=Paper=
{{Paper
|id=None
|storemode=property
|title=Development of RSA with Random Permutation and Inversion Algorithm
to Secure Speech in GSM Networks
|pdfUrl=https://ceur-ws.org/Vol-867/Paper25.pdf
|volume=Vol-867
|dblpUrl=https://dblp.org/rec/conf/icwit/MeritO12
}}
==Development of RSA with Random Permutation and Inversion Algorithm
to Secure Speech in GSM Networks==
https://ceur-ws.org/Vol-867/Paper25.pdf
Development of RSA with random permutation and
inversion algorithm to secure speech in GSM networks
Khaled Merit1 and Abdelazziz Ouamri2
1
National Institute of Telecommunications and Information and communication technologies,
INT&TIC Oran, Algeria
merit1984@gmail.com
2
Signals and Images Laboratory, University of Sciences and Technology of Oran
USTO Oran, Algeria
ouamri@univ-usto.dz
ABSTRACT
Global System for Mobile Communications (GSM) is one of the most commonly used cellular
technologies in the world. One of the objectives in mobile communication systems is the
security of the exchanged data. GSM employs many cryptographic algorithms for security like
A5/1, A5/2 and A5/3. Even so, these algorithms do not provide sufficient level of security for
protecting the confidentiality of GSM. Therefore, it is desirable to increase security by
additional encryption methods. This paper presents a voice encryption method called: “RSA
with Random permutation and Inversion”, based on current voice channel, which overcomes
data channel's insufficiencies and solves the problem of penetrating the RPE-LTP vocoder by
the encrypted voice. The proposed method fulfils an end-to-end secured communication in the
GSM; insure a good compatibility to all GSM networks, and easy implementation without any
modification in these systems.
KEYWORDS : SECURITY, GSM, SPEECH CHANNEL, RSA, SPEECH CODEC
1. INTRODUCTION
Security presents a very important axis in wireless communication systems. This is
obviously because of the ubiquitous wireless medium’s nature that makes it more
susceptible to attacks. Any eavesdropper can get over to whatever is being sent over
the network through the wireless medium. In addition, the presence of communication
does not uniquely identify the originator. Besides this, any eavesdropping or tapping
cannot even be detected in a medium as ubiquitous as the wireless medium which
makes the latter situation even worse. Hence, security plays a fundamental task for the
successful operation of a mobile communication system.
To secure data in GSM, encryptions and mechanisms to grant it are obligatory. In this
paper, a new approach has been proposed which includes extra encryption RSA with
random permutation and inversion algorithm. GSM employs stream ciphers for
encryption which requires the data to be in its binary form [1]. Our encryption
technique processes directly on symbols without passing to the bit level. In addition,
this technique does not need any hardware; it is totally based on software. This
technique is much simpler than existing techniques, thus a more robust and efficient
system is achieved. The following sections discuss the proposed scheme: Section 2
enumerates the security requirements of mobile networks. Section 3 gives a quick
overview of existing GSM encryption algorithms and a variety of attacks on these
Proceedings ICWIT 2012 240
algorithms. Section 4 illustrates the proposed End-To-End encryption method. Section
5 the simulation results, and Finally, concludes this paper by summarizing the key
points and proposing related suggestions.
.
2. SECURITY REQUIREMENTS OF MOBILE NETWORKS
Security has become an essential topic in current mobile and wireless networks. As
the security procedures for such networks elevates, the tools and techniques used to
attack such networks also increases. Wireless communications security is the
measures or methods used to protect the communication between certain entities. To
protect the entity from any third party attacks, such as revealing a particular identity,
data modification or data-hijacking, eavesdropping, impersonating an identity,
Protection mechanisms are used. Devoted technologies for securing data and
communication are mandatory in wireless networks, which vary according to the
category of wireless technology deployed. Security in mobile networks handles a
diversity of issues, from authenticating a user accessing a network, to data integrity
and data encryption. GSM, like a lot of other systems with huge users’ numbers,
contains numerous precious resources that need protection against misuse and
deliberate attacks. This section highlights the GSM Network precious resources,
which are important to protect for the best of the system’s shareholders.
The facilities listed below are provided to insure security to the users of the
communication networks [3]-[7]:
Confidentiality: This means that the transmitted information is only disclosed to the
authorized parties. Sensitive information disclosed to an adversary could have severe
consequences.
Integrity: This assumes that a message is not altered in transit between sender and
receiver. Messages could be corrupted due to network malfunctioning or malicious
attacks.
Authentication: Authentication guarantees the identity of the entity with which
communications are established, before granting it the access to the resources of the
network. In the absence of authentication mechanisms, an attacker could masquerade
as a legitimate entity and attempt to violate the security of the network.
