INTRODUCTION
An internet video streaming provides different features such as digital television,
mobile TV, video conferencing and video in demand. Once the video stream goes
beyond simple public communications and we start thinking about the possibilities,
we realize that a lot of applications are not possible without some kind of
data security (Abomhara et al., 2010a). We should
not send information over public networks without some kind of reliable protection.
The answer to cover this need in video streaming is no different than the answer
anywhere else and it is cryptography (Naji et al.,
2009a, b).
Cryptography can protect video streaming in different ways. It can provide
encryption and decryption so that the video can be read only by authorized receivers.
It provides a means to ensure that video reach their destinations without being
tempered with. It provides ways to ensure the identity of communicating parties,
making sure that none of them can deny that he/she had sent or received a specific
video (Khalifa et al., 2004; Alaa
et al., 2009).
AN OVERVIEW OF CRYPTOGRAPHY
Secured transmission/storage media against eavesdroppers is a very important
task in applications such as commercial TV broadcast and video conferencing.
However, cryptography (encryption and decryption) is being the science of protecting
data. It can be applied to video streams at the transmitters and receivers (Rabah,
2006). Cryptanalysis is when hackers break the cryptography algorithms and
decipher the encoded data (Naji et al., 2009a).
It is a very sophisticated science to break the secrecy of encryption algorithms
and reveal the encoded data. Thus, the main goal of cryptography is to keep
the data secure from unauthorized individuals. Since cryptography’s first
known usage in ancient Egypt (Kessler, 1998), it has
passed through various stages and has been affected by the ways in which people
handled information. In the World War II, for instance, Cryptography played
an important role and was a key element that gave the allied forces the upper
hand, enabling them to win the war sooner, as they were able to dissolve the
Enigma cipher machine, which the Germans used to encrypt their military secret
communications (Kahn, 1980).
In modern days, cryptography is no longer limited to secure sensitive military
information (Abomhara et al., 2010a). It was
recognized as one of the major components of the security policy of any organization
and considered industry standard for providing information security, trust,
controlling access to resources and electronic financial transactions. The original
data which is to be transmitted or stored is called plaintext and this can be
read and understood by either a person (a document) or by a computer. The disguised
data, meanwhile, is called ciphertext and neither a human nor a machine can
read it or properly process it until it is decrypted. A system or product that
provides encryption and decryption is called cryptosystem (Zaidan
et al., 2009a). Cryptosystem uses encryption algorithms to determine
how simple or complex the encryption process will be, the necessary software
component and the key (usually a long string of bits), which work with the algorithm
to encrypt and decrypt the data (White, 2003; Harris,
2007).
In encryption, the key is a piece of information which specifies the particular
transformation of plaintext to ciphertext, or vice versa during decryption.
Encryption key is based on the keyspace, which is the range of the values that
can be used to assemble a key. The larger the keyspace, the more possible keys
can be constructed (e.g., today we commonly use key sizes of 128, 192,or 256
bit , so the key size of 256 would provide a 2^{256 }keyspace) (Abomhara
et al., 2010a). Moreover, the strength of the encryption algorithm
relies on the secrecy of the key, the length of the key, the initialization
vector and how they all work together. Depending on the algorithm and the length
of the key, the strength of encryption can be considered (Zaidan
et al., 2009b, 2010).
THE BASIC CONCEPTS OF VIDEO ENCRYPTION
The encryption and decryption of a plain text or a video stream can be done in two ways. In some of the encryption technologies, when two end points need to communicate with one another via encryption, they must use the same algorithm and most of the time, the same key. In other encryption technologies, they must use different but related keys for encryption and decryption purposes. Cryptography algorithms are either symmetric algorithms, which use symmetric keys (also called secret keys), or asymmetric algorithms, which use asymmetric keys (also called public and private keys).
Symmetric key algorithms: As shown in Fig. 1, in symmetric
key encryption, the sender and receiver use the same key for encryption and
decryption. Symmetric key encryption is also called secret key because both
the sender and receiver have to keep the key secret and properly protected.
If two users want to exchange data using secret key encryption, both of them
must obtain a copy of the same key (Khalifa et al.,
2004).
