1. A Secure Cloud Gateway
based upon XML and Web Services
PhD Symposium, ECOWS ’11
2011-09-16
Sebastian Graf
supervised by Prof. Marcel Waldvogel
Distributed Systems Group
University of Konstanz
sebastian.graf@uni-konstanz.de 1
Sonntag, 18. September 11

2. Problem Statement
What approaches support secure storage of
data in the cloud ?
2
Sonntag, 18. September 11

3. Problem Statement
What approaches support secure storage of
data in the cloud ?
Availability: Integrity:
• access to data • System Integrity
• no unauthorized deletion • Data Integrity
Accountability: Confidentiality:
• traceability of changes • closure of private data
G. Stoneburner: Underlaying technical models for information technology security
National Institute of Standards and Technology
2
Sonntag, 18. September 11

4. Security and Cloud Storage
Web Web
Services Services
Integrity Auditing
Rev874
Web Web
Services Services
Rev25
3
Sonntag, 18. September 11

5. How to provide…
Goals Measurements
(Availability Cloud-based Service)
R1: How can integrity be ensured
Integrity within a distributed environment
with focus on fast processing?
Security Confidentiality R2: How can key handling be
adapted to support collaborative
use cases?
Accountability R3: How can current versioning
approaches be adapted to fit a
distributed environment?
4
Sonntag, 18. September 11

6. Integrity
‣ Data must be consistent while
→ in transfer
→ in process
→ in storage
‣ Data is handled in decrypted form within client but
stored encrypted into the cloud
→ Consistency check must guard data in all
representations
5
Sonntag, 18. September 11

7. Dual Integrity
Decrypted Data Encrypted Data
‣ Data is wrapped in XML ‣ XML is mapped to pages
‣ Tree-structure to offer ‣ Pages are encrypted
recursive checksums ‣ Pages are ordered within
hierarchy to offer
versioning and
consistency checks
6
Sonntag, 18. September 11

8. Decrypted Integrity Check
<?xml version="1.0" encoding="UTF-8"?>
...
<country id='f0_404'
3 lr9c
name='Switzerland'
capital='f0_1627'
population='7207060' sl24 19ak
datacode='SZ'
4 7
total_area='41290'
...>
...
</country>
5 6 67 8 9
...
56bd 234g mbkl 1117 345v
3 lr9c 3 997d
19ak 8h5y 19ak
sl24 4 7 4 7
5 6 67 8 9 5 6 8 9
56bd 234g mbkl 1117 345v 56bd 234g 1117 345v
7
Sonntag, 18. September 11

9. Decrypted Integrity Check
<?xml version="1.0" encoding="UTF-8"?>
...
<country id='f0_404'
3 lr9c
name='Switzerland'
capital='f0_1627'
population='7207060' sl24 19ak
datacode='SZ'
4 7
total_area='41290'
...>
...
</country>
5 6 67 8 9
...
56bd 234g mbkl 1117 345v
3 lr9c 3 997d
19ak 8h5y 19ak
sl24 4 7 4 7
5 6 67 8 9 5 6 8 9
56bd 234g mbkl 1117 345v 56bd 234g 1117 345v
R. Merkle: A digital signature based on a conventional encryption function
Advances in Cryptology, ’86
7
Sonntag, 18. September 11

10. Decrypted Integrity Check
5e+05
! Merkle−Hash !
!
!
!
Incremental Checksum !
!
No Hashing !
!
!
!
!
!
5e+04
!
!
!
!
Time[ms]
5e+03
!
!
5e+02
!
5e+01
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
xmark factor[f*0.001]
8
Sonntag, 18. September 11

11. Decrypted Integrity Check
5e+05
! Merkle−Hash !
!
!
!
Incremental Checksum !
!
No Hashing !
!
!
!
!
!
5e+04
!
!
!
!
Time[ms]
5e+03
!
!
5e+02
!
5e+01
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
xmark factor[f*0.001]
S. Graf, S. K. Belle, and M. Waldvogel, Rolling boles, optimal XML structure integrity for updating
operations, in Poster on the 20th International Conference on World Wide Web
8
Sonntag, 18. September 11

