As common as birds are, and as photogenic as many of them are, they are maddeningly difficult to photograph. It’s challenging to get close enough for a decent shot, and birds never seem to sit still for the camera. Lighting, focus, and exposure can be tricky, fooling even the best automatic exposure and focus mechanisms. So what do you do?

Technique
As with any photography, knowing your subject will improve your photographs. Spend some time watching the behavior of birds around your feeder. You’ll notice certain patterns. Do they first land on a nearby branch before going to your feeder? Do they return once they have grabbed a morsel? Do some birds feed off the spilled seed on the ground? Do they favor a certain spot on your feeder? What time of day do various species feed? When will the best light fall on your birds?

Getting close to the subject is the key to successful bird photography, or any photography, for that matter. Cameras have a funny way of making things look larger through the viewfinder than they will on the print. If you’ve tried to photograph birds, you know that the bird often appears only as a small dark spot on the print. But with a little ingenuity, you can get close enough for decent photographs. Here are some ideas for getting close to your subject.

The most common solution is the blind. You can buy blinds made for the purpose from professional camera stores, but an old tent or tarp can be just as effective. A visit to your big-box sporting goods section can be worth while as well. Even a car can be an effective blind. What ever you use put it in place two or three days before you plan to photograph so the birds will get used to it.

Patience is a virtue. When you go into your blind, the birds will leave, but only temporarily. But you should plan to be in your blind for some time, maybe a few hours. So have some water and snacks with you, and take care of necessities before you go to the blind. You will be sitting still, so in cold weather layers, gloves and a hat will be needed.

A great solution is to place your feeders near a clear window, and you can photograph from the comfort of your own home! Feeders are even available that fit on the window sill or stick to the window glass. You will need to darken the room as much as possible to avoid reflections and, of course, clean the glass.

It is only necessary that the camera be close – you don’t necessarily have to be close yourself. Remote releases, both wired and wireless, are available for most popular camera brands. To use this method, you will need to set your exposure and focus the camera on a spot where you know birds will land. Experiment with your aperture setting to provide some focusing leeway for birds that aren’t exactly at your focus point.

Before we leave the topic of blinds, keep in mind that the safety of the birds is more important than any photograph you may get. Do not position yourself or your blind where you will interfere with nesting birds.

As mentioned, getting close is the most important technique, but here are some other tips to help you get the knock-out image. First, get familiar with your camera’s aperture and shutter priority modes, as well as its manual mode because you will want to set the shutter speed and aperture yourself. Try a shutter speed of about 1/1000 or 2/1000 seconds to stop the motion of the birds. Open your lens almost – but not quite – as wide as it will go. This will help “fuzz out” the back ground, and moving down from wide open improves image sharpness. Set your focusing mode to use its center point focus – don’t let the camera pick the focus point, because it will likely pick the wrong one.

Exposure can be tricky as well. A bright bird against a dark wooded background that fills most of the frame is sure to cause over exposure of the bird. As mentioned above, don’t rely solely on the automatic exposure modes (automatic, program, aperture priority, shutter priority) of your camera. You will need to learn the exposure compensation feature, or better yet, the manual mode, in which you set the aperture and shutter speed yourself.

If you use flash, practice with it before hand so that you can get the right amount of flash dialed in. You want just enough flash to open the shadow areas, but too much flash overpowers the background and makes the entire image look unrealistic. The right amount of flash will make the bird “pop” against its background and will provide that pleasing catch-light in the eye.

Photography is painting with light. You simply cannot – cannot – take good images in bad light. Study the lighting of your bird photography area. Look for times when your birds will be side lit, generally in the morning or afternoon. At those times the light will have a pleasing “warm” characteristic that will enhance your image. Avoid the middle of the day. The strong down light and harsh shadows don’t make for good photos. Cloudy days are great too, because the light is so even and colors pop. (Hint: exposure is much simpler on cloudy days.)

Equipment
Professional wildlife photographers will spend many thousands of dollars on cameras and big telephoto lenses, and will go to enormous effort and personal discomfort to get “the shot.” However, you can get great bird photos with relatively modest equipment and without wading through swamps.

Although it is possible to photograph birds with any film or digital point and shoot (PS), a digital single-lens-reflex (DSLR) will be more satisfying and produce superior results. The DSLR does not suffer from the frustrating shutter delay of most digital PS cameras. This delay may be up to one second, rendering them unsuitable for fast moving birds. The DSLR will also offer burst exposures of three or more frames per second, and the capability of accepting telephoto lenses, which will help with the getting close part.

The sensor in a DSLR is many times larger than the sensor in a PS, meaning that the DSLR images will enlarge better with less noise. The DSLR has much higher ISO capability (800, 1600 and beyond) than PS cameras, allowing you to use fast shutter speeds even in relatively dim light. Finally, with a DSLR, you can shoot as much as you want – for free.

