1. ISSN: 2277 – 9043
International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 5, July 2012
Data Security and Authentication using
Steganography and STS protocol
1 2 3
Shaik Riyaz J. Rajakala M RamaKrishna
Abstract: Steganography differs from cryptography in the For many years Information Hiding has captured the
sense that where cryptography focuses on keeping the contents imagination of researchers. Digital watermarking and
of a message secret, steganography focuses on keeping the steganography techniques are used to address digital rights
existence of a message secret. Steganography and management, protect information, and conceal secrets.
cryptography are both ways to protect information from Information hiding techniques provide an interesting
unwanted parties but neither technology alone is perfect and
challenge for digital forensic investigations. Information
can be compromised. Once the presence of hidden information
is revealed or even suspected, the purpose of steganography is can easily traverse through firewalls undetected.
partly defeated. The strength of steganography can thus be
amplified by combining it with cryptography. This paper proposes a new approach to public-key
In this paper, we design a system, which uses features of steganography based on matching method to hide the secret
both cryptography as well as steganography. We proposed a information inside 24-bit image file. In the proposed
method which describes two stages for sending the method, the stego-key is generated by applying a public
information securely by using the Steganography based on key exchange protocol which is based on STS protocol.
matching method and to perform key exchange using STS
protocol which guarantees authentication. This is done in Diffie-Hellman session key agreement is the first key
following steps: exchange protocol, proposed by Diffie and Hellman.
1.Encrypt the message using any one of the popular Public-
Diffie-Hellman key exchange by itself achieves perfect
Key Encryption Algorithms, so that only authorized parties
can only be able to read the message. forward secrecy because no long-term keying material
2. Find and share stego-key between the two exists at the end of the session to be disclosed. However, it
communication parties over insecure networks by applying does not provide authentication of the communicating
Station To Station(STS) Protocol. parties; hence it is vulnerable to a man-in-the-middle
3. Sender uses the secret stego-key to select pixels that it attack.
will be used to hide the message obtained in first step. Each
selected pixel used to hide 8 bits of information. In order to fix the security flaw in the Diffie-Hellman
protocol, the Station-To-Station (STS) protocol was
Keywords: Public-Key Cryptography, Steganography, Stego- proposed in [12]. To add authentication, the STS protocol
key, STS protocol. requires both the parties to have a pair of public keys for
signature generation and verification, and to know a
1. INTRODUCTION publicly released symmetric key encryption. In contrast,
note that the Diffie-Hellman protocol does not have these
Steganography is the science of hiding selected assumptions. These assumptions can be included into the
information from a third party. Therefore, steganography in protocol by sending public key certificates if the keys are
contrast with cryptography, where the existence of the not known in advance. In the STS protocol, STS protocol
message is clear, but the meaning is obscured. uses signatures to authenticate the communicating parties.
Steganography applications conceal information in other, It encrypts the signatures with the session key
seemingly innocent media. Steganographic results may subsequently to show the knowledge of this session key.
masquerade as other file for data types, be concealed within However, signatures and certificates cause the messages to
various media, or even hidden in network traffic or disk increase considerably in size.
space. There are many ways in which information and data
can be exploited to conceal additional information. The goal of an authentication protocol is to provide the
communicating parties with some assurance that they know
Manuscript received July 20, 2012. each other’s true identities. In an authenticated key
Shaik Riyaz M.Tech Student in CSE dept, A.S.R college of exchange, there is the additional goal that the two parties
Engineering, riyaz.shaik62@gmail.com,Tetali,Tanuku,West Godavari end up sharing a common key known only to them. This
(DT),Andhra Pradesh, India.
J.Rajakala, Asst Prof in CSE dept, A.S.R college of Engineering secret key can then be used for some time thereafter to
rajajaladi@gmail.com, Tetali, Tanuku, West Godavari (DT),Andhra provide privacy, data integrity, or both. In this paper, we
Pradesh, India discuss the security of public-key based authentication
M.Ramakrishna, M.Tech Student in CSE dept, Sana engineering protocols, with and without an associated key exchange.
college mathe.ramakrishna@gmail.com, kodad, Nalgonda(dt),Andhra
Pradesh,India
We restrict our attention to two-party mutual
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International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 5, July 2012
authentication, rather than multi-party and one-way Diffie–Hellman key exchange is the most widely used
authentication protocols. We assume that individual public key distribution system. Diffie–Hellman key
underlying cryptographic mechanisms are not vulnerable, exchange is a specific method of exchanging cryptographic
and restrict our attention to attacks on protocols themselves. keys. It is one of the earliest practical examples of key
An enemy (attacker, intruder, adversary) can see all exchange implemented within the field of cryptography.
