1. Chap 13. Representing Identity
정보보호 알고리즘 연구실 이 민 호
Chap 13. Representing Identity

2. Representing Identity
13.1 What Is identity? 13.2 Files and Objects 13.3 Users 13.4 Groups and Roles 13.5 Naming and Certificates 13.6 Identity on the Web

3. 13.1 What is Identity? Principal : a unique entity
Identity: specifies a principal
Authentication : binds a principal to a representation of identity internal to the computer
Purpose of Identities
For accountability
For access control

4. 13.2 Files and Objects
The identity of file or object depends on system that containing object
Local systems identify objects by assigning names
Name may be intended
For human use (e.g file name)
For process use (e.g file descriptor or handle)
For kernel use (e.g file allocation table entry, inode)
Each name may have different semantics
If the object resides on a different system, the name must encode the location of the object

5. 13.3 Users
Systems represent user identity in a number of different ways
EXAMPLE : UNIX operating system
- Represent user identity as an integer (0 ~ usually 65,535)
- User Identification Number (UID)
- Each login name uniquely corresponds to a single UID, but multiple login names have a common UID
(Kernel uses UID to identify users)

6. 13.3 Users (cont.)
The same principal may have many different identities.
Each identity serves a particular function
EXAMPLE : UNIX operating system
-Real UID : user identity at login, but changeable
-Effective UID : user identity used for access control
Setuid changes effective UID
-Saved UID : UID before last change of UID
Used to implement least privilege
Work with privileges, drop them, reclaim them later
-Audit/Login UID : user identity used to track original UID
Cannot be altered; used to tie actions to login identity

7. 13.4 Groups and Roles Groups Two implementations of groups
The members of the set must be distinguishable
Used to share access to file
Two implementations of groups
alias for a set of principals
- Processes assigned to groups and stay in those groups for their lifetime of their sessions
principals can change groups
- Rights due to old group discarded; rights due to new group added

8. 13.4 Groups and Roles (cont.)
A type of group that membership tied to function
Rights given are consistent with rights needed to perform function
EXAMPLE : DG/UX system
System administrator privileges are in sysadmin role
Network administrator privileges are in netadmin role
Users can assume either role as needed

9. 13.5 Naming and Certificates
The identifier must uniquely identify the principal to avoid confusion
Problem: names may be ambiguous
- Does the name “Matt Bishop” refer to
We don’t know that the "Matt Bishop" who teaches at UC Davis from the "Matt Bishop" who works at Microsoft Corporation
Identifiers must include ancillary information to distinguish

10. 13.5 Naming and Certificates (cont.)
EXAMPLE : X.509v3 Distinguished Names
Identifies a principal
Consists of a series of fields
(e.g.) /O=University of Sogang/OU=Seoul campus/OU=Department of Computer Science/CN=Lee Minho/
(CN is common name, OU is organizational unit, O is organization)

11. 13.5 Naming and Certificates (cont.)
Certification authorities (CAs) vouch, at some level, for the identity of the principal to which the certificate is issued
CA’s authentication policy says the level of authentication required to identify the principal the certificate is to be issued
CA’s issuance policy says to which principals the CA will issue certificates

12. 13.5 Naming and Certificates (cont.)
Certification Hierarchy
Tree structured arrangement of CAs
Root is Internet Policy Registration Authority, or IPRA
Sets policies all subordinate CAs must follow
Certifies subordinate CAs (called policy certification authorities, or PCAs), each of which has own authentication, issuance policies
Does not issue certificates to individuals or organizations other than subordinate Cas
PCAs issue certificates to ordinary CAs
Does not issue certificates to individuals or organizations other than subordinate CAs

13. 13.5 Naming and Certificates (cont.)
University of Valmont issues certificates to students, staff
Students must present valid reg cards (considered low assurance)
First PCA: requires subordinate CAs to make good-faith effort to verify identities of principals
Staff must present proof of employment and fingerprints, which are compared to those taken when staff member hired (considered high assurance)
Second PCA: requires use of biometrics to verify identity

14. 13.5 Naming and Certificates (cont.)
UValmont and Certification Hierarchy
IPRA
PCA-1
UValmont
Student CA
student
PCA-2
Staff CA
staff
high assurance
PCA
low assurance

15. 13.5 Naming and Certificates (cont.)
Certificate tied to a role
EXAMPLE : A company wishes to have its comptroller authorized to digitally sign documents
Distinguished Name
/O=University of Valmont/OU=Office of the Big Bucks/RN=Comptroller
where “RN” is role name; note the individual using the certificate is not named, so no CN
Even if the current comptroller leaves, the same certificate can be used

16. 13.5.1 The Meaning of the Identity
The authentication policy defines that principals prove their identities
If incorrect, CA may misidentify entity unintentionally
Cause trust problem

17. 13.5.1 The Meaning of the Identity (cont.)
Persona Certificate
With the erosion of privacy, comes the need for anonymity
Persona Certificate supply the requisite anonymity
Certificate with meaningless Distinguished Name

18. Trust
Goal of certificate : bind the correct identity to the public key
The critical question : What is the degree of that assurance?
X.509v3, certificate hierarchy
Depends on policy of CA issuing certificate
Depends on how well CA follows that policy
Really, estimate based on the above factors

19. 13.5.2 Trust (cont.) EXAMPLE : Passport required
CA require name on passport, number and issuer of passport
What are points of trust?
Passport not forged and name on it not altered
Passport issued to person named in passport
Person presenting passport is person to whom it was issued
CA has checked passport and individual using passport

20. 13.6 Identity on the Web 13.6.1 Host identity 13.6.2 State and Cookies
Static identifiers: do not change over time
Dynamic identifiers: changes as a result of an event or the passing of time
State and Cookies
Anonymity
Anonymous
Anonymity: good or bad?

