This document discusses cryptography and searchable encryption. It defines cryptography as the science of secret communication and encryption as the process of hiding information using ciphers and keys. It describes the different types of cryptosystems including symmetric, asymmetric, and searchable encryption. It also explains the basic concepts and algorithms involved in searchable encryption such as encryption, indexing, trapdoor generation, and searching on encrypted data without decryption. Several examples of searchable encryption schemes are provided.

2. O Cryptography, derived from Greek words
krptos- secret and graphy- writing
O It is the science of using mathematics to hide
information.
O With the help of Cryptography we can store
sensitive information, or transmit it over insecure
networks (such as the internet) so that it can only
be read by the intended recipient.

3. O Cryptography is the study or science of secret
communication, while encryption is simply a
component of that science. Encryption is the
process of hiding information, through the use of
ciphers, from everybody except for the one who
has the key.
O Encryption is a direct application of cryptography
O In today's digital world, there are two major types
of ciphers--one based on symmetric and one
based on asymmetric key algorithms.

4. O Plaintext: message to be sent, in readable
form
O Ciphertext: message in coded form,
unreadable without special information such
as a key
O cryptanalysis: cracking a code - attempting
to decrypt without the required special
information
O Key: A key in cryptography is a long
sequence of bits used by encryption and
decryption algorithms.

5. O Encryption is the process of converting
readable data (called the plaintext) into a
form which hides its content, called the
ciphertext.
O Decryption is the reverse process, with a
ciphertext converted back into the
corresponding plaintext.

6. O A given encryption algorithm takes the
plaintext , and a key, and alters the original
message mathematically based on the key's
bits to create a new encrypted message.
O Likewise, a decryption algorithm takes an
encrypted message and restores it to its
original form using one or more keys
O When a user encodes a file, another user
cannot decode and read the file without the
decryption key

7. O To decode cipher, a user must possess the
appropriate decryption key.
O A decryption key consists of a random string of
numbers, from 40 through 2,000 bits in length.
The key imposes a decryption algorithm onto the
data.
O This decryption algorithm reverses the
encryption algorithm, returning the data to
plaintext.
O The longer the encryption key is, the more
difficult it is to decode. For a 40-bit encryption
key, over one trillion possible decryption keys
exist.

8. O SYMMETRIC ENCRYPTION: This is the most
common type of encryption and uses the
same key for encoding and decoding data.
This key is known as a session key.
O ASYMMETRIC ENCRYPTION(Public-key
encryption): uses two different keys, a
public key and a private key. One key
encodes the message and the other
decodes it. The public key is widely
distributed while the private key is secret.

11. O Searchable encryption is a cryptography primitive
that enables users to search through outsourced
encrypted data without exposing keywords to the
untrusted server [song et al[2]]
O Having the ability of secure search through
outsourced encrypted data should be the core
feature of any searchable encryption
O Secure search through outsourced encrypted data
means authorized user is the only one can search
for any keywords within outsourced data and
unauthorized parties should not learn anything
during search procedure.

12. O The immediate application for searchable
encryption is cloud storage where the client
outsources its storage, but encrypts its files for
confidentiality beforehand and retains the key.
O The advantage compared to standard encryption
is that the cloud can perform the search operation
without the key and only return a matching subset
for a query.
O Hence, the client does not have to download the
entire data set and search himself.
O In many cases this is an enormous efficiency gain.

13. O SE consists of three operations.
O ENCRYPTION: Encryption transforms a keyword/file pair
using a secret key into a ciphertext.
O TOKEN GENERATION: Using the secret key one can
generate a search token for a specific keyword. Using this
token,
O SEARCH: one can then search in a set of ciphertexts for
those that match the keyword. Hence, one can encrypt, but
still search without decryption.
O searchable encryption is composed of two steps
O Storing a special encryption of data on the untrusted third
party (Store phase),
O Make an encrypted search query to retrieve the desired
information (Search phase).

15. O Searchable encryption can be categorized in
two fields according to number of involved
key:
O Symmetric searchable encryption (SSE)
O Asymmetric searchable encryption (ASE)

16. O The main feature of this model of private
search is that the user, who encrypts data, is
the only user who can perform a search or
an update.
O This fact is due to the use of a private key
that belongs only to the user and who
cannot share with others.
O The symmetric scheme was introduced by
GOLDREICH AND R. OSTROVSKY [1] and
supposes that the user encrypts his data
with a secret key, stores it in the untrusted
server, can retrieve his encrypted data then
decrypts it with the same key.