exchanged messages, can delete, alter, inject, and redirect The Diffie–Hellman key exchange method allows two
messages, can initiate communications with another party, parties that have no prior knowledge of each other to jointly
and can reuse messages from past communications. establish a shared secret key over an insecure
communications channel. This key can then be used to
We are concerned with both authentication and key encrypt subsequent communications using a symmetric key
exchange. It is now well accepted that these topics should cipher.
be considered jointly rather than separately [Baus89]. A
protocol providing authentication without key exchange is Diffie-Hellman key agreement is not based on
susceptible to an enemy who waits until the authentication encryption and decryption, but instead relies on
is complete and then takes over one end of the mathematical functions that enable two parties to generate a
communications line. Such an attack is not precluded by a shared secret key for exchanging information confidentially
key exchange that is independent of authentication. Key online. Essentially, each party agrees on a public value g
exchange should be linked to authentication so that a party and a large prime number p . Next, one party chooses a
has assurances that an exchanged key (which might be used secret value x and the other party chooses a secret value y .
to facilitate privacy or integrity and thus keep authenticity Both parties use their secret values to derive public values,
alive) is in fact shared with the authenticated party, and not g x mod p and g y mod p, and they exchange the public
an impostor. For these reasons, it is essential to keep key values. Each party then uses the other party's public value
exchange in mind in the design and analysis of to calculate the shared secret key that is used by both
authentication protocols. parties for confidential communications. A third party
cannot derive the shared secret key because they do not
know either of the secret values, x or y .
2. RELATED WORK
2.1 Public-key cryptography: For example, Alice chooses secret value x and sends the
public value gx mod p to Bob. Bob chooses secret value y
Public-key cryptography refers to a cryptographic system and sends the public value g y mod p to Alice. Alice uses
requiring two separate keys, one of which is secret and one the value g xy mod p as her secret key for confidential
of which is public. Although different, the two parts of the communications with Bob. Bob uses the value g yx mod p
key pair are mathematically linked. One key locks or as his secret key. Because g xy mod p equals g yx mod p ,
encrypts the plaintext, and the other unlocks or decrypts the Alice and Bob can use their secret keys with a symmetric
cyphertext. Neither key can perform both functions. One of key algorithm to conduct confidential online
these keys is published or public and the other is kept communications. The use of the modulo function ensures
private. that both parties can calculate the same secret key value,
but an eavesdropper cannot. An eavesdropper can intercept
Public key cryptography uses asymmetric key algorithms the values of g and p , but because of the extremely
be referred to by the more generic name of "asymmetric difficult mathematical problem created by the use of a large
key cryptography." The algorithms used for public key prime number in mod p, the eavesdropper cannot feasibly
cryptography are based on mathematical relationships (the calculate either secret value x or secret value y . The secret
most notable ones being the integer factorization and key is known only to each party and is never visible on the
discrete logarithm problems) that have no efficient solution. network.
Although it is computationally easy for the intended
recipient to generate the public and private keys, to decrypt
the message using the private key, and easy for the sender
to encrypt the message using the public key, it is extremely
difficult for anyone to derive the private key based on their
knowledge of the public key. This is why, unlike symmetric
key algorithms, a public key algorithm does not require a
secure initial exchange of one, or more, secret keys between
the sender and receiver.
In contrast, symmetric-key algorithms, variations of
which have been used for thousands of years, use a single
secret key — which must be shared and kept private by
both sender and receiver — for both encryption and
decryption. To use a symmetric encryption scheme, the
sender and receiver must securely share a key in advance.
Figure 1 Diffie–Hellman key exchange Algorithm
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International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 5, July 2012
the exponential gy.
The main problem with Diffie–Hellman exchange is that, 3) Bob computes the shared secret key K = (gx)y.
it does not provide authentication of the communicating 4) Bob concatenates the exponentials (gy, gx) (order is
parties and is thus vulnerable to a man-in-the-middle attack. important), signs them using his asymmetric key B,
A person in the middle may establish two distinct Diffie– and then encrypts them with K. He sends the cipher
Hellman key exchanges, one with Alice and the other with text along with his own exponential gy to Alice.
