Thursday, September 30, 2010

What is a Digital Signature?



What is a Digital Signature?
An introduction to Digital Signatures


Tom


(Tom's public key)

(Tom's private key)
Tom has been given two keys. One of Tom's keys is called a Public Key, the other is called a Private Key.
Tom's Co-workers:



   Anyone can get Tom's Public Key, but Tom keeps his Private Key to himself
Jerry
Henry
Jenny
Tom's Public key is available to anyone who needs it, but he keeps his Private Key to himself. Keys are used to encrypt information. Encrypting information means "scrambling it up", so that only a person with the appropriate key can make it readable again. Either one of Tom's two keys can encrypt data, and the other key can decrypt that data.
Jenny (shown below) can encrypt a message using Tom's Public Key. Tom uses his Private Key to decrypt the message. Any of Tom's coworkers might have access to the message Jenny encrypted, but without Tom's Private Key, the data is worthless.

"Hey Tom, how about lunch at Taco Bell. I hear they have free refills!"
HNFmsEm6Un BejhhyCGKOK JUxhiygSBCEiC 0QYIh/Hn3xgiK BcyLK1UcYiY lxx2lCFHDC/A


HNFmsEm6Un BejhhyCGKOK JUxhiygSBCEiC 0QYIh/Hn3xgiK BcyLK1UcYiY lxx2lCFHDC/A
"Hey Tom, how about lunch at Taco Bell. I hear they have free refills!"
With his private key and the right software, Tom can put digital signatures on documents and other data. A digital signature is a "stamp" Tom places on the data which is unique to Tom, and is very difficult to forge. In addition, the signature assures that any changes made to the data that has been signed can not go undetected.


To sign a document, Tom's software will crunch down the data into just a few lines by a process called "hashing". These few lines are called a message digest. (It is not possible to change a message digest back into the original data from which it was created.)


Tom's software then encrypts the message digest with his private key. The result is the digital signature.
Finally, Tom's software appends the digital signature to document. All of the data that was hashed has been signed.

Tom now passes the document on to Jerry.
First, Jerry's software decrypts the signature (using Tom's public key) changing it back into a message digest. If this worked, then it proves that Tom signed the document, because only Tom has his private key. Jerry's software then hashes the document data into a message digest. If the message digest is the same as the message digest created when the signature was decrypted, then Jerry knows that the signed data has not been changed.
Plot complication...

Henry (our disgruntled employee) wishes to deceive Jerry. Henry makes sure that Jerry receives a signed message and a public key that appears to belong to Tom. Unbeknownst to Jerry, Henry deceitfully sent a key pair he created using Tom's name. Short of receiving Tom's public key from him in person, how can Jerry be sure that Tom's public key is authentic?
It just so happens that Jenny works at the company's certificate authority center. Jenny can create a digital certificate for Tom simply by signing Tom's public key as well as some information about Tom.

Tom Info:
    Name
    Department
    Cubical Number
Certificate Info:
    Expiration Date
    Serial Number
Tom's Public Key:
         


Now Tom's co-workers can check Tom's trusted certificate to make sure that his public key truly belongs to him. In fact, no one at Tom's company accepts a signature for which there does not exist a certificate generated by Jenny. This gives Jenny the power to revoke signatures if private keys are compromised, or no longer needed. There are even more widely accepted certificate authorities that certify Jenny.
Let's say that Tom sends a signed document to Jerry. To verify the signature on the document, Jerry's software first uses Jenny's (the certificate authority's) public key to check the signature on Tom's certificate. Successful de-encryption of the certificate proves that Jenny created it. After the certificate is de-encrypted, Jerry's software can check if Tom is in good standing with the certificate authority and that all of the certificate information concerning Tom's identity has not been altered.
Jerry's software then takes Tom's public key from the certificate and uses it to check Tom's signature. If Tom's public key de-encrypts the signature successfully, then Jerry is assured that the signature was created using Tom's private key, for Jenny has certified the matching public key. And of course, if the signature is valid, then we know that Henry didn't try to change the signed content.
Although these steps may sound complicated, they are all handled behind the scenes by Jerry's user-friendly software. To verify a signature, Jerry need only click on it.