17. O The first searchable symmetric encryption
scheme proposed by Song et al[2]. In the
Song’s scheme, all the words within every
document have to be encrypted in a double
layer ciphertext form called inner layer and
outer layer.
O Server strips the outer layer by using the
trapdoor and checks the inner layer. In order
to make the same verifiable inner layer
structure trapdoor and ciphertext have to be
generated by using the same keyword.

18. O The first index-based SSE proposed by Goh
[3]. Goh’s scheme is based on making a
secure index of all the words in a document,
which uses multiple different hash
functions.
O The method of making that index and
searching within that is called bloom filter,
which is used in spread spectrum of
applications in various areas.
O Then Curtmola [8] proposed another two
inverted index-based SSEs, where its search
time cost is O(1).

19. O SSE Consists of four (4) algorithms:
1. Keygen (k): This algorithm takes a security
parameter k and generates a secret key K.
2. BuildIndex (K, D): The index I is produced by
using the secret key K and the document
collection D.
3. Trapdoor(K, w): The trapdoor Tw of the word
w is produced from w and the secret key K.
4. Search(I, T w): Search documents in the
collection D that contain the keyword w
using the trapdoor Tw.

20. 1. Fuzzy Keyword Search over Encrypted Data
in Cloud Computing (FKSEDCC) proposed by
Jin Li. et al. [7]
2. Phrase Search over Encrypted Data with
Symmetric Encryption Scheme proposed by
Tang et al.[9]

21. O The multiuser search allows the owner of the
encrypted data to share with other users the
right to search over these encrypted data.

22. O In the M-SSE, the data owner can grant or revoke a
user from the group of privileged users.
O To add a user, the owner has to give him a key Ku
O The server manages the user revocation by
checking for each search queries whether the user
has his privilege or not.
O Indeed, each time the owner adds or deletes a user,
he generates a new value R, and then he sends it to
the server. Using this information, the server may
verify the permission of the user.
the group of revoked users changes
dynamically, and thus even if a user has a key he
cannot retrieve the documents using his trapdoor.

23. O ASE is a scheme that enables other parties
besides the data owner to make queries to
the server, as long as having access to the
owner’s public key
O Public key Encryption with Keyword Search
(PEKS) is a primitive with such functionality
that provides delegation of exact-match
searches. Boneh et al. in 2004 and Baek et
al
O ASE schemes are appropriate in an setting
where the party searching over the data is
different from the party that generates it.

24. O ASE allows searches to be carried over
cipher-texts, through delegation, and by
means of trapdoors issued by the owner of
the data.
O As it is important that cipher-texts preserve
data privacy, it is also important that
trapdoors do not expose the user's search
criteria.
O The main advantage of ASE is functionality
while the main disadvantages are
inefficiency and weaker security guarantees.

25. O ESE schemes are appropriate in any setting where the
party that searches over the data is different from the
party that generates it and where the keywords are
hard to guess
O The main advantage of efficient ASE is that search is
more efficient than (plain) ASE.
O The main disadvantage, however,is that ESE schemes
are also vulnerable to dictionary attacks.
O mSSE schemes are appropriate in any setting where
many parties wish to search over data that is
generated

26. O In a mSSE scheme, in addition to being able to
encrypt indexes and generate tokens, the owner of the
data can also add and revoke users’ search privileges
over his data.

27. O The concept of a PEKS scheme was proposed by
Boneh et al.[10] and Baek et al[11]. who extended
PEKS scheme into a secure channel free PEKS
scheme (SCF-PEKS) which removes the assumption,
a secure channel between users and a server.
O Public Key Encryption with Keyword Search (PEKS in
short) scheme, which is also name searchable public-
key encryption scheme, enables one to search
encrypted documents on the untrusted server without
revealing any information.

28. O To construct a secure PEKS or SCF-PEKS scheme with
privacy protection, there are some security
requirements needed to achieve as follows:
O Trapdoor indistinguishability
O Ciphertext indistinguishability
O Authorized identity protection (Anonymity)
O User authentication

29. O Each user may have access to a different set of
documents stored on the server; this can be achieved
by ensuring that each document is encrypted with a
separate per-document key, and arranging for each
user’s client machine to have access to the keys of
the documents that the corresponding user has
access to. Popa et al [15]
O Many applications, such as document sharing, chat,
forums, and calendars, support search over
documents shared by different users.

30. O This cryptographic scheme that allows a client to
provide a single search token to the server, but still
allows the server to search for that token’s word in
documents encrypted with different keys
O The scheme hides the content of the document and
the words one searches for, and the only information
the server learns is whether some word being
searched for matches a word in a document.

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