Bob, effectively masquerading as Alice to Bob, and vice 5) Alice computes the shared secret key K = (gy)x.
versa, allowing the attacker to decrypt (and read or store) 6) Alice decrypts and verifies Bob's signature.
then re-encrypt the messages passed between them. The 7) Alice concatenates the exponentials (gx, gy) (order
man-in-the-middle attack in cryptography and computer is important), signs them using her asymmetric key
security is a form of active eavesdropping in which the A, and then encrypts them with K. She sends the
attacker makes independent connections with the victims cipher text to Bob.
and relays messages between them, making them believe 8) Bob decrypts and verifies Alice's signature.
that they are talking directly to each other over a private
connection, when in fact the entire conversation is Alice and Bob are now mutually authenticated and have
controlled by the attacker. a shared secret. This secret, K, can then be used to encrypt
further communication. The basic form of the protocol is
A key exchange protocol enables two parties to share a formalized in the following three steps:
common key for encrypting a large amount of data.
Authentication is an essential requirement prior to the key (1) Alice → Bob : gx
exchange process in order to prevent man-in-the-middle (2) Alice ← Bob : gy, EK(SB(gy, gx))
attack. The goal of an authentication protocol is to provide (3) Alice → Bob : EK(SA(gx, gy))
the communicating parties with some assurance that they
know each other’s true identities. In an authenticated key Full STS setup data can also be incorporated into the
exchange, there is the additional goal that the two parties protocol itself. Public key certificates may be sent in steps
end up sharing a common key known only to them. This 2 and 3 if the keys are not known in advance.
secret key can then be used for some time thereafter to
provide privacy, data integrity, or both. (1) Alice → Bob : gx
(2) Alice ← Bob : gy, CertB, EK(SB(gy, gx))
A method to authenticate the communicating parties to (3) Alice → Bob : CertA, EK(SA(gx, gy))
each other is generally needed to prevent this type of attack.
Variants of Diffie-Hellman, such as STS, may be used If system-wide key establishment parameters are not
instead to avoid these types of attacks. used, the initiator and responder may create and send their
own parameters. In this case, parameters should be sent
2.2 STS protocol: with the exponential.
1) Alice → Bob : g, p, gx
The STS protocol consists of Diffie-Hellman key They must also be verified by Bob to prevent an active
establishment [Diff76], followed by an exchange of attacker from inserting weak parameters (and thus a weak
authentication signatures. In public-key cryptography, the key K). Diffie, van Oorschot & Wiener (1992) recommend
Station-to-Station (STS) protocol is a cryptographic key against special checks to prevent this and instead suggest
agreement scheme based on classic Diffie-Hellman that including the group parameters in Alice's certificate. In
provides mutual key and entity authentication. STS protocol, for just authentication is as follows.
The basic idea of STS protocol is as follows. Prior to 1. Alice generates a random number x sends it to Bob.
execution of the protocol, the two parties Alice and Bob 2. Bob generates a random number y.
each obtain a public/private key pair and a certificate for the 3. Bob concatenates the random numbers (y, x) (order is
public key. During the protocol, Alice computes a signature important) and signs them using his asymmetric key B.
on certain messages, covering the public value ga mod p. He sends the signature along with his own random
Bob proceeds in a similar way. Even though Carol is still number to Alice.
able to intercept messages between Alice and Bob, she 4. Alice verifies Bob's signature.
cannot forge signatures without Alice's private key and 5. Alice concatenates the random numbers (x, y) (order is
Bob's private key. Hence, the enhanced protocol defeats the important) and signs them using her asymmetric key
man-in-the-middle attack. A. She sends the signature to Bob.
6. Bob verifies Alice's signature.
Supposing all setup data has been shared, the STS Formally:
protocol proceeds as follows. If a step cannot be completed, (1) Alice → Bob : x
the protocol immediately stops. All exponentials are in the (2) Alice ← Bob : y, SB(y, x)
group specified by p. (3) Alice → Bob : SA(x, y)
1) Alice generates a random number x and computes
and sends the exponential gx to Bob. 2 .3 Steganography:
2) Bob generates a random number y and computes
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Steganography is the art and science of hiding files. If an 8-bit image is viewed as a grid and the grid is
information into covert channels so as to conceal the made up of cells, these cells are called pixels. Each pixel
information and prevent the detection of the hidden consists of an 8-bit binary number (or a single byte), and
message (refer to figure2). The goal of steganography is to each 8-bit binary number refers to the color palette (a set of
avoid drawing attention to the transmission of a hidden colors defined within the image). All color variations for
message. If suspicion is raised, then this goal is defeated. the pixels are derived from three primary colors: red, green,
Today, steganography refers to hiding information in digital and blue. Each primary color is represented by 1 byte (= 8
picture files and audio files. Modern steganography refers bits).
to hiding information in digital picture files and audio files.