12. Encrypted Integrity Check
Uber Uber
Indirect Indirect Indirect
Indirect Indirect Indirect
RevRoot,0 RevRoot,0 RevRoot,1
Indirect Indirect Indirect
Name
Name
Name
Indirect Indirect Indirect
Node,1 Node,2 Node,1 Node,2 Node,1 Node,3
J. Bonwick, M. Ahrens,V. Henson, M. Maybee, and M. Shellenbaum, “The zettabyte file
system,” in FAST 2003: 2nd Usenix Conference on File and Storage Technologies.
9
Sonntag, 18. September 11

13. Summary of archiving integrity
Goals Measurements
Integrity Dual Integrity
Security ✓ Checks of decrypted data on XML within
Treetank
✓ Checks of encrypted data in the cloud and
within the transfer based on pages
✓ All integrity structures are persisted
10
Sonntag, 18. September 11

14. Confidentiality
‣ Achieved due to encryption of the data
→ Straightforward
‣ Supporting collaborative workflows
→ Key-Management must take place on a trusted third-
party
11
Sonntag, 18. September 11

15. Versakey
23 3 23 3
TEK 2 TEK' 2
1
01 0 01' 0
E0 (010 )
0
E 010 (T EK )
E23 (T EK 0 )
M. Waldvogel, G. Caronni, D.Sun, N. Weiler, B. Plattner: “The VersaKey framework:Versatile
group key management” IEEE Journal on Selected Areas of Communication 1999
12
Sonntag, 18. September 11

16. Key Management on the Data
DEK
01
DEK
0
01
1
DEK
23
2 Key Manager
DEK 23 3
23
DEK 2
3 1
01 0
Sonntag, 18. September 11

17. Key Management on the Data
DEK
01
DEK
0
01
E0 (010 )
E010 (DEK 0 )
1
DEK E23 (DEK 0 )
23
2 Key Manager Key Manager Key Trails
DEK 23 23
3 3
23 DEK DEK' E0 (010 )
2 2
E010 (DEK 0 )
1 E23 (DEK 0 )
3
01 0 01' 0
Sonntag, 18. September 11

18. Key Management on the Data
DEK'
01' E0 (010 )
E010 (DEK 0 )
DEK
0
01
E0 (010 )
E010 (DEK 0 )
1
DEK' E23 (DEK 0 )
23
2 E23 (DEK 0 ) Key Manager
DEK' 23 3
23 DEK' 2
3
15 01' 0
Sonntag, 18. September 11

19. Summary of archiving confidentiality
Goals Measurements
Confidentiality VersaKey
Security ✓ Encryption itself straightforward
✓ Key handling supports changing client-sets
✓ Exposing & supporting versioning
✓ Different handling of keys (within Key Manager)
and updates (within the cloud storage)
16
Sonntag, 18. September 11

20. Accountability
“Accountability is the requirement that actions of an entity may
be traced uniquely to that entity.”
‣ Achieved due to versioning the data
→ Tracing of insertions, deletions and modifications
‣ Together with signatures on the action
→ Non-Repudiation of modifications
G.Stoneburner: Underlaying technical models for information technology security
National Institute of Standards and Technology
17
Sonntag, 18. September 11

21. Classic Approaches
Differential Differential
4 5
3 4
2 4
1 …
0 0
18
Sonntag, 18. September 11

22. Classic Approaches
Differential Differential
4 5
3 4
2 4
1 …
0 0
Incremental Incremental
4 6
3 5
2 4
1 …
0 0
18
Sonntag, 18. September 11

23. Paging the data
Uber Uber
Indirect Indirect Indirect
Indirect Indirect Indirect
RevRoot,0 RevRoot,0 RevRoot,1
Indirect Indirect Indirect
Name
Name
Name
Indirect Indirect Indirect
Node,1 Node,2 Node,1 Node,2 Node,1 Node,3
J. Bonwick, M. Ahrens,V. Henson, M. Maybee, and M. Shellenbaum, “The zettabyte file
system,” in FAST 2003: 2nd Usenix Conference on File and Storage Technologies.
19
Sonntag, 18. September 11

