with TP– Visa– Mastercard, etc

• Similar to account registration

• Certified Documentation (CD) transferred to the merchant from the TP for storage on merchant computer

Customer Ordering• Customer must have copy of merchant

public key for particular account type• Customer asked what type of account• CD for that account sent• Customer certifies CD using key• Customer allowed to shop in the on-line

environment• Customer fills out appropriate information

when ordering products

Customer Software

• 1. Encrypts account information with the TP public key

• 2. Attaches encrypted account info to order form

• 3. Creates message digest of order form; digitally signs it with customer private key

Customer Software

• 4. Secret-key encryption for– order form– digital signature– customer CD

• 5. Secret key encrypted with merchants public key

• 6. Secret-key encrypted message transmitted to merchant

Customer Ordering - Order sent to merchant

Account TPpublic

key

Encryptedaccount

HashingMessage

digestCustomer

privatekey

Encryptedmessage

Secret key

Merchantpublic key

Transmission

1 2 3

4

5

6

CustomersCD

Merchant Software Functions• 1. Decrypt secret key using private key of

merchant• 2. Decrypt order form, digital signature and

customer CD using secret ket• 3. Decrypt MD using customer public key

obtained from customer CD (to verify digital signature)

• 4. Calculate MD from order form and compare with customer decrypted MD

Customer Ordering - Merchant receives order

Encryptedmessage

Reception Encryptedmessage

Hashing

Messagedigest

Messagedigest

Merchantprivate

keyCustomers

CD

TPpublic key

Customer’s public key

Customer’s public key

Compare

1

2

3

4

Certificates: Need for Authentication

• Before using public-key cryptography, need to make sure other party is authenticated– want to make sure other party’s public

key is really theirs and not an imposter’s– impractical to receive this information

directly from the other party over a secure channel

Certificates: Need for Authentication

• Alternative is to use a trusted third party– Certificate Authority (CA) used to

authenticate public key– authenticate based on published policies– certificate generated which includes

name and public key and digitally signed by CA

Certificate Classes• Class 1

– automated unambiguous name and e-mail address search

• Class 2– Class 1 plus automated enrollment information

check (driver’s license, SSN, DOB) and automated address check (US and Canada)

• Class 3– Class 1 plus personal presence and ID documents

plus Class 2 automated ID check for individuals (credit check); business records for organizations

Certificate Classes

• Primary commercial issuers– Verisign– CyberTrust

• Issuance through the Web

• Free 6 month Class 1 offerred

• Postal Service entering market

Security Protocol Layers

Physical

Data Link

TCP/IP

Higher-levelnet protocols

Email

Applications

Physical

Data Link

TCP/IP

Higher-levelnet protocols

Email

Applications

Internet

Hardware link encryption

IPSEC

SSL, SSH, Kerberos

S/MIME, PGP

E-commerce protocols

The further down you go, the more transparent it is.The further up you go, the easier it is to deploy

Key Management and Certificates

Key Management

• Hardest part of cryptography• Two classes of keys

– Short term session keys (called ephermal keys)

• generated automatically and invisibly• used for one message or session and

discarded

– Long term keys• generated explicitly by the user

Key Management

• Long term keys are used for two purposes– authentication

• access control

• integrity

• non-repudiation

– confidentiality• establish session keys

• protect stored data

Key Management Problems

• Key certification

• Distributing keys– obtaining someone else’s public key– distributing your own public key

• Establishing a shared key with another party– confidentiality: is it really known by the other

party?– Authentication: is it really shared with the other

party?

Key Management Problems

• Key storage– secure storage of keys

• Revocation– revoking published keys– determining whether a published key is

still valid

Key Lifetimes and Key Compromise

• Authentication keys– public keys may have an extremely long

lifetime (decades)– private keys/conventional keys have shorter

lifetimes (year or two)

• Confidentiality– should have as short a lifetime as possible

• If the key is compromised– revoke the key

Key Lifetimes and Key Compromise

• Effects of compromise– authentication; signed documents are

rendered invalid unless timestamped– confidentiality; all data encrypted with it

is compromised

Browser Encryption Capabilities