It works by replacing bits of unused data in regular digital
files with bits of invisible information. To embed hidden
information into an image requires two files - the cover
image file that will hold the hidden data and the secret
message file. A message may be plain text, cypher text (or
another image). When combined, the cover image and the
hidden message makes a stego image. A stego-key or
password may be used to hide and decode the message.
Special software is needed for steganography. In this
tutorial we will look at two programs that hide text within
images(refer to figure 3).
Figure 4:A Graphical Version of the Steganographic System
There are many ways to hide information in digital
images. We look at the following approaches :
1. least significant bit insertion
2. masking and filtering
3. algorithms and transformations
Least significant bit insertion:
Many stego tools make use of least significant bit (LSB).
For example, 11111111 is an 8-bit binary number. The
Figure 2: Inaccessible information using steganography rightmost bit is called the LSB because changing it has the
least effect on the value of the number. The idea is that the
LSB of every byte can be replaced with little change to the
overall file. The binary data of the secret message is broken
up and then inserted into the LSB of each pixel in the
image file.
Hiding the data
Using the Red, Green, Blue (RGB) model a stegno tool
makes a copy of an image palette, say, an 8-bit image. The
copy is rearranged so that colors near each other in the
RGB model are near each other in the palette. The LSB of
each pixels 8-bit binary number is replaced with one bit
Figure 3: A Steganographic system from the hidden message. A new RGB color in the copied
palette is found. A new 8-bit binary number of the new
RGB color in the original palette is found. The pixel is
Steganographic messages may first be encrypted and changed to the 8-bit binary number of the new RGB color.
then a cover message is modified to contain the encrypted
message, resulting in stegno text. Only those who know the Recovering the data can be done by using stegno tool,
technique used can recover the message and, if required, stegno tool finds the 8-bit binary number of each pixels
decrypt it. The message may be a few thousand bits (often RGB color. The LSB of each pixel's 8-bit binary number is
at 7 or 8 bits per text character) embedded in millions of one bit of the hidden data file. Each LSB is then written to
other bits. Probably the most typical use is digital images. an output file.
Digital images are commonly stored in either 24-bit or 8-bit A simplified example with an 8-bit image
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pixel: Secret data
(00 01 10 11)
white red green blue Public key encryption
Insert(0011)
(00 00 11 11) Cipher text Image file
message
white white blue blue
Convert Ascii Pixels to binary
As can be seen from the example, with an 8-bit image,
to binary
the cover image must be carefully selected since LSB
manipulation is not as forgiving because of the color
limitations. To hide information in the LSBs of each byte of
a 24-bit image, it is possible to store 3 bits in each pixel.
A simplified example with a 24-bit image
Encoding using
Matching method and
pixel: stegno key
(00100111 11101001 11001000)
Binary form to pixel
Insert(101)
(00100111 11101000 11001001)
Stegno image file, sent
to the receiver
red green blue
LSB insertion works well with gray-scale images as well.
It is possible to hide data in the least and second least Figure 5. Proposed steganography mechanism for sender
significant bits and the human eye would still not be able to
discern it. B. Receiver Side
After reception of Stego image the receiver at the start
3. PROPOSED METHOD converts the pixels into the binary values. The decoder
using Matching method and stegno key then removing the
3.1 Public Steganography in various selected regions of encrypted data from image pixel values. The encrypted data
an image: is decrypted using decryption algorithms. This is how, the
A. Sender Side plain text is recovered from image. Fig. 6 shows the whole
process at the receiver side.
The proposed scheme uses any public key encryption
algorithm to encrypt secret information, encrypted ASCII Stegno image file
value is converted in binary form.
The pixels in the image at the same time are also
Pixels to binary
converted into binary form. The same image is now used as
a cover to embed the encrypted information. The process
starts as a Encoder using Matching method and stegno key
which replaces the three least significant bits of pixel values Decoding using
with the encrypted information bits. The modified picture is Matching method and
now termed as Stego image. The whole process is stegno key
explained in Fig. 5.
As shown below, STS protocol is used to exchange
secret stegno key between two communication parties. Convert binary Binary to pixels
to ASCII
Sender
Cipher text Cover Image
message file
STS Protocol
Secret message
Receiver Figure 6. Proposed steganography mechanism for receiver
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The proposed method describes two steps for hiding the steganography, our aim is to improve it by removing one of
secret information by using the public steganography based its problems that is: the ease of extraction. We don't want
on matching method in different regions of an image. that a malicious attacker is able to read everything we are
sending.