Wednesday, September 29, 2010

Base Station Installation Best Practices

split-mount installation with single-point building ground

GPS Installation Guidelines


A GPS unit is required for the synchronization of the TDD radio network. With the Time
Division Duplex TDD operation each sector should transmit and receive at the same
time, to avoid intra-cell and inter-cell interference. The GPS unit must be installed
outdoors. The receiver’s antenna should be installed somewhere on the supporting
structure without any obstructions on the way to the open sky.
For details about the installation of the GPS unit, please refer to the guidelines of the
GPS manufacturer as there is difference in mounting assembly of different manufacturers.

Figure shows standard guidelines for GPS installation.

Thursday, September 23, 2010

Basic Antenna concepts

(note that basic antenna concepts may make reference to vehicular installation.  While this installation situation is not applicable to wireless broadband, the fundamental theory is very similar)
Antenna
An antenna is a device that transmits and/or receives electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned to the same frequency band that the radio system to which it is connected operates in, otherwise reception and/or transmission will be impaired.



WavelengthWe often refer to antenna size relative to wavelength. For example: a half-wave dipole, which is approximately a half-wavelength long. Wavelength is the distance a radio wave will travel during one cycle. The formula for wavelength is:




Note: The length of a half-wave dipole is slightly less than a half-wavelength due to end effect. The speed of propagation in coaxial cable is slower than in air, so the wavelength in the cable is shorter. The velocity of propagation of electromagnetic waves in coax is usually given as a percentage of free space velocity, and is different for different types of coax.
Impedance Matching
For efficient transfer of energy, the impedance of the radio, the antenna, and the transmission line connecting the radio to the antenna must be the same. Radios typically are designed for 50 ohms impedance and the coaxial cables (transmission lines) used with them also have a 50 ohm impedance. Efficient antenna configurations often have an impedance other than 50 ohms, some sort of impedance matching circuit is then required to transform the antenna impedance to 50 ohms. Radiall/Larsen antennas come with the necessary impedance matching circuitry as part of the antenna. We use low loss components in our matching circuits to provide the maximum transfer of energy between the transmission line and the antenna.

VSWR and Reflected Power
The Voltage Standing Wave Ratio (VSWR) is an indication of how good the impedance match is. VSWR is often abbreviated as SWR. A high VSWR is an indication that the signal is reflected prior to being radiated by the antenna. VSWR and reflected power are different ways of measuring and expressing the same thing.

A VSWR of 2.0:1 or less is considered good. Most commercial antennas, however, are specified to be 1.5:1 or less over some bandwidth. Based on a 100 watt radio, a 1.5:1 VSWR equates to a forward power of 96 watts and a reflected power of 4 watts, or the reflected power is 4.2% of the forward power.
Bandwith
Bandwidth can be defined in terms of radiation patterns or VSWR/reflected power. The definition used in this book is based on VSWR. Bandwidth is often expressed in terms of percent bandwidth, because the percent bandwidth is constant relative to frequency. If bandwidth is expressed in absolute units of frequency, for example MHz, the bandwidth is then different depending upon whether the frequencies in question are near 150, 450, or 825 MHz.


Directivity and Gain
Directivity is the ability of an antenna to focus energy in a particular direction when transmitting or to receive energy better from a particular direction when receiving. The relationship between gain and directivity: Gain = efficiency/Directivity. We see the phenomena of increased directivity when comparing a light bulb to a spotlight. A 100 watt spotlight will provide more light in a particular direction than a 100 watt light bulb, and less light in other directions. We could say the spotlight has more "directivity" than the light bulb. The spotlight is comparable to an antenna with increased directivity. An antenna with increased directivity is hopefully implemented efficiently, is low loss, and therefore exhibits both increased directivity and gain.
Gain is given in reference to a standard antenna. The two most common reference antennas are the isotropic antenna and the resonant half-wave dipole antenna. The isotropic antenna radiates equally well in "all" directions. Real isotropic antennas do not exist, but they provide useful and simple theoretical antenna patterns with which to compare real antennas. An antenna gain of 2 (3 dB) compared to an isotropic antenna would be written as 3 dBi. The resonant half-wave dipole can be a useful standard for comparing to other antennas at one frequency or over a very narrow band of frequencies. To compare the dipole to an antenna over a range of frequencies requires an adjustable dipole or a number of dipoles of different lengths. An antenna gain of 1 (0 dB) compared to a dipole antenna would be written as 0 dBd.
Gain Measurement
One method of measuring gain is by comparing the antenna under test against a known standard antenna. This is technically known as a gain transfer technique. At lower frequencies, it is convenient to use a 1/2-wave dipole as the standard. At higher frequencies, it is common to use a calibrated gain horn as a gain standard, with gain typically expressed in dBi.