21. 13.6.1 Host Identity Bound up to networking
Not connected: pick any name (only locally)
Connected: one or more names depending on interfaces, network structure, context
Each principal on an individual host can gave different names
“Name” identifies a principal, “Address” identifies where that principal is located

22. 13.6.1 Host Identity (cont.) EXAMPLE : Layered network -MAC layer
Ethernet address: 00:05:02:6B:A8:21
AppleTalk address: network 51, node 235
-Network layer
IP address:
-Transport layer
Host name: cherry.orchard.chekhov.ru

23. 13.6.1.1 Static and Dynamic Identifiers
Static identifier : Not change over time
Dynamic identifier : changes either as a result of an event or over time
Domain Name Server(DNS) : associates hostnames and IP addresses
Maps transport identifiers (host names) to network identifiers (host addresses)
Forward records: host names  IP addresses
Reverse records: IP addresses  host names

24. 13.6.1.1 Static and Dynamic Identifiers (cont.)
If dynamic identifiers,
Assigned to principals for a limited time
Server maintains pool of identifiers
Client contacts server using local identifier
Only client, server need to know this identifier
Server sends client global identifier
Client uses global identifier in other contexts, for example to talk to other hosts
Server notifies intermediate hosts of new client, global identifier association

25. 13.6.1.2 Security Issues with the Domain Name Service
Trust is that name/IP address binding is correct
Goal of attacker: associate incorrectly an particular IP address with a host name
Assume attacker controls name server, or can intercept queries and send responses

26. 13.6.1.2 Security Issues with the Domain Name Service
Attacker can change records on server
- Called “control”
Add extra record to response, giving incorrect name/IP address association
- Called “cache poisoning”
Attacker sends victim request that must be resolved by asking attacker
- Attacker responds with answer plus two records for address spoofing (1 forward, 1 reverse)
- Called “ask me”

27. State and Cookies
Cookie : a token that contains information about the state of a transaction on a network
The Cookies consist of several values
name, value: name has given value
expires: how long cookie valid
Expired cookies discarded, not sent to server
If omitted, cookie deleted at end of session
domain: domain for which cookie intended
Consists of last n fields of domain name of server
Must have at least one “.” in it (.com )
secure: send only over secured (SSL, HTTPS) connection

28. Anonymity on the Web
Only one person is using the original host, and the address is not spoofed, someone could guess the identity of the sender with high accuracy
Anonymizer: a site that hides origins of connections
Usually a proxy server
User connects to anonymizer, tells it destination
Anonymizer makes connection, sends traffic in both directions
Destination host sees only anonymizer

29. 13.6.3 Anonymity on the Web (cont.)
EXAMPLE : anon.penet.fi
Offered anonymous service
Sender sends letter to it, naming another destination
Anonymizer strips headers, forwards message
Assigns an ID (say, 1234) to sender, records real sender and ID in database
Letter delivered as if from
Recipient replies to that address
Anonymizer strips headers, forwards message as indicated by database entry

30. 13.6.3 Anonymity on the Web (cont.)
Cypherpunk R er : R er that deletes header of incoming message, forwards body to destination
Also called Type I R er
No record kept of association between sender address, r er’s user name
Prevents tracing, as happened with anon.penet.fi
Usually used in a chain, to obfuscate trail
For privacy, body of message may be enciphered

31. 13.6.3 Anonymity on the Web (cont.)
Encipher message
Add destination header
Add header for r er n …
Add header for r er 2
send to r er 1
send to r er 2
send to Alice
Hi, Alice,
It’s SQUEAMISH
OSSIFRIGE
Bob

32. 13.6.3 Anonymity on the Web (cont.)
Mixmaster R er : Cypherpunk r er that handles only enciphered mail and pads (or fragments) messages to fixed size before sending them
Also called Type II R er
Designed to hinder attacks on Cypherpunk r ers
Messages uniquely numbered
Fragments reassembled only at last r er for sending to recipient

33. 13.6.3 Anonymity on the Web (cont.)
Mixmaster message
enciphered with RSA for r er #1
r er #2 address
packet ID: 135
Triple DES key: 1
enciphered with Triple DES key #1
enciphered with RSA for r er #2
final hop address
packet ID: 168
message ID: 7839
Triple DES key: 2
random garbage
enciphered with Triple DES key #2
recipent’s address
any mail headers to add
message
padding if needed

34. 13.6.3.1 Anonymity for Better or Worse
Benefit
Removes personalities from debate
Anonymity allows whistleblowers considerable protection
Anonymity protects privacy
Incorrect conclusions from misinterpreted data
Harm from erroneous information
Not being let alone
Are these benefits or drawbacks?
Depends on society, and who is involved