Modeling and Analysis of Networked Secure Systems with Application to Trusted Computing

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Modeling and Analysis of Networked Secure Systems
with Application to Trusted Computing

• Jason Franklin
• Joint work with Deepak Garg, Dilsun Kaynar, and
  Anupam Datta

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Motivation

• Despite progress in cryptography and security
  protocols, designing secure systems remains
  difficult
• Network protocols and cryptography rely on
• secure system assumption
• Problem Attackers violate secure system
  assumption by exploiting system design flaws
• Solution Prove security of system designs

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Motivating Example
Client System
Network
Server System
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Trusted Computing

• Question How do we trust remote servers?
• Proposed Solution Ask servers what programs they
  are running?
• Server attests to (vouches for) its running
  software

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Details of Proposed Solution
Since I last rebooted, Ive run 0 Operating
System 1 Web Server 2 Shopping Cart Server
System and Network Protocol Analysis
Ill ask what programs they have run!
How do I know if I can trust that server?
Hey server! What code have you run?
I trust those programs to protect my data!
Network Protocol Analysis
Network
Server System
Client System
Request
Reply
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In this talk

• Describe Logic of Secure Systems (LS2)
• Modeling system designs
• Analysis of system designs
• Analyze trusted computing protocol
• Limitations, work in progress, and conclusion

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Modeling Protocols as Programs
Since I last rebooted, Ive run 0 Operating
System 1 Web Server 2 Shopping Cart Server
Hey server! What code have you run?
I trust those programs to protect my data!
Network
Server System
Client System
request
Reply
Client request read question send
request reply receive match /reply/trusted/
Server r receive send reply
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Components of Model
Key
Added
Existing
Explicit Time
Memory Protection
Memory
Control Flow
Hardware Resets
Trusted Computing
Encryption Decryption
Hash
Sign/Verify
Network Comm.
Threads
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Modeling Details

• Memory
• Read and write named memory locations
• Example d read Memx
• Memory Protection
• Exclusive write locks
• Example lock(Memx)
• Explicit time
• Ordering events
• Read at time TR, Write at time TW and TR
• Expressing invariants
• Memx contains value V during interval (TR,TW)

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New Adversary Model
I can remove, reorder, or replay messages.
I can generate messages if I know their
components!
Im a malicious local thread! I can write to any
unprotected memory location! I can read any
memory location! I collude with the network
adversary!
Network
Server System
Client System
Standard Network Adversary
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Overview of Protocol Analysis
Modeling
Analysis
Client request read question send
request reply receive match /reply/trusted/
Suppose Client code executed then Server is
executing trusted programs
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LS2, what are you good for?

• Describe Logic of Secure Systems (LS2)
• Analyze trusted computing protocol
• Limitations, work in progress, and conclusion

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Problems with Trusted Computing Protocol
Since I last rebooted, Ive run malicious
code. Mwhahaha!!!!
Hey server! What code have you run?
I trust those programs to protect my data!
Since I last rebooted, Ive run linux. Really, I
have!
Network
Malicious Server System
Client System
Reply
request
Client request read question send
request reply receive match /reply/trusted/
MALICIOUS_Server r receive send linux
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Solution Trusted Coprocessor
Server System
Trusted Coprocessor
Client System
Network
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Trusted Coprocessor

• Trusted Computing
• Augments platform with trusted co-processor
• Coprocessor includes
• Crypto. Primitives
• Keys
• Append-only log

Coprocessor

Log
A
B
Log.append(A) Log.append(B)
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Server with Coprocessor

Server System
Booting 1. append log, os_code os read
os_code 2. call os 3. append log,
web_server_code ws read web_server_code 4.
call ws
Web Server
3. Append
4. Call
OS
2. Call
1. Append
Coprocessor

Log
os_code
web_server_code
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Coprocessor Vouching for Log
Signer _ receive w read log s
sign w, K send s

os_code
ws_code
Log
s SIGN K, (os_code, ws_code)
Client System
Client_Receive s receive verify s,
seq(os_code, ws_code), K
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Protocol Analysis
Modeling
Analysis
Client_Receive s receive verify s,
seq(os_code, ws_code), K
Suppose Client code executed then Server is
executing trusted programs
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Trusted Computing Analysis

• Suppose that clientss thread executes code
  Client_Receive in time interval Begin, End) then
  the following properties hold

Read Log _at_ TRead
Verify Log _at_ TV
Call to OS Code _at_ TOS
Reset _at_ TR
Begin
End
No Resets during TR to TRead
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Anyone See Security Vulnerability?

• Reset attack possible after read of log

Verification of Stale Data
Read Log _at_ TRead
Verify Log _at_ TV
Client believes server can be trusted!
Call to OS Code _at_ TOS
Reset _at_ TR
Begin
End
No Resets during TR to TRead
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Adversaries Attack Assumptions

• Assumptions
• Client is not running in coprocessor
• Coprocessor does not reveal its private key
• Coprocessor only executes Signer
• Programs in memory are write locked

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Up next

• Describe Logic of Secure Systems (LS2)
• Designed for modeling and analysis of system
  specifications
• Use LS2 to analyze trusted computing protocols
• Specify trusted computing protocols
• State security properties and security proof
• Discuss implications and analysis of proof
• Limitations, work in progress, and conclusion

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Limitations and Work In Progress

• Control flow and context sensitive analysis
• Composition theorems for modular analysis of
  complex systems
• Expanding access control policies
• Analysis of other trusted computing protocols
• Modeling layer diagrams

User Level Programs
Operating System
Hardware
Layer Diagram
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Related Work

• LS2 is derived from Protocol Composition Logic
  Datta05-07
• Related work on program correctness
• Concurrent separation logic Brookes04
• Verification of concurrent systems TLA,
  Lamport84, OwickiGries76

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Conclusion

• LS2 is logic for analysis of networked secure
  systems including local and network adversaries
• LS2 models abstract memory protections, time,
  hardware resets, and security primitives
• Described formal analysis of trusted computing
• Identified reset attack
• Ongoing work seeks to expand scope of LS2

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Take Home Points

• If you are designing secure systems
• LS2 enables security analysis of system design
  before you implement
• Avoids costly and embarrassing vulnerabilities
• Clarifies security properties of design
• Even if you are just using secure systems
• LS2 can provide stronger guarantees about your
  security

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Questions?

• Theory of Secure Systems Project (ToSS)
• http//www.cs.cmu.edu/jfrankli/toss
• Publications and Manuscripts
• D. Garg, J. Franklin, D. Kaynar, A. Datta.
  Towards a Theory of Secure Systems Cylab
  Technical Report, Feb. 2008.
• D. Garg, J. Franklin, D. Kaynar, A. Datta. A
  Logic for Reasoning about Networked Secure
  Systems. Under submission to FCS 08.
• J. Franklin, D. Garg, D. Kaynar, A. Datta.
  Modeling and Security Analysis of Trusted
  Computing. In progress.

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