Another method for measuring gain is the 3 antenna method. Transmitted and received power at the antenna terminals is measured between three arbitrary antennas at a known fixed distance. The Friis transmission formula is used to develop three equations and three unknowns. The equations are solved to find the gain expressed in dBi of all three antennas.
Radiall/Larsen uses both methods for measurement of gain. The method is selected based on antenna type, frequency, and customer requirement.
Use the following conversion factor to convert between dBd and dBi: 0 dBd = 2.15 dBi.
Antenna Placement
Correct antenna placement is critical to the performance of an antenna. An antenna mounted on the roof will function better than the same antenna installed on the hood or trunk of a car. Knowledge of the vehicle may also be an important factor in determining what type of antenna to use. You do not want to install a glass mount antenna on the rear window of a vehicle in which metal has been used to tint the glass. The metal tinting will work as a shield and not allow signals to pass through the glass.

Radiation Patterns
The radiation or antenna pattern describes the relative strength of the radiated field in various directions from the antenna, at a fixed or constant distance. The radiation pattern is a "reception pattern" as well, since it also describes the receiving properties of the antenna. The radiation pattern is three-dimensional, but it is difficult to display the three-dimensional radiation pattern in a meaningful manner, it is also time consuming to measure a three-dimensional radiation pattern. Often radiation patterns are measured that are a slice of the three-dimensional pattern, which is of course a two-dimensional radiation pattern which can be displayed easily on a screen or piece of paper. These pattern measurements are presented in either a rectangular or a polar format.

Absolute and Relative Patterns
Absolute radiation patterns are presented in absolute units of field strength or power. Relative radiation patterns are referenced in relative units of field strength or power. Most radiation pattern measurements are relative pattern measurements, and then the gain transfer method is then used to establish the absolute gain of the antenna.

Near-Field and Far-Field Patterns
The radiation pattern in the region close to the antenna is not exactly the same as the pattern at large distances. The term near-field refers to the field pattern that exists close to the antenna; the term far-field refers to the field pattern at large distances. The far-field is also called the radiation field, and is what is most commonly of interest. The near-field is called the induction field (although it also has a radiation component).

Ordinarily, it is the radiated power that is of interest, and so antenna patterns are usually measured in the far-field region. For pattern measurement it is important to choose a distance sufficiently large to be in the far-field, well out of the near-field. The minimum permissible distance depends on the dimensions of the antenna in relation to the wavelength. The accepted formula for this distance is:

When extremely high power is being radiated (as from some modern radar antennas), the near-field pattern is needed to determine what regions near the antenna, if any, are hazardous to human beings.
Beamwidth
Depending on the radio system in which an antenna is being employed there can be many definitions of beamwidth. A common definition is the half power beamwidth. The peak radiation intensity is found and then the points on either side of the peak represent half the power of the peak intensity are located. The angular distance between the half power points traveling through the peak is the beamwidth. Half the power is —3dB, so the half power beamwidth is sometimes referred to as the 3dB beamwidth.

Antenna Pattern Types
Omnidirectional Antennas
For mobile, portable, and some base station applications the type of antenna needed has an omnidirectional radiation pattern. The omnidirectional antenna radiates and receives equally well in all horizontal directions. The gain of an omnidirectional antenna can be increased by narrowing the beamwidth in the vertical or elevation plane. The net effect is to focus the antenna’s energy toward the horizon.

Selecting the right antenna gain for the application is the subject of much analysis and investigation. Gain is achieved at the expense of beamwidth: higher-gain antennas feature narrow beamwidths while the opposite is also true.
Omnidirectional antennas with different gains are used to improve reception and transmission in certain types of terrain. A 0 dBd gain antenna radiates more energy higher in the vertical plane to reach radio communication sites that are located in higher places. Therefore they are more useful in mountainous and metropolitan areas with tall buildings. A 3 dBd gain antenna is the compromise in suburban and general settings. A 5 dBd gain antenna radiates more energy toward the horizon compared to the 0 and 3 dBd antennas to reach radio communication sites that are further apart and less obstructed. Therefore they are best used in deserts, plains, flatlands, and open farm areas.
Directional Antennas
Directional antennas focus energy in a particular direction. Directional antennas are used in some base station applications where coverage over a sector by separate antennas is desired. Point to point links also benefit from directional antennas. Yagi and panel antennas are directional antennas.