Nonrepudiation: This means that the source of a message cannot deny having sent
the message. An attacker could generate a wrong message that appears to be initiated
from an authorized party, with the aim of making that party the guilty one. If non-
repudiation is guaranteed, the receiver of a wrong message can prove that the
originator has transmitted it, and that, therefore, the originator misbehaved.
Access control: Access control means that only authorized parties can be allowed to
access a service on the network, use a resource, or participate in the communications;
Proceedings ICWIT 2012 241
any other entity is denied access. The access control assumes the authentication of the
entity trying to get access to the network.
Network availability: Availability ensures that all resources of the communications
network are always utilizable by authorized parties. An attacker may launch a Denial
of Service (DoS) attack by saturating the medium, jamming the communications, or
keeping the system resources busy in any other way or by any other means. The aim
here is just to slow down or stop authorized parties from having access to the
resources, thereby making the network unusable.
3. GSM encryption and attacks
In GSM, A5 stream cipher is used [5]. Versions A5/1 and A5/2 were kept secret for a
long period of time. Briceno et al reverse-engineered A5/1 and A5/2 from a GSM
handset and published them. After which, attacks were rapidly found for these
algorithms. The principal problem is the small key length of the session key Kc. The
actual length of Kc is 64 bits. However, only 54 bits are effective. Even though this
key size is sufficiently big to protect against real-time attacks, the hardware state
available today makes it possible to record the packets between the mobile subscriber
and the BTS and then decrypt them afterward [6].
Biryukov et al. found a known-key stream attack on A5/1 that needed about two
seconds of the key stream and recovers Kc in a few minutes on a PC after a large pre-
processing stage. Barkan et al. [5] have proposed a ciphertext-only attack on A5/1 that
also recovers Kc using only four frames; the problem was its complexity. A5/2 was
also cracked and proved to be totally vulnerable. The attack needed very few pseudo
random hits and only 216 steps [5].
A new security algorithm, known as A5/3 provides users of GSM mobile phones with
an even higher level of protection against eavesdropping than they have already. A5/3
is based on the Kasumi algorithm, specified by 3GPP for use in 3G mobile systems.
The A5/3 encryption algorithm particularly provides signaling protection to protect
important information such as telephone numbers as well as user data protection to
secure voice calls and other user generated data. This algorithm were so far assumed
to be stronger than A5/1 and A5/2, but the Biham et al attack shows that the key can
be obtained quickly without applying exhaustive key search.
4. END-TO-END encryption method
4.1. Review of GSM Voice Transmission
The process of GSM voice channel transmission illustrated in Figure 1, includes five
components: A/D module, RPE-LTP vocoder, channel coding/decoding module,
wireless encryption /decryption module, and GMSK modulation/demodulation
module. The wireless encryption/decryption module only works on wireless channel.
So it cannot provide end-to-end secure communication in GSM system.
Proceedings ICWIT 2012 242
Channel
Voice A/D RPE-LTP Coding/
Vocoder Decoding
GMSK Wireless
Modulation/ Encryption /
Demodulation Decryption
Figure 1. GSM voice transmission process.
4.2. Review of GSM Voice Transmission
The RPE-LTP [4]-[2] is an important algorithm in the field of voice encoding. It is not
only used in GSM, but also used in Internet.
At the transmitter, the processing in the RPE-LTP Encoder includes pre-processing,
LPC analysis, short-term analysis filtering, Long-Term Prediction and Regular Pulse
Excitation sequence coding. The details is described as following: first Encoder
samples original digital voice signal at 8kHz sampling rate, and removes the direct
current component, then it can make use of FIR filter to pre-emphasis the high
frequency. Secondly, LPC analysis takes every 160 sample points (20ms) as one frame
and figures out 8 logarithm acreage ratio parameter for each frame. Short-time analysis
filter produces LPC residual signal. It removes redundancy farther coding with RPE-
LTP, and outputs 260 bits coding every frame at last. At the receiver, it practices a
reverse processing and rebuilds the original speech signal.
4.3. Voice Encryption Method
In a general way, encryption/decryption module is put before the RPE-LTP vocoder,
which is easy to implement in MT (Mobile Terminal). But it cannot accomplish the
end-to-end secured communication, and need to be modified in BS (Base Station). So a
novel voice encryption/decryption method is proposed based on voice channel, which
can fulfill the end-to-end secured communication without any modification in BS. The
novel voice encryption/decryption scheme is depicted in Figure 2.
Proceedings ICWIT 2012 243
Voice
Voice A/D Encryption/ RPE-LTP
Decryption Vocoder
Module
Wireless Channel
GMSK Encryption / Coding/
Modulation/ Decryption Decoding
Demodulation
Figure 2.Voice encryption module access point in MT.