If one wants to communicate with the other person, then he needs to have three
separate keys, one for each one. It sounds like is not a big deal, but if one
wants to communicate with hundreds of people, keeping track and using the correct
key that corresponds to each specific receiver can be a daunting task.

Fig. 1: 
Symmetric key algorithm 
The more people he wants to communicate with, the more number of keys he needs
to keep. The equation used to calculate the number of symmetric keys needed
is N (N1)/2 = number of keys, where, N represents the number of users (Harris,
2007) (Abomhara et al., 2010a). Basically,
the security level of the symmetric keys encryption method is totally dependent
on how well the users keep the keys protected. If the key is known by an intruder,
then all the data encrypted using that key can be decrypted. This is what makes
it more complicated how symmetric keys are practically shared and updated when
necessary. If one wants to communicate with another for the first time, he has
to figure out how to send the right key to the second person securely. It is
not safe to send the key by an email or have it delivered by courier to get
it to the user as the key will not be protected and can be easily intercepted
and used by attackers.
Symmetric keys can provide confidentiality but they cannot provide authentication
because there is no way one can prove, through cryptography, who actually sent
a massage if two people are using the same key. Despite all the problems and
defects that symmetric keys have, they are still used in many applications as
they are fast and can be hard to break if using a large key size. Symmetric
keys can handle a large amount of data that would take an unacceptable amount
of time with an asymmetric key to encrypt and decrypt (Kessler,
1998).
The following are the advantages and disadvantages of the symmetric key systems:
Advantages:
• 
It is much faster than asymmetric systems 
• 
Its security is dependent on the length of the key. If using
a large key size, the algorithm will be hard to break because symmetric
algorithms carry out relatively simplistic mathematical functions on the
bits during the encryption and decryption processes 
• 
It doesn’t consume too much computing power 
Disadvantages:
• 
It requires a secure mechanism to deliver keys properly 
• 
Each pair of users needs a unique key; if a user has N trading
partners, then N secret keys must be maintained so that as the number of
individuals increases, so does the number of keys 
• 
The management of the symmetric keys becomes problematic 
• 
Provides confidentiality but not authenticity because the
secret key is shared 
According to the data on which they act, the secret key cryptography schemes
are further divided into main type of subdivided. If an algorithm encrypts all
the bits in a group of blocks at the same time, then the algorithm is called
block encryption algorithm. However, if the algorithm is applied to each bit
individually, it is called stream encryption algorithm. Stream encryption dose
treats the input data as a stream of bits. The bits are then subjected to mathematical
functions individually. A key generator is needed to provide a bit key to be
XORed with the data bits and produce the encrypted stream. In block encryption
algorithms, meanwhile, the data are divided into a group of blocks of bits and
each block is encrypted one at a time (Harris, 2007).
The most popular secret key encryption algorithms are Data Encryption Standard
(DES), Triple DES (Khalifa et al., 2004) and
Advance Encryption Standard (AES).
Data Encryption Standard (DES): is one of the most important examples
of a block cipher. DES was the result of a contest set by the U.S. National
Bureau of Standards (now called the NIST) in 1973 and adopted as a standard
application in 1977. The winning standard was developed at IBM as a modification
of the previous system called LUCIFER (Alanazi et al.,
2010a; Abomhara et al., 2010a). The DES is
widely used for encryption of PIN numbers, bank transactions and the likes.
The DES is an example of a block cipher, which operates on blocks of 64 bits
at a time, with an input key of 64 bits. Every 8th bit in the input key is a
parity check bit, which means that in fact the key size is effectively reduced
to 56 bits (Rabah, 2005a).
Triple DES: was developed to address the obvious flaws in DES without
designing a whole new cryptosystem. Triple DES simply extends the key size of
DES by applying the algorithm three times in succession with three different
keys. The combined key size is thus 168 bits (3 times 56), beyond the reach
of bruteforce techniques, which are used by the EFF DES Cracker. Triple DES
has always been regarded with some suspicion since the original algorithm was
never designed to be used in this way, but no serious flaws have been uncovered
in its design and today, it is used in a number of Internet protocols (Alanazi
et al., 2010a).