Antenna Polarization
Polarization is defined as the orientation of the electric field of an electromagnetic wave. Polarization is in general described by an ellipse. Two often used special cases of elliptical polarization are linear polarization and circular polarization. The initial polarization of a radio wave is determined by the antenna that launches the waves into space. The environment through which the radio wave passes on its way from the transmit antenna to the receive antenna may cause a change in polarization.

With linear polarization the electric field vector stays in the same plane. In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation, making one full turn for each RF cycle. The rotation may be right-hand or left-hand.
Choice of polarization is one of the design choices available to the RF system designer. For example, low frequency (< 1 MHz) vertically polarized radio waves propagate much more successfully near the earth than horizontally polarized radio waves, because horizontally polarized waves will be cancelled out by reflections from the earth. Mobile radio systems waves generally are vertically polarized. TV broadcasting has adopted horizontal polarization as a standard. This choice was made to maximize signal-to-noise ratios. At frequencies above 1 GHz, there is little basis for a choice of horizontal or vertical polarization, although in specific applications, there may be some possible advantage in one or the other. Circular polarization has also been found to be of advantage in some microwave radar applications to minimize the "clutter" echoes received from raindrops, in relation to the echoes from larger targets such as aircraft. Circular polarization can also be used to reduce multipath.

Friday, September 3, 2010

Convert an IP address to an IP Number

IP address (IPv4 / IPv6) is divided into 4 sub-blocks. Each sub-block has a different weight number each powered by 256.

IP number is being used in the database because it is efficient to search between a range of number in database.

Beginning IP number and Ending IP Number are calculated based on following formula: IP Number = 16777216*w + 65536*x + 256*y + z

(Formula 1) where IP Address = w.x.y.z

For example, if IP address is "202.186.13.4", then its IP Number "3401190660" is based on the Formula 1.

IP Address = 202.186.13.4

So, w = 202, x = 186, y = 13 and z = 4

IP Number = 16777216*202 + 65536*186 + 256*13 + 4

= 3388997632 + 12189696 + 3328 + 4 = 3401190660

To reverse IP number to IP address,

w = int ( IP Number / 16777216 ) % 256

x = int ( IP Number / 65536 ) % 256

y = int ( IP Number / 256 ) % 256

z = int ( IP Number ) % 256

IP Address = w.x.y.z

where % is the mod operator and int is return the integer part of the division.

In Microsoft Excel you can easily find out computation of  above formula for calculating answer of w by typing formula =MOD(int ( IP Number / 16777216 ),256) same method is applicable for x,y & z.

Thursday, September 2, 2010

Common factors to the Establishment of an FTP Server

Presuppositions to the examples

(1) Required functions

  • No other service except for ftp runs on the server.
  • Only specific registered users are allowed to access the server (no anonymous user accounts are allowed).
  • Authenticating the user in not only a user ID and a password but also a source IP address
  • The server provides ftp service for each user equally.

(2) Platform

  • Hardware with Linux operating system tolerable for 24 hour continuous operation
  • Fundamental settings as an Internet server have already been done.

(3) Network configuration (see Figure 9.1)

  • Permanent connection to the Internet ( via ISP) is prepared.
  • The server is placed in the DMZ (De-Militarized Zone) branch from a firewall.
  • The firewall allows only ftp connections for the server.
  • Data stored in the server are transferred from data source system(s) in intranet through the firewall.
Figure 9-1. Presuppositions to examples

2 Process to set up FTP server software

This section introduces an outline of practical process of setting up FTP service in case of "proftpd" on Linux environment. Please refer to the "installation guide" of the software on the details.

(1) Installation

Things to do first are download the archived file of the latest version of the software from the official site or its mirror sites, extraction, compilation and then installation.
To store server modules and related files to the proper directories by make install command, you should become root or super user as you have to write some files to protected directories or installation might fail.

