Accumulo requires its users to trust each Accumulo installation with their data — a malicious server or user could easily compromise critical data or learn secrets they are not authorized to access. One particular threat is a malicious Accumulo server compromising data’s integrity, by tampering with query results and returning forged, modified, or incomplete results to a user. In prior work, we implemented a lightweight client-side tool to protect against this kind of threat. We now present improvements to this tool that handle a wider range of attacks by a malicious server and reduce overhead for the client. In our solution, Accumulo clients use Authenticated Data Structures (ADSs) to verify their range queries’ integrity. ADS metadata is stored in Accumulo, so that after each query, the server must construct a proof that the query has not been tampered with. We use Accumulo iterators to compute these proofs on the server without requiring an unnecessary computational burden from the client. We will present our approach to adding ADSs to Accumulo, our schema for storing the ADS metadata, and opportunities for future work in efficiency and expressiveness. – Speaker –  Leo St. Amour Military Fellow, MIT Lincoln Laboratory Leo St. Amour is a master’s student at Northeastern University and a military fellow at MIT Lincoln Laboratory. He graduated from the United States Military Academy in May 2015, where he worked on a TLS library with enhanced usability and security. In addition to his work on TLS and Accumulo, he is currently working on binary analysis, with a focus on discovering and hardening security properties. — More Information — For more information see http://www.accumulosummit.com/

1. Embedding Authenticated Data Structures in
Accumulo
Leo St. Amour
Cassandra Sparks, Robert K. Cunningham, Scott Ruoti, Emily Shen
October 11, 2016
DISTRIBUTION STATEMENT A. Approved for public release: distribution unlimited.
This material is based upon work supported by the Department of Defense under Air Force Contract
No. FA8721-05-C-0002 and/or FA8702-15-D-0001. Any opinions, findings, conclusions or
recommendations expressed in this material are those of the author(s) and do not necessarily reflect
the views of the Department of Defense.
© 2016 Massachusetts Institute of Technology.
Delivered to the U.S. Government with Unlimited Rights, as defined in DFARS Part 252.227-7013 or
7014 (Feb 2014). Notwithstanding any copyright notice, U.S. Government rights in this work are
defined by DFARS 252.227-7013 or DFARS 252.227-7014 as detailed above. Use of this work other
than as specifically authorized by the U.S. Government may violate any copyrights that exist in this
work.

2. Embedded ADSs in Accumulo - 2
LS 10/11/16
Querying
Clients
Threats to Accumulo
Network
Data
Sources
Threat 1: Malicious clients
Threat 2: External attacker Threat 3: Malicious cloud or admins
• Make unauthorized queries
• Learn stored data
• Learn other clients’ queries
• Insert malware
• Hack or compromise
system
• Learn contents of data & queries
• Modify, forge, or delete data
• Misattribute data to honest clients
Today’s focus: security against a malicious cloud or system
administrators
Malicious System Administrator System Administrator

3. Embedded ADSs in Accumulo - 3
LS 10/11/16
Secure Accumulo Overview
Querying
Clients
Network
Data
Sources
Data in motion
encryption
Accumulo provides
no safeguards!
Data at rest encryption
Malicious System Administrator System Administrator

4. Embedded ADSs in Accumulo - 4
LS 10/11/16
Secure Accumulo Overview
Querying
Clients
Network
Data
Sources
Digital
signatures
Field-level
encryption
Verifiable
query results
Our goal: use cryptography to protect against malicious insiders
Cryptographically
enforced access control
Malicious System Administrator System Administrator

5. Embedded ADSs in Accumulo - 5
LS 10/11/16
Outline
• Introduction
• Prior Work: Accumulo Summit 2015
– Digital Signatures
• Authenticated Data Structures
• Embedding Authenticated Data Structures in Accumulo
• Conclusion

6. Embedded ADSs in Accumulo - 6
LS 10/11/16
Using Digital Signatures with Accumulo
Case 1: Correct Response

7. Embedded ADSs in Accumulo - 7
LS 10/11/16
Using Digital Signatures with Accumulo
Case 2: Partially Forged Response

8. Embedded ADSs in Accumulo - 8
LS 10/11/16
Using Digital Signatures with Accumulo
Case 3: Incomplete Response
Clients cannot use signatures to detect maliciously omitted query results

9. Embedded ADSs in Accumulo - 9
LS 10/11/16
Integrity Protection Security Enforcement
Integrity
Protection
Insertion Modification Omission
Signatures
Authenticated Data
Structures

10. Embedded ADSs in Accumulo - 10
LS 10/11/16
Microbenchmark: Authenticated Skip Lists*
vs Signatures
* Goodrich, and Tamassia. "Efficient authenticated dictionaries with skip
lists and commutative hashing." Johns Hopkins Information Security
Institute (2000).
Authenticated data structures are capable of improving both performance and
security over digital signatures

11. Embedded ADSs in Accumulo - 11
LS 10/11/16
Our Contributions
• Scheme for embedding an Authenticated Data Structure in Accumulo
– Allows for clients to verify that query results are correct and complete
• Prototype client-side library as proof-of-concept
– Written in Python
– Coming soon
• Java library
– Leverages iterators for improved efficiency
– Designed as “drop-in” API
– Coming 1st Quarter of CY-17
Contact: pace-contact@ll.mit.edu

12. Embedded ADSs in Accumulo - 12
LS 10/11/16
Outline
• Introduction
• Prior Work: Accumulo Summit 2015
• Authenticated Data Structures
– Skip List Properties
– Querying Over Skip Lists
• Embedding Authenticated Data Structures in Accumulo
• Conclusion