Advanced Encryption Standard (AES): In 1997, the NIST called for the
submission of a new standard to replace the aging DES. The contest terminated
in November 2001with the selection of the Rijndael cryptosystem as the Advanced
Encryption Standard (AES) (Naji et al., 2009a).
The Rijndael cryptosystem operates on 128bit blocks, arranged as 4x4 matrices
with 8bit entries. The algorithm can use a variable block length and key length
and the latest specification allows for the combination of any key lengths at
128, 192, or 256 bits and blocks of length at 128, 192, or 256 bits (Abomhara
et al., 2010b; Alanazi et al., 2010a).
AES VS DES and 3DES: Advance Encryption Standard (AES) and DES (or 3DES)
are commonly used block ciphers. Whether you choose AES or 3DES depends on your
needs. In this section, we would like to highlight their differences in terms
of security, performance and summaries, as shown in Table 1.
Since 3DES is based on DES algorithm, it will talk first about DES (Alanazi
et al., 2010b; Khalifa et al., 2004).
DES, which was developed in 1977, was carefully designed to work better in
hardware than software. DES performs lots of bit manipulation in substitution
and permutation boxes in each of the 16 rounds. For example, switching bit 30
with 16 is much simpler in hardware than software. DES encrypts data in 64 bit
block size and uses a 56 bit key effectively. A 56 bit key space amounts to
approximately 72 quadrillion possibilities (Xu and Dereje,
2004). Although, it seems large, according to today’s computing power,
it is still insufficient and vulnerable to brute force attack (Alanazi
et al., 2010b). Thus, DES is not able to cope with the technological
advancement and is no longer appropriate for security. Since DES was widely
used at that time, a quick solution was to introduce 3DES, which was secure
enough for most purposes today. 3DES is a construction of applying DES three
times in sequence. 3DES, with three different keys (K1, K2 and K3), has effective
key length of 168 bits (The use of three distinct keys is recommended in 3DES).
Table 1: 
Comparative results of the symmetric encryption algorithm
(Alanazi et al., 2010b) 

Another variation is called twokey (K1 and K3 are the same) 3DES, which reduces
the effective key size to 112 bits, making it less secure. The twokey 3DES
is widely used in electronic payments industry. 3DES takes three times as much
CPU power compared to its predecessor, which is a significant performance hit.
The AES outperforms 3DES both in software and in hardware (Alanazi
et al., 2010b).
The Rijndael algorithm has been selected as the Advance Encryption Standard
(AES) to replace 3DES. The AES is a modified version of Rijndael algorithm.
It was submitted by Joan Daemen and Vincent Rijmen. When considered together,
Rijndael’s combination of security, performance, efficiency, implement
ability and flexibility made it an appropriate selection for the AES. By design,
the AES is faster in software and works more efficiently in hardware. It also
works fast in small devices such as smart phones, smart cards etc. The AES provides
more security due to a larger block size and longer keys. It uses 128 bits fixed
block size and works with 128, 192 and 256 bit keys. Generally, Rigndael algorithm
is flexible enough to work with keys and block size of any multiple of 32 bits,
with a minimum of 128 bits and maximum of 256 bits. Although the AES has abstract
advantages over 3DES for speed and efficiency, in some hardware implementation,
where support for 3DES is mature, 3DES may operate faster (Alanazi
et al., 2010b).
Asymmetric key algorithm: is also called public key algorithm. Public
Key Cryptography was first described publicly by Stanford University Professor
Martin Hellman and graduate student Whitfield Diffie in 1976 (Diffie
and Hellman, 1976). They described a two key crypto system, in which two
parties could securely communicate over a nonsecure communications channel
without having to share a secret key and address the problem of secret key distribution
by using two keys instead of a single key. In public key algorithm, two keys
are used (Rabah, 2005b). A public key is a key which
is known by everyone, while a private key should be kept secret and known only
by the owner.

Fig. 2: 
Asymmetric key algorithm 
This is shown in Fig. 2.
If the message is encrypted by one key, then the other key is required in order
to decrypt the message. The public key and private key are mathematically related.
