Electronic Payment Systems
20-763
Lecture 6
Digital Certificates
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Outline
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Trust infrastructures
Identity documents
Digital certificates
Certificate hierarchy
Certification chains
Remote authentication
Public key infrastructure (PKI)
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Trust Infrastructures
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OS (Windows, Linux, BSD…)
Device (BIOS, CPU, Video/Audio, Storage)
User (Biometrics, smart cards, digital signatures)
Applications (Virus checkers, code authentication)
Server (Secure Email, SSL)
Content (Copy/tamper protection, document
authentication)
• Network (VPNs, firewalls, proxy servers, intrusion
detectors)
• Enterprise (Central management procedures)
• External organization (Gov’t agency, CA)
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FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Identity Documents
• What is an identity document? (Passport, birth
certificate, driver’s license)
– A piece of paper
– Issued by a trusted third party
– With information verifying the identity of the holder
• An identity document is useless unless the holder can
be CHALLENGED to demonstrate that he is the
person named in the document
– Photograph
– Signature
– Fingerprint
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FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Digital Certificate
• A digital identity document binding a public-private
key pair to a specific person or organization
• Verifying a digital signature only proves that the
signer had the private key corresponding to the public
key used to decrypt the signature
• Does not prove that the public-private key pair
belonged to the claimed individual
• We need an independent third party to verify the
person’s identity (through non-electronic means) and
issue a digital certificate
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Digital Certificate Contents
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Name of holder
Public key of holder
Name of trusted third party (certificate authority)
DIGITAL SIGNATURE OF CERTIFICATE
AUTHORITY
• Data on which hash and public-key algorithms have
been used
• Other business or personal information
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FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
X.509 Version 2 Certificate
VERSION # OF X.509
UNIQUE # ASSIGNED BY CA
EXAMPLES: MD5RSA,
sha1RSA
USUALLY A DOMAIN NAME
EXAMPLES: RSA
SOURCE: FORD & BAUM,
SECURE ELECTRON IC COMMERCE
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FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Digital Certificate Verification
• Do I trust the CA? (Is it in my list of trust root
certification authorities?)
• Is the certificate genuine?
– Look up the CA’s public key; use it to decrypt the signature
– Compute the certificate’s hash; compare with decrypted sig
• Is the holder genuine? This requires a challenge
• If the holder is genuine, he must know the private key
corresponding to the pubic key in the certificate
• Having the certificate is not enough. (They are
exchanged over the Internet all the time)
• Send him a nonce (random 128-bit number)
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FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Challenge by Nonce
• If you’re really Shamos, you must know his private
key
• So please encrypt this nonce:
“A87B1003 9F60EA46 71A837BC 1E07B371”
• When the answer comes back, decrypt it using the
public key in the certificate
• If the result matches, the remote user knew the
correct private key
• Never use the same nonce twice
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FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
ISO X.500 Directory Standard
STANDARD FOR HIERARCHICAL
DIRECTORIES
RDN: RELATIVE DISTINGUISHED NAME
C: ISO COUNTRY CODE
O: ORGANIZATION
CN: COMMON NAME
EACH RDN MAY HAVE ATTRIBUTES
20-763 ELECTRONIC PAYMENT SYSTEMS
SOURCE: XCERT.COM
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Certification Hierarchy
• What happens if you don’t recognize the CA in a
certificate or it is not a trusted CA?
• Suppose CA1 has a certificate issued by trusted
CA2?
• You may choose to trust CA1
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FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Certificate Authority Hierarchy
Root CA issues its own certificate!
RCA
RCA : Root Certificate Authority
BCA : Brand Certificate Authority
GCA : Geo-political Certificate Authority
CCA : Cardholder Certificate Authority
MCA : Merchant Certificate Authority
PCA : Payment Gateway
Certificate Authority
BCA
GCA
CERTIFICATE ISSUANCE
CCA
MCA
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PCA
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Certification Chains
X.500 Name Directory
similar to domain naming
Children have unique
relative names
SOURCE: FORD & BAUM,
SECURE ELECTRON IC
COMMERCE
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FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Certification Paths
• Alice has a certificate issued by authority D
• To verify Alice’s certificate, Bob needs the public key
of authority D (to decrypt D’s signature on the
certificate)
• How does Bob get it so he is sure it is really the
public key of D? This is another verification problem.
