Computer Security Integrity Policies 2242021 1 Integrity Policies

Computer Security Integrity Policies 2/24/2021 1

Integrity Policies Commercial requirement • Users should not write their own programs • Programmers will develop and test programs on a non production system. • A special process must be followed to install a program from the development system onto the production system. • This must be controlled and audited. • Managers and auditors must have access to both the system state and log state. 2/24/2021 2

Integrity Policies Goals • Separation of duties – • If two or more steps are required to perform a critical function at least two people should perform the steps. Separation of function – Developers do not develop new programs on production systems Developers do not process production data on production systems – • Auditing – – 2/24/2021 Commercial systems emphasize recovery and accountability Auditing involves analyzing systems to determine what actions took place and who was involved. 3

Biba Integrity model Basically a dual of the Bell-La. Padula model. We have a subject set S, an object set O, a set of integrity levels I, and a relation on I. Let i : S O I return the integrity level, Relations • • • 2/24/2021 r : ability to read an object w : ability to write an object x : ability to execute a subject 4

Information transfer path A Information transfer path is a sequence of objects o 1, … , on+1 and a corresponding sequence of subject s 1, … , sn such that sj r oj and sj w oj+1 for all i 2/24/2021 5

Low-Water-Mark Policy 1. s S can write to o O iff i (o) i (s). 2. If s S reads o O then i ’(s)= min(i (s) , i (o)), where i ’(s) is the integrity level of s after the read. 3. s 1 S can execute s 2 S iff i (s 2) i (s 1). So • write up is prevented (prevents implant of corrupted data) • Integrity level drops on read access to lower level objects (prevents contaminating the subject: relying on less trustworthy 2/24/2021 data) 6

Low-Water-Mark Policy Theorem: If there is an information path from o 1 O to on+1 O , then enforcement of the low-water-mark policy requires that i (on+1) i (o 1) for all i>n. Proof The integrity level cannot go up. Proof by induction. 2/24/2021 7

Low-Water-Mark Policy Problem The integrity level of a subject is non-increasing, resulting in some subjects being eventually unable to access certain objects. 2/24/2021 8

Ring Policy This ignores indirect modifications and focuses on direct modifications. • s S can write to o O iff i (o) i (s). • • s S can read any o O. s 1 S can execute s 2 S iff i (s 2) i (s 1). Difference: Subjects can read any object. 2/24/2021 9

Biba’s strict integrity Policy 1. 2. 3. s S can write to o O iff i (o) i (s). s S can read o O iff i (s) i (o). s 1 S can execute s 2 S iff i (s 2) i (s 1). So • write up is prevented • read down is prevented (prevents relying on less trustworthy data) • execute up is prevented. 2/24/2021 10

Lipner’s Integrity Matrix Model Combines BLP and Biba Two basic Security levels • • Audit Manager (AM): system and management functions System Low (SL): any process can read info at this level. Five categories • • • Development (D) Production Code (PC) Production Data (PD) System Development (SD) Software Tools (T) 2/24/2021 11

Lipner’s Integrity Matrix Model Users Ordinary users Application Developers System Programmers System Managers & Auditors System Controllers downgrade privileges. Clearance levels (SL, {PC, PD}) (SL, {D, T}) (SL, {SD, T}) (AM, {D, PC, PD, ST, T}) (SL, {D, PC, PD, ST, T}) and Objects Class Development code/test data (SL, {D, T}) Production Code (SL, {PC}) Production Data (SL, {PC, PD}) Software Tools (SL, {T}) System Programs (SL, { }) System programs in modification(SL, {SD, T}) System and Application logs (AM, {appropriate categories}) 2/24/2021 12

Lipner’s full Model Users Security clearance Integrity clearance Ordinary users {IP}) Application Developers (ISL, {ID}) System Programmers (ISL, {ID}) System Managers & Auditors (ISL, {IP, ID}) System Controllers (ISP, {IP, ID}) Repair (ISL, {IP}) (SL, {SP}) (ISL, (SL, {SD}) (SL, {SSD}) (AM, {D, PC, PD, ST, T}) (SL, {SP, SD}) & downgrade privileges (SL, {SP}) Objects Integrity level Security level Development code/test data (ISL, {IP}) Production Code (IO, {IP}) Production Data (ISL, {IP}) Software 2/24/2021 Tools (IO, {ID}) (SL, {SP}) (SL, { }) 13