$ tar zxvf proftpd-1.2.9.tar.gz
$ cd proftpd-1.2.9
$ ./configure
$ make
$ su
Password: xxxxxx
# make install
# exit
$

(2) Configuration

There are typical procedures and examples for configuration of "proftpd" in the user's guide in the official site. According to the procedures, it is possible to complete the general configuration. In addition the following procedures (including explicit descriptions) are required to customize the configuration on the presuppositions. Descriptions for configuration are including in a specific configuration file named "proftpd.conf". A directory where the configuration file is installed depends on each platform. In this example, it is installed under the directory of " /user/local/etc/".

File Transfer Methods & Protocols

There are many protocols that could transfer data between hosts over TCP/IP networks. Each protocol has its characteristics, Some examples are as follow

1 File Transfer Protocols

(1) TFTP
The Trivial File Transfer Protocol (TFTP) is a simple file transfer protocol defined by RFC1350. The mechanism of TFTP is completely different from the FTP. TFTP service uses single TCP port 69 instead o f 21 and 20 of the FTP. As the TFTP has no mechanism for authentication, it is inadequate to use it for Internet server because anybody could read/write data from/to the server. The TFTP is mainly used to boot disk-less equipment like X-11 terminals.

(2) FTPS

The FTPS is a extended version of the FTP. The extension to the standard FTP is define by the RFC2228 which introduce optional security function to the control and data connection and some new command and reply to the FTP are added. The encryption algorithms are not the part of the FTPS definition and installation dependent, and Secure Socket Layer (SSL) or Transport Layer Security (TLS) might be used.

FTP Clients

1 Built in FTP Clients

Linux and Windows have their built in FTP client software. Those have traditional text-based user interface that could be controlled by some scripting software, like shell, but not suit for operational unattended purposes because their user interface is full duplex and scripting languages are not good at handling those asynchronous communications.


#!/bin/sh
ftp -n ftp.wmo.int << EOF >> logfile 2>&1
user username passrowd
binary
get remote-file1
bye
EOF
Example: Simple Shell Script for FTP

2 GUI Clients

You can find much free FTP client software on the Internet. Most of them are Graphical User Interface (GUI) based client and easy to use, but not suit for unattended operation because they need human interaction to transfer files. Some shareware or commercial software has batch or automatic operation capability with error retry or/and error recovery. Net Vampire and Core FTP Pro are examples.

FTP Server Software

There are kinds of software products for FTP server. The server administrator should choose the appropriate one in consideration of security and reliability where the authors respond quickly to security issues or bugs, performance to handle necessary concurrent connections, easiness in management and maintenance at server side, compliance with RFC959, and convenience from the user view.
The following software products are predominant examples in diffusion and useful features.
Be sure to install the most recent version of the software and to apply most recent patches to it, whichever product you choose.

1 wu-ftpd (http://www.wu-ftpd.org/)

The "wu-ftpd" had merits of having functions for archiving and compressing entire elements under a directory and running on many supported platforms. The last version of the "wu-ftpd" is 2.6.2 released in Nov 2001. Since the "wu-ftpd" used to be most commonly used server software, information on security flaw and its patch is adequately released to support a great number of the implementers. But "wu-ftpd" is being taken over by some other server software recently.

2 proftpd (http://www.proftpd.org/)

Since the "proftpd" runs as a configurable non-privileged process in stand-alone mode in order to decrease chances of attacks, which might exploit its "root" abilities, it is more secure than other server software, which runs in privileged mode like "wu-ftpd".
The "proftpd" has single main configuration file similar to "Apache". Therefore directives and directive groups in the file are intuitive to an administrator who has ever used the "Apache" Web server. This feature provides easy and flexible configuration.

3 vsftpd (http://vsftpd.beasts.org/)

The vsftpd is a high performance and secure FTP server for Unix like systems, including Linux. The name "vsftpd" stands for Very Secure ftpd. Literally its significant features of security are emphasized. Other good features are that it is extremely fast and that it does not consume a lot of resources.
Lately RedHat, one of most famous Linux distributors, has replaced "proftpd" by "vsftpd" as standard FTP server software included in their distributions. Major Linux/Unix distributions include vsftpd as its default FTP server. The vsftpd has been upgraded to version 2 since July 2004, including TSL/SSL features to support FTPS service.