13. Embedded ADSs in Accumulo - 13
LS 10/11/16
digest
The digest is a small value
(constant size) that represents
the entire dataset
• Data structures that allow provably correct and complete query results
– Correctness defined by a trusted data owner
– Need to support range queries
VO
Data
Source
Querying
Client ?
VO
ADS
Authenticated Data Structures
ADS: Authenticated DataStructure
VO: Verification Object

14. Embedded ADSs in Accumulo - 14
LS 10/11/16
Skip Lists
6
-∞ ∞
-∞
-∞
-∞
∞
∞
∞
2 4 86
2
6
10 12
-∞ ∞
8
8
8
12
Elements can be inserted in
any order: insertion takes
expected O(log(n)) time

15. Embedded ADSs in Accumulo - 15
LS 10/11/16
Authenticated Skip Lists*
6
-∞ ∞
-∞
-∞
-∞
∞
∞
∞
2 4 86
2
6
10 12
-∞ ∞
8
8
8
12
1
2
Pruning edges based on query
paths lets authenticated
operations be O(log(n))
* Goodrich, and Tamassia. "Efficient authenticated dictionaries with skip
lists and commutative hashing." Johns Hopkins Information Security
Institute (2000).

16. Embedded ADSs in Accumulo - 16
LS 10/11/16
… …
… …
…
…
Node Labeling
6
2 4 6
2 0
0
a = h(4, 6)
b = h(2, a)
h(b, 0)
A node’s label depends on all subsequent nodes
6
642
2
Note: h is a cryptographic hash function

17. Embedded ADSs in Accumulo - 17
LS 10/11/16
Authenticated Skip Lists*
digest
6
-∞ ∞
-∞
-∞
-∞
∞
∞
∞
2 4 86
2
6
10 12
-∞ ∞
8
8
8
12
range(1, 5)
* Goodrich, and Tamassia. "Efficient authenticated dictionaries with skip
lists and commutative hashing." Johns Hopkins Information Security
Institute (2000).

18. Embedded ADSs in Accumulo - 18
LS 10/11/16
Outline
• Introduction
• Prior Work: Accumulo Summit 2015
• Authenticated Data Structures
• Embedding Authenticated Data Structures in Accumulo
– API Overview
• Conclusion

19. Embedded ADSs in Accumulo - 19
LS 10/11/16
Storing Skip Lists in Accumulo
6
8
8
8
12
Skip List Structure Accumulo
n2
n5
n3
n1
n4
Row Column Value
ADS n1
ADS n2
ADS n3
ADS n4
ADS n5
All data for each node is stored server-side in a single column
{elem : 8,
label : bGFiZWwgZm9yIG5vZGUgbjM=,
down : “n5”,
right : “n4”,
parent : {name: “n1”, dir: “from_up”}}

20. Embedded ADSs in Accumulo - 20
LS 10/11/16
Client-side ADS Computations
• ADS prototype has clients compute VOs based on data stored
in Accumulo, resulting in high communication overhead
Accumulo
Query
Parent node
ADS metadata
Parent node

21. Embedded ADSs in Accumulo - 21
LS 10/11/16
Server-side ADS Computations
• Using Accumulo iterators, we can perform all computation
server-side and return the result
• This avoids the communication overhead of client-side
computations
Accumulo
Query
Compute
verification
object
Return verification object

22. Embedded ADSs in Accumulo - 22
LS 10/11/16
Outline
• Introduction
• Prior Work - Accumulo Summit 2015
• Authenticated Data Structures
• Embedding Authenticated Data Structures in Accumulo
– API Overview
• Conclusion

23. Embedded ADSs in Accumulo - 23
LS 10/11/16
API Overview

24. Embedded ADSs in Accumulo - 24
LS 10/11/16
API Overview

25. Embedded ADSs in Accumulo - 25
LS 10/11/16
Java Implementation

26. Embedded ADSs in Accumulo - 26
LS 10/11/16
Java Implementation

27. Embedded ADSs in Accumulo - 27
LS 10/11/16
Java Implementation
• Iterating over VerifyScanner
– Applies custom Accumulo Iterator to metadata scanner
– Custom Iterator extends RowEncodingIterator

28. Embedded ADSs in Accumulo - 28
LS 10/11/16
Java Implementation
• VerifyScanner only returns results if the query has been verified

29. Embedded ADSs in Accumulo - 29
LS 10/11/16
Outline
• Introduction
• Prior Work: Accumulo Summit 2015
• Authenticated Data Structures
• Embedding Authenticated Data Structures in Accumulo
• Conclusion

30. Embedded ADSs in Accumulo - 30
LS 10/11/16
Limitations
• Current implementation returns range query boundaries in verification object
– Leaks boundary elements
• Verification objects do not support visibility fields
– Proof may contain data that the client is not authorized to access

31. Embedded ADSs in Accumulo - 31
LS 10/11/16
Future Work
• Develop a method for verifying completeness without leaking boundaries
• Develop a “visibility aware” authenticated data structure

32. Embedded ADSs in Accumulo - 32
LS 10/11/16
Conclusion
• Scheme for embedding an Authenticated Data Structure in Accumulo
– Allows for clients to verify that query results are correct and complete
• Prototype client-side library as proof-of-concept
– Written in Python
– Coming soon
• Java library
– Leverages iterators for improved efficiency
– Designed as “drop-in” API
– Coming 1st Quarter of CY-17
Contact: pace-contact@ll.mit.edu