The next required piece of equipment is a tripod. The tripod is necessary to stabilize even moderate telephoto lenses if you want sharp images. Holding the camera to your eye for some period of time will tire your arms, and lifting the camera to point it out of the blind will scatter the birds. Make certain you get one that will solidly support your camera, the largest lens you anticipate using, and possibly a flash unit. The tripod head (the part that attaches to the camera) should allow the camera to be aimed and re aimed without making any adjustments to the head itself. Tripods come in all sizes and prices, and the better ones do not come with a head. Your best bet is to consult a good, professional photo shop.

Using flash can help a bird’s natural colors “pop,” even in bright daylight. The purpose of flash is not so much to supply the light, as it is to reduce contrast by “opening up” shadow areas. Most entry level and “prosumer” DSLRs come with a built in flash, but an external flash will provide better results. They have more power, and are less likely to cause shadows in the image from long lenses and lens shades. Good flash units are expensive, so the built in flash is certainly worth a try.

Many books have been written about photographing birds, and there’s a huge amount of information on the Internet. While it’s not possible to cover the entire topic in one article, these tips should get you off and shooting. Bird photography is challenging, but with the right equipment, technique, and some practice, you can produce wonderful images.

The flight arrived at around 11:30 am at CDG and a driver was waiting for the group with a plaque with the family name. The group headed directly to the Marais where they they were welcomed into a 5 bedroom private house featuring an elevator and pool and best of all included a butler. The butler gave them a brief tour of their Paris home for the next week. Yes Tom, did his homework and found the Bauderlaire apartment on the Paris Luxe Apartments.

To some it may see like an extravagant getaway but when you consider ultra luxury accommodations in the heart of Paris able to cater to 10 guests for only $22,000, it comes out to less than $3,000,00 per person.

After the group settled, they walked approximately a half mile to Arc de Triomphe where they purchased tickets for the 2 day hop-on/hop-off bus tour of the city. The weather was warm and beautiful ideal for cruising on top and was the best way to experience the amazing sites.

The next stop was Notre Dame, as a massive, very impressive structure, but it was a bit dark and it’s hard for amateur photographers to do justice to such a vast, impressive site. After that, the party walked around the Ile St-Louis and enjoyed lunch (croque monsMonday they spent 3.5 hours at the Louvre, which is amazing, but utterly overwhelming. they managed to see all the most famous pieces (Mona Lisa, Winged Victory of Samothrace, Venus de Milo, etc.) and also visited the old French crown (Louis XIV) and the recreation of Napoleon old house. They had lunch at Cafe Marly and walked around the area a bit and made way back to the Opera House and, naturally, arrived right at the time it was closing (4 PM). It was cocktail hour and they wre pretty tired so, fortunately, as they Harry’s Bar where they popped in for a couple of Bloody Marys.

They went to Cafe de la Paix for dinner and had a very good meal and then they back to the townhouse and popped into the James Joyce pub where they sat at an outside table and were accosted by a total drunk patron.

On Tuesday , they took the metro to the Orsay in the morning and spent 2.5 hours. They raved about the Orsay , the building is amazing and the collection is unforgettable. They took the RER to the Tour Eiffel and walked around and took some photos. They had a long lunch at Jules Verne, which was totally delicious and quite an experience.

They walked from Eiffel Tower to Les Invalides and looked at the WWI and WWII exhibits and then visited Napoleon crypt. From there they walked to Rue de Cler and saw all the fruit and vegetable markets and had a drink at a local bar. They took the metro back to the townhouse and had a light snack at Chez Clement.

On Wednesday, they took the metro to Montmarte to visit Sacre Coeur, which all thought was way more interesting that Notre Dame. They walked over to Place de Tetre and walked around and looked at all the artist stalls. They had lunch at a fun restaurant called La Cremaillerie, which is right on the Square. Montmartre is a really interesting area with fun shops and nice gardens and interesting looking apartments.

They followed a walking tour and walked to Au Lapin Agile (which, disappointingly, is apparently closed during the day) and then continued on to the Moulin Rouge. They did have drinks on Boulevard Blanche (an area full of prostitutes!).

They took the metro to Place de Concorde which is a very impressive spot and then They walked up the Champs-Elysees and had dinner at a restaurant called Cafe Fouquet on the Champs-Elysees which is supposed to be the hot, hip place for locals. They were on the early side so they missed a lot of that but the food and wine was very nice.

On Thursday, they took the metro to St. Germaine, an area everyone loved. They kept saying that if they lived in Paris, this is the neighborhood to live in. They shopped on the main Boulevard and found a great shop, La Fragonard, and bought some beautiful soap and perfumes (both of us) and Louise bought some lovely salad plates.