However, it does not mean that if someone got the public key, he/she will be
able to figure out the private key. But if someone obtains the private key,
then there is big trouble as the private key should only be accessed by the
owner and no one else (Harris, 2007; Abomhara
et al., 2010b).
Each key in asymmetric key algorithm can be used to encrypt and decrypt as
they both have the capability to do so. If a data is encrypted with a private
key, it cannot be decrypted with a private key; it must be decrypted by the
corresponding public key. Public key encryption provides both confidentiality
and authentication. In the event that confidentiality is required, the sender
would encrypt the data with the receiver’s public key as in this matter,
only the person who has the corresponding private key will be able to decrypt
the data. This is called secure message format (Abomhara
et al., 2010b). On the condition that authentication is required;
the data would be encrypted with the sender’s private key. Then each person
who has the corresponding public key will be able to decrypt the data.
This allows the receiver to know that the data has been encrypted by the one
who has the possession of that private key. Encrypting the data with a private
key is called open message format, since confidentiality is not ensured. Anyone
with a copy of the corresponding public key can decrypt the data.
Asymmetric algorithms work much slower than the symmetric algorithm because they use more complex mathematics to perform their functions, which require more processing time. With public key, you can just send out your public key to all of the people whom you need to communicate with, instead of keeping track of a unique key for each one of them.
The following are the advantages and disadvantages of the Asymmetric Key Systems:
Advantages:
• 
Better key distribution than symmetric systems 
• 
Better scalability than the symmetric systems 
• 
It provides authentication and confidentiality 
Disadvantages:
• 
Works much more slower than symmetric systems 
• 
It provides mathematically intensive tasks 
One of the most used public key algorithms today is RivestShamir Adelman (RSA). This algorithm was invented in 1977 by Ron Rivest, Adi Shamir and Len Adelman. The RSA is based on the idea of factorization of integers into their prime. Assuming that A and B want to communicate with one another and B chooses two distinct large primes p and q and multiplies them together to form N, N = p*q. He also chooses an encryption exponent e, such that the greatest common divisor of e and [(p1)*(q1)] is 1. That is gcd(e,[(p1)*(q1)])=1. He computes his decryption key d, d=1/e (mod [(p1)*(q1)]). Now he makes the pair (N,e) public and keeps p and q secret. This is how to generate keys and encryption and decryption are of the following forms. For some plain text block M and ciphertext block C: C = M^{e }mod n, M = C^{d}mod n = (M^{e}) mod n = M^{ed}mod n. Both the sender and receiver must know the values of n and e and only the receiver knows the value of d. This makes a public key encryption of KU = {e, n} and private of KR {d, n}.
THE SUITABILITY OF USING SYMMETRIC KEY TO SECURE MULTIMEDIA DATA
Although, asymmetric encryption provides far more functionality, there are still many applications in which symmetric encryption are the best solution, as it does the job securely and more efficiently. Due to its nature, symmetric technology is also far less expensive to implement. Public key cryptography is not applicable for securing real time video conferencing, as its operations require a large amount of time, which is unsuitable for video conferencing. The principal aspects of the two methods are compared in Table 2.
CONCLUSION
In this study, a comparative study between symmetric and asymmetric key encryption was presented. First of all, a brief discussion about the popular cryptography algorithms was made. A general survey about the use of cryptography and how it started was highlighted. Next, the current known methods of cryptography (Symmetric key encryption and Asymmetric key encryption) were discussed and evaluated in terms of their security level and encryption speed. Subsequently, the advantages and disadvantages of each type were presented, while illustrating their usage in different applications. Apart from this, brief synopses regarding the difference between block encryption and stream encryption was given. Last but not least, some examples of the encryption algorithms were provided, whereby the AES and DES were evaluated in terms of their speed and security level, as well as their suitability to secure video data.
ACKNOWLEDGMENTS
Authors would like to express our heartfelt appreciation to all those who have helped us understand the importance of knowledge and showed us the best way to attain it. We would also like to extend our gratitude to the Multimedia University, Malaysia for their ceaseless support in making our study successful.