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SiRiUS : Securing Remote Untrusted Storage. Eu -Jin Goh , Hovav Shacham , Nagendra Modadugu , Dan Boneh. Motivation. To make existing Network Files Systems more secure with no changes to the infrastructure. Easy to use software to implement security for any legacy file systems. Overview.

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sirius securing remote untrusted storage

SiRiUS: Securing Remote Untrusted Storage

Eu-Jin Goh, HovavShacham, NagendraModadugu, Dan Boneh

  • To make existing Network Files Systems more secure with no changes to the infrastructure.
  • Easy to use software to implement security for any legacy file systems.
  • Design Criteria
  • Design Overview
  • FS services
  • Implementation
  • Weaknesses
  • Extensions
  • Conclusion
system design
System Design
  • No changes to file server
    • Adds security to the existing network file system
  • File sharing
    • Ability to share files with distinct read/write access
  • Confidentiality & Integrity
    • File data must be protected from unauthorized users
  • Untrusted server
    • Any requests coming to the server can be spoofed, hence, SiRiUS must have its own access control information
system design cont d
System Design (cont’d)
  • Access Control
    • Not limited by the underlying File system and provide its own access control
  • Key management
    • Easy key management and must be compatible with use in other applications simultaneously
    • Key distribution and access revocation
  • Freshness Guarantees
    • Timely revocation of access control
    • Latest version of the file
file system design overview
File System design Overview
  • User’s have one Master Encryption key (MEK) and one Master Signature Key (MSK)
  • On server : md-file and d-file
    • Each file data is encrypted using symmetric cipher (FEK) and signed (FSK)
  • Meta Data Freshness File (mdf-file) on server
    • periodically update time stamp the root mdf-file for freshness and sign using MSK
file structure
File Structure
  • md-file:
  • d-file:
  • mdf-file: A tree structure of hashed md-files

File Data Encrypted (EFEK)

Signature[Hash of data]FSK

Hash(md-files) in the sub-directory

Hash(md-files) in the directory

Hash(md-files) in the root ||timestamp

creating a file
Creating a File
  • Steps for creating a file

Create hash and sign with DSA using user’s MSK

Encrypt Keys with owner’s MEK

Create Keys (FSK & FEK)

Step 3

Step 1

Step 2

Create md-file

Step 4

Encrypt file data using FEK and create d-file

Update root mdf-file

Step 5

Step 6

creating a file detailed
Creating a File : detailed
  • Step 1: DSA File signing key & AES File Encryption key
  • Step 2: Encrypt the keys using owner’s MEK and tag it with username
  • Step 3: Apply SHA-1 to

{(Encrypted block|| FSK || timestamp || filename), MSK} - Signature

creating a file detailed1
Creating a File : detailed
  • Step 4: md-file creation

{Encrypted block || FSK || timestamp || filename || Signature}

  • Step 5:


EFEK(data) || {(hash(EFEK(data))), FSK}

  • Step 6: Update root mdf-file
operations on fs



Operations on FS
  • File sharing
  • Read/Write
  • Renaming



Encrypted with MEK of user



Encrypted with MEK of user

FSK public key




File Links

    • Supports sym links if supported by underlying FS
    • Cannot support hard links
  • Key Management
    • Simple because it needs to track only MSK & MEK
    • Block removed from md-file and new FSK & FEK generated for encryption
  • SiRiUS on Linux over NFS v3
    • User level NFS loopback server
    • NFS client

NFS File Handle Cache

    • In NFS all file system objects are identified by unique NFS file handle
    • Cache the results of regular NFS calls for arguments and results
  • Changing Access Controls
    • User level permission tool
  • Random access and Low bandwidth
    • Implemented to deal with large files with size more than 8192 bytes.
  • Caching
  • Deleting contents

Attack : Attacker breaks into the server and deletes all files

  • Seizing File System Control

Attack : Replace an existing file and update the root mdf-file and sign it

  • d-file Rollback

Attack : Freshness guarantees checks the rollback on md-files but not on d-files

  • Non-essential capabilities that improve performance/security of basic SiRiUS
    • Random Access & Low Bandwidth
    • Encrypted Pathnames
    • Large scale Group Sharing using NNL
    • Maintaining traditional file system semantics


Thank you