EE515/IS523 Think Like an AdversaryLecture 5

Access Control in a Nutshell

Yongdae Kim

Recap http://security101.kr

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Challenge-response authentication

Alice is identified by a secret she possessesBob needs to know that Alice does indeed possess this secret

Alice provides response to a time-variant challenge

Response depends on both secret and challenge

UsingSymmetric encryptionOne way functions

Challenge Response using SKE

Alice and Bob share a key KTaxonomy

Unidirectional authentication using timestampsUnidirectional authentication using random numbers

Mutual authentication using random numbers

Unilateral authentication using timestampsAlice Bob: EK(tA, B)Bob decrypts and verified that timestamp is OKParameter B prevents replay of same message in B A direction

Challenge Response using SKEUnilateral authentication using random numbersBob Alice: rb

Alice Bob: EK(rb, B)

Bob checks to see if rb is the one it sent out Also checks “B” - prevents reflection attack

rb must be non-repeating

Mutual authentication using random numbersBob Alice: rb

Alice Bob: EK(ra, rb, B)

Bob Alice: EK(ra, rb)

Alice checks that ra, rb are the ones used earlier

Challenge-response using OWF

Instead of encryption, used keyed MAC hK

Check: compute MAC from known quantities, and check with message

SKID3Bob Alice: rb

Alice Bob: ra, hK(ra, rb, B)

Bob Alice: hK(ra, rb, A)

Key Establishment, Management

Key establishmentProcess to whereby a shared secret key becomes available to two or more parties

Subdivided into key agreement and key transport.

Key managementThe set of processes and mechanisms which support key establishment

The maintenance of ongoing keying relationships between parties

Access Control in a Nutshell

Yongdae Kim

Kerberos vs. PKI vs. IBE

Still debating Let’s see one by one!

Kerberos (cnt.)

T

A B

A, B, NA

EEKBT

KBT(k, A, L), E

(k, A, L), E K

AT

KAT(k, N

(k, N AA, L, B)

, L, B)

EEKBTKBT(k, A, L), E(k, A, L), Ekk(A, T(A, TAA, A, Asubkeysubkey))

EEkk(T(TAA, B, Bsubkeysubkey))

•EEKBTKBT(k, A, L): Token for B(k, A, L): Token for B•EEKATKAT(k, N(k, NAA, L, B): Token for A, L, B): Token for A•L: Life-timeL: Life-time•NNAA??

•EEkk(A, T(A, TAA, A, Asubkeysubkey): To prove B that A knows k): To prove B that A knows k•TTAA: Time-stamp: Time-stamp

•EEkk(B, T(B, TAA, B, Bsubkeysubkey): To prove A that B knows k): To prove A that B knows k

Kerberos (Scalable)

T (AS)

A B

A, G, NA

EEKGT

KGT(k(k A

GAG, A, L), E

, A, L), E K

AT

KAT(k(k A

GAG, N

, N AA, L, G)

, L, G)

EEKGB KGB (k(kABAB, A, L, N, A, L, NAA’’), E), EkABkAB(A, T(A, TAA’’, A, Asubkeysubkey))

EEkk(T(TAA’’, B, Bsubkeysubkey))

G (TGS)

EE KGT

KGT(k(k AGAG

, A, L), E

, A, L), E kA

GkAG(A,

(A,

TT AA), B, N

), B, N AA

’’

EE KAG

KAG(k(k ABAB

, N

, N AA’’, L, B), E

, L, B), E kG

BkGB(k(k ABAB

, A, L, N

, A, L, N AA’’), B, NA

), B, NA

’’

Public Key Certificate Public-key certificates are a vehicle

public keys may be stored, distributed or forwarded over unsecured media

The objective make one entity’s public key available to others such that its authenticity and validity are verifiable.

A public-key certificate is a data structure data part

cleartext data including a public key and a string identifying the party (subject entity) to be associated therewith.

signature part digital signature of a certification authority over the data part

binding the subject entity’s identity to the specified public key.

CA

a trusted third party whose signature on the certificate vouches for the authenticity of the public key bound to the subject entityThe significance of this binding must be provided by additional means, such as an attribute certificate or policy statement.

the subject entity must be a unique name within the system (distinguished name)

The CA requires its own signature key pair, the authentic public key.

Can be off-line!

ID-based CryptographyNo public keyPublic key = ID (email, name, etc.)PKG

Private key generation centerSKID = PKGS(ID)PKG’s public key is public.distributes private key associated with the ID

Encryption: C= EID(M)

Decryption: DSK(C) = M

Discussion (PKI vs. Kerberos vs. IBE)

On-line vs. off-line TTPImplication?

Non-reputation?Revocation?Scalability?Trust issue?

OS SecurityOS Security is essentially concerned with four problems:User authentication links users to processes.

Access control is about deciding whether a process can access a resource.

Protection is the task of enforcing these decisions: ensuring a process does not access resources improperly.

Isolation is the separation of processes’ resources from other processes.