4 Internet Information server (IIS)

Microsoft provides the Internet Information Server (IIS) as an optional module of the Windows Servers. The IIS has capability of server functions including the FTP.
The IIS 5.0, which is provided with Windows Server 2000, is not recommended as it is automatically installed with other additional modules in default, may include undesirable flaws unconsciously. On the contrary, the IIS 6.0, which is provided with Windows Server 2003 but not installed in default, is recommended as it is considered for security well.

5 GuildFTPd (http://www.guildftpd.com/)

  The guildFTPd provides capability of ordinary FTP daemon running on the UNIX like platforms, including user and user-group managements, IP filtering, connection control, change root, throughput control, etc.
  The most recent version of the guildFTPd is 0.999.14 (26-May-2006) as of June 2006.

Previous_Page Next_Page


Management of the FTP Server

In this section, server management issues such as security, availability, and service constructions are described.

Execution Environment (Run Mode on the Unix like OS)

One major concern on establishing FTP service is the run mode of the server, i.e., stand alone or run by xinetd.

(1) Stand Alone Mode (Run as daemon)

On the stand-alone mode, server runs as a daemon that directly accepts connections from Clients by itself and serves for it. A daemon is a process that waits for some event and act for some service on the Linux. A daemon runs as a root process on the Linux. It is not recommended to run a service as daemon because daemon consumes memory resource even it only waits for some event and do nothing. Also it has security risk because daemon runs as root, the privileged process that tend to be the target of attacks, if once the process is compromised by using some technique like buffer overflow attack, the server may defect to the enemy and might be completely controlled by the hijacker.

Designing FTP Service

In this section, FTP related things to design the service are considered.

FTP Solutions

On the view of risk management within the recent Internet world, it is indispensable to adopt countermeasures against new risks continuously to keep the server secure after the operation. Fundamentals of security issues should be referred to the "WMO Guidance on Information Technology Security at WWW Centres" (being developed).
 Generally, clustering and duplication of the serving system and multi-access circuits preparing for physical troubles are effective to improve availability, although it should be noted that their implementation range and details influence installation and recurrent costs.
Considering human resource and cost for security and appropriate availability, self-management of the server is not always the sole solution. It might be worth to study the use of outsourcing services from ASP (Application Service Provider) / ISP (Internet Service Provider) such as hosting or housing services.
Figure 3-1. FTP Solutions

It is important to examine requirements in operation and management of the server and then to find an appropriate solution within possible conditions in human resource and cost for outsourcing, access circuits and security.

Wednesday, September 1, 2010

How FTP works?

The File Transfer Protocol (FTP) is defined by the RFC959 based on the FTP Model illustrated in the Figure 2-3. The FTP uses two TCP/IP connections, i.e., control connection and data connection between the User and the Server. The control connection manages and controls the Server to transfer files between the Server and the User through the data connection.
It is important for understanding the FTP that the RFC959 defines the protocol between User-FTP Process and Server-FTP Process, not between User and User-FTP Process nor User and Server-FTP Process, in the Model. You cannot see the conversation between the User-FTP Process and the Server-FTP Process, i.e., FTP protocol, directly, as you operate FTP through the User Interface.
Figure 2-3. The FTP Model (Session Layer)

File Transfer Protocol (FTP)

Basics on the File Transfer Protocol (FTP)

In this section, mechanism of the File Transfer Protocol (FTP) is illustrated briefly. If you are familiar with the FTP protocol, you can skip this section.

What is the FTP

   The FTP is basic and common service to exchange files between computers, namely hosts, over TCP/IP networks e.g., private networks or Internet. The FTP supports file transmission and character code conversion when exchanging text or binary files. The use of FTP is effective in exchanging or distributing of large volume of data over private networks and/or the Internet. A structural outline of FTP service is illustrated in Figure 2-1.
Figure 2-1. Structural outline of FTP service

  Basically, FTP is defined in the RFC959 as a communication protocol between Server and User for exchanging files. The FTP Server stores files to be exchanged or exchanged. Users, who want to exchange files, will login to the server and PUT/GET files to/from the server (Figure 2-2). A User may be a person or an autonomous process on behalf of a person who wishing to exchange files.

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