24. Summary of archiving accountability
Goals Measurements
Accountability Adaptive Pagelayer
✓ Each modification on the data results in one
Security subtree
✓ Versions consists out of modifications &
injected full-dumps
✓ Robustness and consistent read-write-effort
20
Sonntag, 18. September 11

25. Putting everything together
Client Server
Treetank PaaS-Implementation
XML Data Interf. Storage
Data Store
node layer Key Trail Propag.
Key Trails
page layer
storage layer
Local Keys Key Manager
DEK Key Trail Propag.
01
NoSQL File 0
Access Ctrl. Key Mgmt.
21
Sonntag, 18. September 11

26. Workflow of Change on Clientset
Ext.Tigger Cloud Storage Key Mg
23 3
DEK 2
1
Delete
Client "1" 01 0
E0 (010 )
E010 (DEK 0 )
E23 (DEK 0 )
POST
Key Trails
22
Sonntag, 18. September 11

27. Workflow of Storage
Client Cloud Storage
Session
begin
Keycheck
Challenge
Keycheck Validate
Hash
H(H(DEK) ⊕ Challenge)
Recompute
Keys
Key Sync
[Keys differ]
Send Key Trails
Data Validate
Request
Send read/write request
Data
[Hashs differ]
Request resend
Session
close
23
Sonntag, 18. September 11

28. the new insertion operation.
Figure 7. Shredding and Serializing of XMark
XMark, Time
XMark serialize, f=0.1
XMark serialize, f=1.0
XMark shredding, f=0.1
2e+05
5e+04 XMark shredding, f=1.0
[ms]
2e+04
5e+03
0 20 40 60 80 100
revisions
24
Sonntag, 18. September 11 Figure 7 shows the result. The shredding represents the insertion process while the serializing stand

29. Figure 8. Performing random insert, Time
Random Insert, Time
20000
5000
2000
250 nodes per commit
[ms]
500 nodes per commit
1000 nodes per commit
1000
500
200
100
0 200 400 600 800 1000
revisions
25
Sonntag, 18. September 11

30. Figure 9. Performing random insert, File
Random Insert, Space
2e+06
5e+05
[bytes]
250 nodes per commit
1e+05
500 nodes per commit
1000 nodes per commit
2e+04
5e+03
0 200 400 600 800 1000
revisions
26
Sonntag, 18. September 11

31. Next Steps
Dual Integrity
✓Implementation of XML-check
‣ Improving performance within cryptographic checksums
‣ Extending with signatures
‣ Introducing page-based integrity-check
VersaKey
✓Implementation of encryption
✓Versakey implementation
‣ Access to old revisions within new client-joins
27
Sonntag, 18. September 11

32. Next Steps, .cont
Versioning
✓Implementation and first results
✓Analysis of read-/write-effort
‣ Working directly on Versioning
Framework
✓ Client partly released
✓ Key Management
‣ Server
28
Sonntag, 18. September 11

33. Thanks for your attention…
Questions?
(or even better: Suggestions!)
Sebastian Graf
Distributed Systems Group
University of Konstanz
sebastian.graf@uni-konstanz.de
29
Sonntag, 18. September 11

34. Publications
1. S.Graf, M.Kramis, M.Waldvogel, "Distributing XML with Focus on Parallel
Evaluation" in Proceedings of the 6th Workshop on DBISP2P
2. S. Graf, L. Lewandowski, and M. Waldvogel, “Integrity assurance for
RESTful XML,” in Proceedings of the 7th Workshop on Web Information
Systems
3. S. Graf, M. Kramis, and M. Waldvogel, “Treetank: Designing a versioned
XML storage,” in XMLPrague’11, 2011.
4. S.Graf, V.Zhouldev, L. Lewandowski, and M. Waldvogel, “Hecate, managing
authorization with restful xml,” in Proceedings of the 2nd Workshop on
RESTful Services,
5. S. Graf, S. K. Belle, and M. Waldvogel, “Rolling boles, optimal XML
structure integrity for updating operations,” in Poster on the 20th
International Conference on World Wide Web, ser. WWW ‘11.2011
6. Trailing Versioning (joint work with Marc Kramis, in progress)
7. Versakey on distributed storage (in planning)
30
Sonntag, 18. September 11