The First step is converting the Plain text message into
cipher text using Public-key Encryption algorithm. We can use the following techniques to achieve our
goal:
The next step is to find the shared stego-key between the
two communication parties (SENDER & RECIPIENT) over  Encryption of the message, so that who extracts
insecure networks by applying STS protocol (as explained it must also decrypt it before it makes sense.
above). At the end the protocol, each side recovers his/her
 Randomizing the placement of the bits using a
received public key to reach the shared values between
cryptographical random function (scattering), so
them, that’s mean SENDER & RECIPIENT have arrived
that it's almost impossible to rebuild the message
same sego-key value. without knowing the seed for the random
function.
Next the sender uses the secret stego-key to select pixels
that it will be used to hide. Each selected pixel is then used In this way, the message is protected by two different
to hide 8 bits binary information depending on the matching keys, acquiring much more confidentiality than before.
method which is summarized in four cases as shown by This approach protects also the integrity of the message,
Table 2. Since the 8 bits data will be compared with the being much more difficult (we could say at least
selected pixel's bytes, red, green & blue values respectively computationally infeasible) to counterfeit the message.
to produce an array of binary values as 00, 01, 10, and 11.
SENDER's side, starts comparing to search the equality, Second, there is a problem with the file size that involves
where, he takes data value and compare it with the value of the choice of the format. Unusually big files exchanged
the red color (± 7 – decimal value). As shown by Table 2, between two peers, in fact, are likely to arise suspicion.
case no. 1, if they are equal, then the value zero (00 – Since we need to have small image file sizes, we should
binary value) is set to the array. Table 2, case no. 2, if the resort in using 24-bit images, because their size is more
data value and the red value are not equivalent then the likely to be considered as normal.
value will be compared with the green color, if they are
equals (± 7 – decimal value) then the array is set to be one
(01 - binary value). Table 2, case no. 3, if the data value and
the green value are not equivalent then the value will be
compared with the blue color, if they are equals (± 7 –
decimal value) then the value two (10 – binary value) is set
to the array. Finally (refer to Table 2, case no. 4), If in case
the secret data didn’t equal any of the previous three
conditions then the LSBs method is used to embed the data
inside the selected pixel, and the value three (11 – binary
value) is set to the array. In this case, the data value will be
distributed as follows: FIGURE 4: THE RESULT OF EMBEDDING THE TEXT WITH S-TOOLS
1. The first three bits of the data are replaced by the three
least significant bits of the red byte. 4. RESULTS AND DISCUSSION
2. The second three data bits are replaced by the three least We implemented the public-key steganography based on
significant bits of the green byte. matching method in different selected regions of an image
to show the performance of the proposed method.
3. The last two data bits are replaced by the two least
significant bits of the blue byte. In our implementation, we used 600×400 bitmap image
file to hide 5 KB text data. As discussed earlier, both of the
If 8 bit data ≈ Red Then Red value = two communication parties should find the secret key
Case 1 00
(8 bit ) 8 bit data (stegokey) first by applying STS protocol to perform high
If 8 bit data ≈ Then Green value level of security.
Case 2 01 As in Table 2, the 8 bits data will be hidden inside 1
Green (8 bit ) = 8 bit data
pixel, hence the 600x400, 24 bit image file can accept
If 8 bit data ≈ Blue Then Blue value =
Case 3 10 approximately 240000 bytes of data. This is compared with
(8 bit ) 8 bit data well known stego method such as LSBs (Johnson et al.,
Case 4 Otherwise Use LSBs Method 11 1998) which needs 3 pixels to hide 1 byte of data. We can
also adjust the bit-rate at which we can hide the data in the
TABLE 2: THE FOUR MAIN CASES IN THE PROPOSED PUBLIC-KEY STEGO selected region. Nevertheless, the proposed steganographic
protocol is more efficient than LSBs, since the algorithm
3.2 Problems and Possible solutions used the matching method to get identical pixel's bytes.
As we have seen LSB insertion is good for
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However, the proposed method resorts to the LSBs ACKNOWLEDGMENTS
method to distribute the secret data in case if the 8 bit of
data is not matched with any of the previous three bytes I thank all the staff members of A.S.R college of
(red, green, and blue). engineering for their support. I would also like to thank
my family and friends who encouraged me in doing this
CONCLUSION work.
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