Hardware Assisted Control Flow Obfuscation for Embedded Processors

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Hardware Assisted Control Flow Obfuscation for
Embedded Processors

• Xiaoton Zhuang, Tao Zhang, Hsien-Hsin S. Lee,
  Santosh Pande

HIDE An Infrastructure for Efficiently
Protecting Information Leakage on the Address Bus
Xiaoton Zhuang, Tao Zhang, Santosh Pande
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Overview

• Software Obfuscation
• Obfuscate - v - render obscure, unclear, or
  unintelligible - bewilder (someone)
• Information Leakage
• Layout leakage
• Recurrence leakage
• Hardware Obfuscation Techniques

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Assumptions

• XOM model
• Everything outside the processor chip is assumed
  to be insecure
• Memory contents are encrypted

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Software Obfuscation(and why it doesnt work)

• Lacks of theoretical foundation
• It has been proven the perfect obfuscation does
  not exist
• May incur large overheads in code size
• Performance may be penalized due to carrying out
  extra computations
• History has proven it inefficient

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How is Software Obfuscation Vulnerable to Attack ?

• Layout Leakage
• Spatial vicinity
• Recurrence Leakage
• Recurring addresses

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Layout Leakage
100
101
102
103
104
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Recurrence Leakage
100
101
102
103
104
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So What? Its just Control Flow.

• Control flow info is the essential part of
  algorithms
• Competing
• company ex.
• Can help identify
• reuse code
• Control obfuscation techniques are well known and
  can be reversed

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Hardware Obfuscation Overview (paper 1)

• Encrypt the Address Bus (layout leakage)
• Relocate blocks every time they are written out
  to memory (recurrence leakage)

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Address Bus Encryption
Equates to a fixed mapping
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Shuffle Buffer

• Designed to reorder all writes to memory
• Exclusive to external memory

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Shuffle Buffer

• Indexed array through the block address table
• No address tag
• Smaller size / cheaper
• Blocks can be stored anywhere
• Blocks can be randomly replaced (circuit white
  noise)

• Assume program binary updatable then multi-run
  recurrence prevented

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Block Address Table (BAT) Cache

• Records the current location of blocks
• Use original block address to index into BAT to
  get new address
• Worst case scenario 10 overhead in virtual
  memory space
• Each access request from cache checks with BAT
  use BAT cache to speed things up

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How Secure Is This?

• With a shuffle buffer of 128 blocks 0.8 chance
  of guessing one recurrence correctly
• For n-recurrences the chance of guessing all
  correctly is 1/(Mn) where M is the size of the
  shuffle buffer

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Performance/Cost Summary

• Performance degradation can be below 1
• Hardware costs consist of small on chip shuffle
  buffer and BAT cache

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HIDE(Hardware-support for Leakage-Immune Dynamic
Execution)

• Basic idea is to break the correlation between
  repeated memory addresses
• Achieved by permuting the address space at
  suitable intervals during execution

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Hide Cache

• A cache same as a normal cache except that that
  blocks fetched after the previous permutation are
  all locked
• A locked block cannot be replaced until the
  memory space they belong to is permuted again

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How The Hide Cache Works
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Other Details

• When evicting a block choose the least recently
  used block among the unlocked blocks
• A separately stored bitmap is used to record
  whether a block is locked or not

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Hardware Flowgraph
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HIDE at Chunk Level

• Chunk - one or more pages that are protected and
  permuted together
• Designed to limit size of permutation
• Large memory permutations performance cost
• At chunk level the permutation unit only permutes
  all the blocks within a chunk
• With the smallest chunk size (a page) 75 of
  transition from one address to the next are
  intra-chunk
• Chunks can be specified in the code or at runtime
  with instructions inserted into the header of the
  binary code

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Page Info Cache

• Stores the Page Info Record to speed up access

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How Secure Is this?

• With 64K chunk protection and layout
  optimizations, 87 of address sequence is
  protected, in which 95 of the accesses to code
  and static data are hidden
• Interfaces are provided for the compiler or the
  user to increase the security to achieve almost
  complete protection

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Performance/Cost Summary

• The performance overhead in their experiments was
  at most 1.5 mainly due to permutations
• Most on chip components are small

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References

• Xiaotong Zhuang, Tao Zhang, Hsien-Hsin Lee and
  Santosh Pande. Hardware Assisted Control Flow
  Obfuscation for Embedded Processors. CASES,
  Washington DC, Sept. 2004.
• Zhuang, X., Zhang, T. and Pande, S. HIDE An
  Infrastructure for Efficiently Protecting
  Information Leakage on the Address Bus.
  International Conference on Architectural Support
  for Programming Languages and Operating Systems,
  Boston, MA., Oct 2004.