The lady there was fantastic and gave us tons of free samples of perfumes and lotions and other beauty items. They went to the Swarovski shop and Louise bought a beautiful pendant.

They walked around and visited several other fun shops. They had lunch at Les Deux Magots even though it is famous, the food was average. They made way to the Latin Quarter where they walked around the main Square and also followed some of the guided walk. They walked past the Sorbonne and they to the Pantheon and saw some interesting old graves/crypts.

They walked down to Montparnasse, but many things were closed that day because of Victory in Europe day. They had some drinks at the Coupole de Lila and then took the train to the Trocadero where they took some nice photos of the Tour Eiffel. They walked a bit through the residential area (very posh) and headed for a restaurant called La Fontaine de Mars, a great local place which not touristy, and had a really good dinner.

On Friday, they woke up early to go to Versailles on a guided tour. Versailles is very interesting it has beautiful gardens and the interior of the palace is totally over the top. They got back to townhouse at around 1 PM and changed clothes and headed for the Opera House and took a tour. It is possibly the most magnificent thing they saw on the visit and the photos absolutely do not do it justice. It would be amazing to actually go to an opera there.

After, they all headed to Printemps for shopping and had a late snack at the bistro with the stained glass dome ceiling. They also visited Galleries Lafayette and other shops. They walked to Le Madeleine and visited Fauchon and other shops in that area. They walked down Rue Royale and had cocktails at Maxims and then on to dinner at Le Grand Colbert near Galeries Vivienne which was delicious (especially the smoked salmon on blinis).

They visited Ste. Chappelle, which is absolutely breathtaking and a truly fantastic site. It is another place where photos wil not do it justice, but they actually managed to get a few decent ones. They then headed on to Marais, Les Halles and Bastille and had lunch at Au Pied de Cochon. They walked around (partially following a suggested walk) and also visited Le Cochon, one of the authentic bistros noted in the book that John gave Ali. They walked to Place de Vosges (very impressive) and sat in the square under the trees.

They made their way back to the apartment. They rested and packed and had dinner at a local spot, Chez George, and then hopped over to Tour Eiffel since They wanted to get some nighttime photos. Ali bought George a fun glowy souvenier.

Photoshop CS2 may sound really good. However, for beginners, this will still all be Greek to their ears. Thus, tutorials will really play a big role to expose one to the many possibilities of CS2.

The Photoshop CS2 Program

Photoshop CS2 is the upgraded version of the basic Photoshop program. This now showcases a number of features that you cannot find in the old version. It provides more options to the user and makes creative work an easier task to do.

However, it takes more than just owning and installing the program. You have to learn the various aspects of Photoshop CS2 so you can maximize it. This way, you can also be more productive with your purchase. Here are some of the features that you can enjoy.

1.Adobe Bridge

Adobe Bridge is one convenient feature that makes Photoshop CS2 a very fine companion. With this option, you can easily look at the detail of your artworks and photos even before you open the file.

This is definitely a good tool for organizing your works. It is even user-friendly so you can easily maximize your time and effort.

2.Font Preview

The font preview option is also very desirable. Before, it will take you a lot of guessing game and trials before you can put a good font in your work.

With Photoshop CS2, you can easily choose the right font from the drop-down menu as it showcases the actual font from the preview.

3.Spot Healing Brush

Any imperfections in your graphics can be easily cured by the Spot Healing Brush. You do not need to find any source points anymore. All you have to do is point your mouse to the area and with just a click, you can resolve the problem.

4.Vanishing Point

This aspect lets you cut and paste a photograph in a proper perspective. This is like making a virtual art gallery out of your graphics. This will let you move and skew any windows in places that you deem proper.

5.Preview Video Graphics

For people who are into making videos, they know that coming up with a good graphic will not always translate to a good video output. There are certain qualities that are not retained when the video option is utilized.

Photoshop CS2 will allow you to preview your graphics as to how they will appear in video. Thus, you will never have to spend more time in modifying as you can make your graphics the way you need them to come out.

Photoshop CS2 Tutorials

Now that you have a good idea of the great things about Photoshop CS2, then taking tutorials will help you make the most out of the program in your computer.

There are many options for you to get the right Photoshop CS2 tutorial. Here are some of the things that you can consider.

1. Web Tutorial

There are many web tutorials that you can use. You can search for the right web site and get tutorials for a fee. There are even options for you to view the Photoshop CS2 tutorials in video so you will be guided in every step of the way.

2. PDF Format

Some tutorial services can give you a PDF format option. This is perfect for people who can learn better when reading a printout of the lesson.

3. CD Package

You can also purchase a CD tutorial course. For less than $100, you can have everything you need to learn in your hands.

Conclusion

Photoshop CS2 tutorials are perfect for beginners. You will be taught step-by-step on how to maximize the use of the program. With this, you will surely become more productive and creative.