Access ControlThe OS mediates access requests between subjects and objects.

This mediation should (ideally) be impossible to avoid or circumvent.

? ObjectSubject

Referencemonitor

Definitions Subjects make access requests on objects.

Subjects are the ones doing things in the system, like users, processes, and programs.

Objects are system resources, like memory, data structures, instructions, code, programs, files, sockets, devices, etc…

The type of access determines what to do to the object, for example execute, read, write, allocate, insert, append, list, lock, administer, delete, or transfer

Access Control Discretionary Access Control:

Access to objects (files, directories, devices, etc.) is permitted based on user identity

Each object is owned by a user. Owners can specify freely (at their discretion) how they want to

share their objects with other users, by specifying which other users can have which form of access to their

objects. Discretionary access control is implemented on any multi-user OS

(Unix, Windows NT, etc.). Mandatory Access Control:

Access to objects is controlled by a system-wide policy for example to prevent certain flows of information.

In some forms, the system maintains security labels for both objects and subjects

based on which access is granted or denied.

Labels can change as the result of an access Security policies are enforced without the cooperation of users or

application programs. Mandatory access control for Linux:

http://www.nsa.gov/research/selinux/

Access Control MatrixObj 1 Obj 2 Obj 3 … Obj n

Subj 1 rwl rwlx - - l

Subj 2 rwl rlx rwl - -

Subj 3 - - - rl r

Subj m rl lw rl rw r

RepresentationsAn access control matrix canbe represented internally indifferent ways:

Access Control Lists (ACLs)store the columns with theobjects

Capability lists store the rows with the subjects

Role-based systems group rights according to the “role” of a subject.

O1 O2 …

S1 rwl wl -

S2 ida wlk -

S3 - - rl

…

Sm rwlx wi w

Access Control ListsThe ACL for an object lists the access rights of each subject (usually users).

To check a request, look in the object’s ACL.

ACLs are used by most OSes and network file systems, e.g. NT, Unix, and AFS.

ACL ProblemsTo be secure, the OS must authenticate that the user is who (s)he claims to be.

To revoke a user’s access, we must check every object in the system.

There is often no good way to restrict a process to a subset of the user’s rights.

CapabilitiesCapabilities store the allowed list of object accesses with each subject.

When the subject requests access to object O, it must provide a “ticket” granting access to O.

These tickets are stored in an OS-protected table associated to each process.

No widely-used OS uses pure capabilities. Some systems have “capability-like” features: e.g. Kerberos, NT, OLPC, Android

ACL vs. CapabilitiesCapabilities do not require authentication: the OS just checks each ticket on access requests.

Capabilities can be passed, or delegated, from one process to another.

We can limit the privileges of a process, by removing unnecessary tickets from the table.

Roles

S1 S2 S3 Sm

O1 O2 On…

… S1 S2 S3 Sm

O1 O2 On…

…

R1 R2

Unix/POSIX Access Controlkyd@dio (~) % iduid=3259(kyd) gid=717(faculty)

groups=717(faculty),1686(mess),1847(S07C8271),1910(F07C5471),2038(S08C8271)

kyd@dio (~) % ls -l News_and_Recent_Events.zip -rw-rw-rw- 1 kyd faculty 714904 Feb 22 10:00

News_and_Recent_Events.zip

kyd@dio (/web/classes02/Spring-2011/csci5471) % ls –aldrwxrwsr-x 4 kyd S11C5471 512 Jan 19 10:23 ./drwxr-xr-x 46 root daemon 1024 Feb 17 23:04 ../drwxrwsr-x 3 kyd S11C5471 512 Feb 16 00:36 Assignment/

Mandatory Access Control policies

Restrictions to allowed information flows are not decided at the user’s discretion (as with Unix chmod), but instead enforced by system policies.

Mandatory access control mechanisms are aimed in particular at preventing policy violations by untrusted application software, which typically have at least the same access privileges as the invoking user.

Data Pump/Data DiodeLike “air gap” security, but with one-way communication link that allow users to transfer data from the low-confidentiality to the high- confidentiality environment, but not vice versa.

Examples:Workstations with highly confidential material are configured to have read-only access to low confidentiality file servers.

The covert channel problem

Reference monitors see only intentional communications channels, such as files, sockets, memory.

However, there are many more “covert channels”, which were neither designed nor intended to transfer information at all.

A malicious high-level program can use these to transmit high-level data to a low-level receiving process, who can then leak it to the outside world.

Examples for covert channels: Resource conflicts – If high-level process has already created a file F, a

low-level process will fail when trying to create a file of same name → 1 bit information.

Timing channels – Processes can use system clock to monitor their own progress and infer the current load, into which other processes can modulate information.

Resource state – High-level processes can leave shared resources (disk head position, cache memory content, etc.) in states that influence the service response times for the next process.

Hidden information in downgraded documents – Steganographic embedding techniques can be used to get confidential information past a human downgrader (least-significant bits in digital photos, variations of punctuation/spelling/whitespace in plaintext, etc.).