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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