Lipner’s full Model --- exercise Users Ordinary users Application Developers System Programmers System Managers & Auditors System Controllers Repair Security clearance (SL, {SP}) (SL, {SD}) (SL, {SSD}) (AM, {D, PC, PD, ST, T}) (SL, {SP, SD}) & downgrade privileges (SL, {SP}) Objects Security level Development code/test data (SL, {SD}) Production Code (SL, {SP}) Production Data (SL, {SP}) Software Tools (SL, { }) System Programs (SL, { }) System programs in modification(SL, {SSD}) System and Application logs (AM, {appropriate categories}) Repair (SL, {SP}) Integrity clearance (ISL, {IP}) (ISL, {ID}) (ISL, {IP, ID}) (ISP, {IP, ID}) (ISL, {IP}) Integrity level (ISL, {IP}) (IO, {IP}) (ISL, {IP}) (IO, {ID}) (ISP, {IP, ID}) (ISL, {IP}) For read access: s(subject) s(object) & i(subject) i(object) Controllers do not have read access to development code, or production code For write access: s(subject) s(object) & i(subject) i(object) Controllers do have read access to development code and production code Who can help the controllers move development code to production code? 2/24/2021 14

Reminder: The Bell-La. Padula model ss-property: (s, o, p) S O P satisfies the ss-property relative to the security level f iff one of the following holds: a. p = e or p = a b. p = r or p = w and fc(s) dom fo(o). Also DAC! 2/24/2021 15

Reminder: The Bell-La. Padula model Define b(s: p 1, …, pn) to be the set of objects that s has access to. *-property: For each s S the following hold: a. b(s: a) ≠ [ o b(s: a) [fc(o) dom fc(s)] ] (write-up) b. b(s: w) ≠ [ o b(s: w) [fc(o) = fc(s)] ] (equality for read) c. b(s: r) ≠ [ o b(s: r) [fc(s) dom fo(o)] ] (read-down) Also DAC! 2/24/2021 16

Lipner’s Integrity Matrix Model Lipner’s model combines Biba and Bell-La. Padula model: • Simple security property • * property For example: an ordinary user can execute production code; if he needs to alter production data, the *-property dictates that the data be in (System Low, {Production Code, Production Data}). 2/24/2021 17

Lipner’s Integrity Matrix Model Objects Class Development code/test data (SL, {D, T}) Production code (SL, {PC}) Production data (SL, {PC, PD}) Software tools (SL, {T}) System programs (SL, { }) System programs in modification (SL, {SD, T}) System and application logs (AM, {appropriate catego Logs are append only. By the *-property their class must do those of the subjects that write to them 2/24/2021 18

The Clark-Wilson (CW) Model This model addresses data integrity requirements for commercial applications, e. g. bank transactions. Integrity requirements are divided into, • internal consistency: properties of the internal state that can be enforced by the computer system. • external consistency: the relation of the internal state to the rea world: enforced by means outside the system, e. g. auditing. 2/24/2021 19

The CW Model Integrity is enforced by, • well formed transactions: data items can be manipulated only by a specific set of programs; users have access to programs rather than data items. • separation of duties: users have to collaborate to manipulate data and collude to penetrate the system. 2/24/2021 20

The CW Model In the Clark-Wilson model, • Subjects must be identified and authenticated, • Objects can be manipulated only by a restricted set of programs, • Subjects can execute only a restricted set of programs, • A proper audit log has to be maintained, • The system must be certified to work properly. 2/24/2021 21

The CW Model In the Clark-Wilson model, • Subjects must be identified and authenticated, • Objects can be manipulated only by a restricted set of programs, • Subjects can execute only a restricted set of programs, • A proper audit log has to be maintained, • The system must be certified to work properly. 2/24/2021 22

The CW Model In the Clark-Wilson model • Data items are called Constrained Data Items (CDIs), • Data items not subject to integrity controls are Unconstrained Data Items (UDIs), • A set of integrity constraints constrain the values CDIs, • CDIs can only be manipulated by Transformation Procedures (TPs) • The integrity of a state is checked by Integrity Verification Procedure (IVPs) 2/24/2021 23

The CW Model Security procedures are defined by 5 Certification rules: 1. Integrity Verification Procedures must ensure that all Constrained Data Items are in a valid state when the IVP is run. 2. Transformation Procedures must transform valid CDIs into valid CDIs. 3. The “allowed” access relations must meet the requirements imposed by the principle of separation of duty. 4. All TPs must write to an append-only CDI log. 5. Any TP that takes a UDI as input must either convert it 2/24/2021 24 into

The CW Model Integrity is enforced by the 4 Enforcement rules 1. 2. 3. 4. The system must maintain and protect the certified relations: (TPi: CDIa, CDIb, … ) and ensure that only Transformation Procedures certified to run on a Constrained Data Item manipulate that CDI. The system must maintain and protect the list of entries: (User, TPi: CDIa, CDIb, … ) specifying the TPs that users can execute. The system must authenticate each user requesting to execute a TP. Only the certifier of a TP may modify the respective entities associated with that TP. No certifier of a TP may have execute permission with respect to that entity. 2/24/2021 25