• Solution: Alice sends Bob a certification path, a
sequence of certificates leading from her authority D
to Bob. The public key of D is in D’s certificate
• (D’s certificate is not enough for verification since Bob
may not know D’s certification authority G)
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
SOURCE: SILVA
AND STANTON
Certification Paths
=
CERTIFICATION
AUTHORITY
= END USER
“REVERSE”
CERTIFICATE
BOB
ALICE
CERTIFICATE
ISSUED BY F
F<<B>>
CERTIFICATE
ISSUED BY D
D<<A>>
ALICE WILL TRUST ANY
PARTY TRUSTED BY D
CERTIFICATION PATH:
D<<G>>,
D TRUSTS G
G<<J>>,
G TRUSTS J
J<<H>>,
J TRUSTS H
H<<F>>,
F<<B>>
H TRUSTS F
F TRUSTS B
ALICE NOW HAS (AND TRUSTS) BOB’S CERTIFICATE
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Cryptographic Notation
{ A, B, C, D }
means
strings A, B, C and D concatenated together
SKSENDER( A )
means
string A encrypted with SENDER’s secret (private) key
PKBANK( B )
means
string B encrypted with BANK’s public key
H(A)
means
one-way hash of string A
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Remote Authentication
• B sends a certificate to A (A now knows B’s public key)
• A constructs an authentication token
M = ( TA,
RA ,
IB,
d)
TIMESTAMP
NONCE TO PREVENT
REPLAY ATTACK
ID OF B
DATA TO BE SIGNED
• A sends B the message
( B  A, SKA { M } )
A’S CERTIFICATION PATH
INCLUDING A’S CERTIFICATE
AUTHENTICATION TOKEN ENCRYPTED WITH
A’S PRIVATE KEY (ONLY A CAN DO THIS)
• B obtains A’s public key PKA, trusted because of B  A
• B recovers M by using PKA to decrypt SKA { M }
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Authentication
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B checks IB to make sure he is the intended recipient
B verifies that the timestamp Ta is current
B verifies that RA has not been used before (no replay)
B knows A’s certificate really belongs to A since only A
could have encrypted M with SKA
AT THIS POINT, B HAS AUTHENTICATED A.
THIS IS “ONE-WAY AUTHENTICATION”
• B can send A an authentication token so A will know
that B is authentic
IF A AND B AUTHENTICATE EACH OTHER,
WE HAVE “TWO-WAY AUTHENTICATION”
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Public Key Infrastructure (PKI)
• Digital certificates alone are not enough to establish
security
– Need control over certificate issuance and management
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Certification authorities issue certificates
Who verifies the identify of certification authorities?
Naming of entities
Certification Practice Statement
Certificate Revocation List
The metafunctions of certificate issuance form the
Public Key Infrastructure
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FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Certification Practice Statement
• Satement by a CA of the policies and procedures it
uses to issue certificates
• CA private keys are on hardware cryptomodules
• View Verisign Certification Practice Statement
• INFN (Istituto Nazionale di Fisica Nucleare) CPS
CHRYSALIS LUNA CA3
TRUSTED ROOT KEY SYSTEM
20-763 ELECTRONIC PAYMENT SYSTEMS
IBM S/390 SECURE
CRYPTOGRAPHIC MODULE
FALL 2002
LITRONIC 440
CIPHERACCELERATOR
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Certificate Revocation List
• Online list of revoked certificates
• View Verisign CRL
• Verisign CRL usage agreement
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Functions of a Public Key
Infrastructure (PKI)
• Generate public/private key pairs
• Identify and authenticate key subscribers
• Bind public keys to subscriber by digital certificate
• Issue, maintain, administer, revoke, suspend,
reinstate, and renew digital certificates
• Create and manage a public key repository
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Corporate PKI Components
SOURCE: INFOSEC ENGINEERING
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
eCheck Structure
Payer
Payee
Invoice
Accounts
Payable
Accounts
Receivable
E-Mail or WWW
Electronic
Checkbook
Invoice
Invoice
Check
Signature
Certificates
Electronic
Checkbook
Check
Signature
Certificates
E-Mail
Endorsement
Deposit
Signature
Certificates
Clear and settle echeck
Check
Payer’s Bank
debit account
20-763 ELECTRONIC PAYMENT SYSTEMS
Signature
Certificates
FALL 2002
Payee’s Bank
credit account
COPYRIGHT © 2002 MICHAEL I. SHAMOS
eCheck Signatures & Endorsement
Deposit
Endorsement
action
action
action
deposit
endorsement
check
depositor’s signature
endorser’s signature
payer’s signature
depositor’s account
endorser’s account
payer’s account
depositor’s cert
endorser’s cert
payer’s cert
depositor’s bank’s
signature
depositor’s
bank’s cert
endorser’s bank’s
signature
endorser’s
bank’s cert
public key
references
signatures
public key
references
Check
attachment
invoice
signatures
payer’s bank’s
signature
payer’s
bank’s cert
public key
references
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
signatures
COPYRIGHT © 2002 MICHAEL I. SHAMOS
eCheckbook Distribution & PKI
8. PIN mailer
Customer
1. Sales
contact
Card
initialization
9. Electronic
checkbook,
smart
card reader,
software,
instructions
5. Public key,
certificate
request,
account block
request
6. X.509 certificates,
account block
7. X.509 certificates
and account blocks
2. Account
agreement and
customer data
4. Electronic
checkbook
issuance
instructions
Marketing
and sales
Bank
Certification
Authority
3. Echeck account
information
20-763 ELECTRONIC PAYMENT SYSTEMS
12. CRL
10. Card sent
notification
Bank account
administrative
systems
FALL 2002
11. Account
activation
Bank
echeck
server
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Major Ideas
• Digital certificate is a digital identity document issued
by a trusted third party
• Digital signatures alone do not prove identity
• The holder of a certificate must be challenged to
prove he knows the correct private key
• Certificate authorities form trust hierarchies
• Certification paths lead from sender to recipient,
allowing verification of the trust relationship
• How crucial are certificates to secure eCommerce?
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS
Q&A
20-763 ELECTRONIC PAYMENT SYSTEMS
FALL 2002
COPYRIGHT © 2002 MICHAEL I. SHAMOS