A Searchable Symmetric Key Cipher System

DESIGN AND ANALYSIS OF A
SEARCHABLE SYMMETRIC KEY
CIPHER SYSTEM

By:
Asanka Balasooriya
Kelum Senanayake

SECURITY REQUIREMENTS
A cryptographic scheme which enables searching
on encrypted data without revealing to the server
should have following features.

 Untrusted server should not be able to learn
anything about the plaintext given only the
ciphertext
 Untrusted server should not be able to search for
a word without authorization of the client
 The scheme should be efficient and practical

THE PROPOSED SCHEME

Let’s assume that Alice is the client who owns important data
and wants to store in Bob’s cloud storage which is untrusted.

The scheme is as follows;

 Alice builds an index with keywords that Alice may want to
search for later for all data currently she owns.
 Then she encrypts all the plain data using a symmetric
encryption key.
 All the keywords of the index are encrypted using the same
key.
 Lists of positions of the index are left as plain data.
 Store the complete index and ciphertext of the plain data on
Bob’s server.

Let’s assume the selected encryption mechanism is E with
a key k.

 The list of plaintexts of Alice = {P1, P2, P3 …. Pn}
 Corresponding ciphertexts = {C1, C2, C3, …. Cn}

 The set of selected keywords of the index = {w1, w2, w3
…. wm}
 Then the index with encrypted keywords is as follows for
a keyword wi
 Ek(wi) ---------->> {Cp, Cq, Cr, …. Ct}

SEARCHING FOR A SPECIFIC KEYWORD
WJ IS AS FOLLOWS
 Alice computes Ek(wj) value and send it to Bob.
 Bob searches the index entries for Ek(wj) and finds a
match.
 Bob reads the list of positions associated with
encrypted keyword sent by Alice for the search in the
index.
 Bob sends the list of ciphertexts fetching using the list
of positions to the Alice as the search result.
 Alice decrypts ciphertexts to access the corresponding
plaintexts.

SECURITY STRENGTHS OF THE
SCHEME
 Bob is unable to identify the content of the data
since both data and keywords are encrypted.
 The proposed scheme provides controlled
searching where Bob is not be able to search
without the authorization of Alice
 Bob knows nothing more than the encrypted
search result for an encrypted keyword if he tries
to do a search which is not asked by Alice

SECURITY ATTACKS

Basically statistical attacks can be done on this
scheme in two ways.

 Bob may statistically analyze the index to
identify certain keywords. Although the
keywords are encrypted, Bob still is able to
identify most common words which are
associated with large set of documents.
 Bob may observe number of search requests and
statistically identify access patterns of Alice.

SOLUTIONS
 Statistical attacks on index can be prevented by
maintaining lists of document positions for keywords
in fixed size lists. For words that appear infrequently,
Alice can pad the list to the fixed size. For more
common words, Alice can split the long list into several
lists with the fixed size. Then to search for such a
word, Alice will need to ask Bob to perform and merge
several search queries in parallel.
 The second kind of attacks is hard to avoid unless
performing several unwanted searches together with
regular searches. This way Bob can be misled to
identify false access patterns.

WEAKNESSES OF THE SCHEME
 Alice must update the index whenever she
changes her documents.
 Ideal for read only data storages.

 Bob might be able to learn through updating the
index.
 If Alice does not change the index for a key word
when she adds a new document to the server, then
Bob is able to know that the keyword is not included
in the newly created document.
 Alice must update substantial part of the index to
hide real updates.

REFERENCES

[1] D. X. Song, D. Wagner, and A. Perrig.
Practical techniques for searches on encrypted
data. In IEEE Symposium onSecurity and
Privacy, 2000.

[2] Changyu Dong, Giovanni Russello and
Naranker Dulay. No Shared Keys: Multi-user
Searchable Data Encryption. Department of
Computing, Imperial College London.

A Searchable Symmetric Key Cipher System

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