In Figure 2, the new voice encryption/decryption module is inserted between A/D and
RPE-LTP vocoder in MT. The coming voice signal from the A/D module would
firstly arrive to the newly-added Voice Encryption/Decryption module and finishes
the encryption. After that, it is sent to the RPE-LTP vocoder. Hence, this encryption
method must penetrate the RPE-LTP vocoder and have ideal encryption intensity.
Simultaneously this encrypted signal can be recovered to get the original
understandable speech at the receiver. This new voice encryption method is a kind of
signal source encryption technology, so it could achieve the end-to-end secured
communication.
4.4. Encryption algorithm
Principle of the encryption algorithm:
For implementing encryption algorithm, we follow the following steps:
- Decomposition of a speech signal in sub-frame, each frame is represented by an
index.
- Encrypting data with inversion and random permutation algorithm, which gives the
permutation indexes.
- Encrypting these indexes with RSA algorithm.
- Used these indexes to decrypt the signal.
In this paper, we propose an algorithm that combines between permutation and
inversion of the voice signal samples, giving as a result the permuted indexes. These
indexes are processed in an encryption/decryption module by RSA algorithm, and
finally, these encrypted permuted indexes are added to the compressed encrypted
voice signal samples after the RPE-LTP module. So it has a good recovery character
to RPE-LTP vocoder, and its encryption intensity also can meet the special
requirement. The algorithm is mainly intended to make the encrypted voice signal to
be similar to the natural human voice signal, and can penetrate the RPE-LTP vocoder,
and then it can execute all the encryption and decryption process. (See Figure 3).
Proceedings ICWIT 2012 244
Original
Signal
Segmentation
Random permutation and
inversion
Permuted
Index
Signal
Encryption
Encryption
permuted
index by
RPE-LTP RSA
Novel Encrypted Permuted Index Encrypted
compressed Insertion in the permuted
encrypted compressed encrypted
Index
signal signal
Figure 3. Encryption chart
Proceedings ICWIT 2012 245
5. Simulation results
This section presents the results for the proposed method adopted in Section 3. This
section also discusses the obtained results from implementing the system. In order to
implement such a system, one must go through several steps which were described in
details in the preceding sections. The implementation for this simulated project is
written by MATLAB.
a.1 Original temporal signal b.1 Spectrogram of original Signal
a.2 Encrypted temporal signal b.2 Spectrogram of encrypted
before LPC signal before LPC
Proceedings ICWIT 2012 246
a.3 Synthesized encrypted temporal b.3 Spectrogram of Synthesized
signal. encrypted Signal.
a.4 Synthesized deciphered temporal b.4 Spectrogram of Synthesized
signal. deciphered Signal.
A
Proceedings ICWIT 2012 247
C. Comparison between the original signal and the synthesized signal.
In figure C, the margin between the original signal (blue) and the synthesized one
(red) is due to the reduction of the bit rate imposed by the RPE-LTP module.
6. Conclusion
In this paper, a novel kind of encryption method is proposed to fulfill the end-to-end
secured communication in the GSM voice channel. The new encryption method
solves the problem that traditional encryption algorithms cannot be used in voice
channel directly because of RPE-LTP vocoder requirements in GSM system. In
addition, this encryption method has the advantages of suiting the RPE-LTP
compression module requirements, good compatibility to GSM networks, and suitable
implementation without any adjustment in current GSM signalling system. The
algorithm presented in this paper is made by the RSA algorithm, but it can be done
also by other encryption methods such as: DES, RC4 and AES.
REFERENCES
[1] David G. W. Birch and Ian J. Shaw, “Mobile communications security private or public”,
IEEE, June 1994.
[2] ETSI Speech processing functions General Description, GSM06.01-1999, Version 8.0.1
pp22-53.
[3] H.Imai, M.G. Rahman, K. Kobara "Wireless Communications Security” ARTECH
HOUSE 2006.
[4] K.Hellwig, P.Vary, D. Massaloux, ectl. Speech Codec for the European Mobile Radio
System. IEEE Global Commu Conf, 1989: pplO65-1069.
Proceedings ICWIT 2012 248
[5] Ross Anderson, Mike Roe "A5 - The GSM Encryption Algorithm", 1994.
[6] Dr. S. Muhammad Siddique and Muhammad Amir “GSM Security Issues and
Challenges” (SNPD’06) 2006.
[7] Noureddine Boudriga "Wireless Communications Security”,CRC 2010 by Taylor and
Francis Group, LLC
Proceedings ICWIT 2012 249