Digital photos and digital cameras have many advantages and allow many new usages that were not possible with film. Here are a few:

Low photo cost: Photos taken with a digital camera literally cost nothing. The photos are kept in erasable memory and thus can always be discarded at no cost. Viewing the photos is immediate and does not cost anything (there is no film development cost). Also the photos you would like to keep can be copied to digital media such as a computer’s hard disk. With storage prices going down the cost of saving a photo on disk is practically zero.

The capacity: With ever growing storage capacities digital cameras today can hold hundreds and sometimes thousands of photos on a single media. You can always have a few more in your pocket and changing is very fast. The result is that a digital camera has practically infinite capacity. You can shoot as many photos as you want and at the end of the day just dump them on your computer’s disk. There is no time lost changing film and there is no photo opportunities lost due to lack of film or reserving film for later.

The feedback: One of the great advantages of digital cameras is instant feedback. Almost all digital cameras include a small LCD screen. Once a photo is shot you can go back and watch it on that screen. The ability to see how the photo looks like results in better photos. If the photo is not good you can take another one. Being able to see the photos on the spot results in an educated decision how to fix a photo or how to compose it better. It takes a lot of the guessing away from photography.

New shooting angles: With digital cameras you can take photos without your eye glued to the viewfinder. Overhead shots where you raise the camera over your head are much easier since you can still see what the camera is shooting by just looking up at its LCD screen. You are not limited anymore to angles where you can look through the viewfinder – you can shoot from any angle that the LCD is visible from. You can always shoot blindly like photographers used to do with film cameras.

Correcting photos: With digital cameras photos can be corrected using photo editing software. Some correction abilities are built-in the cameras but many more are available as software packages for your PC. Such corrections include red eye removal, contrast enhancements and more.

Changing conditions: With digital cameras the settings of the sensor can be changed instantly for each photo taken. With a click of a button the camera can be put in an indoor or outdoor mode, low light, night photography etc. Some cameras will automatically sense the scenario and set the sensor mode accordingly. This is literally like changing a roll of film to the optimized one for every photo you shoot – the best ISO, color features and more.

Photo filters are NOT going to change a bad photo to a good one, BUT . . . they may change a good photo into a great one. In other words; filters are like the frosting on the cake, if the cake itself tastes bad, it doesn’t matter how sweet the frosting is. For the sake of this photo article we are going to assume you know how to make a good cake (take a good photo), and now you are ready to go to the next level.

Star Filters – these are available in 4 point, 6 point, or 8 point variety. These photo filters add glamour to nature or highly reflective surfaces (like someone playing a trumpet).

Polarizer’s - this type of photo filter is often thought of for enhancing clouds in the sky, but they also give you more control with reflections on water or glass. They also help with extreme photo lighting situations like snow or sand.

Close-Up Filter Set – not everybody can afford an extra $300 to $400 for a macro lens, this set of photo filters is well worth the investment. This gives you the edge to be able to take advantage of, and photograph the unexpected; whether it is a butterfly on a flower or dew on the morning grass.

Split Field Filter – A Split Field Filter allows you to go beyond the limits of traditional depth of field in photography. Half of the lens is basically a close up filter, half is regular glass. Now you can take an extreme close up photo of a flower at the edge of a vast canyon and still have BOTH sections of your photo look crystal clear.

Split Color Filter – This filter comes in several combinations. Similar to the split field filter, part of the filter is just regular glass, BUT the other half is colored. If half the filter is deep orange for example, you can take a photo of any mountain range and make it look like it was taken at sunset.

Spot Filter – The idea behind a spot filter is that a small portion of your photo will appear sharp (usually a spot in the middle) and the rest of the photo looks soft and dreamy. Often used in Wedding pictures or individual portraits.

Soft Focus Filter – As the name implies this photo filter gives the entire image a soft focus or dreamy effect. Use this VERY sparingly or people will think you can’t focus. Besides photo filters there are some other small items that you should tuck into your camera bag to help you improve photographic opportunities. The cost of these photo gadgets are relatively small but can give you big time (professional looking) results.

Double Sided Poster Board – For many years I carried a 10 inch x 12 inch poster board that was black on one side and white on the other. This can be used to make a small subject stand out from the back ground. A small tree frog on grass is much harder to see than one against a black background. This can also be used for controlling light like a small photo reflector.

Spray Bottle – having a small spray bottle of water means now you can take a “flowers with morning dew” photo, even if it’s the middle of the afternoon. You can also make an athlete sweat whenever you want them to; or you can wet down wild hair that is blowing in the wind for a better outdoor portrait.

The last fairly cheap item for big results is a Tripod or a Monopod. Tripods don’t have to be built so a man can sit on it to be of good quality. And if you are worried about space as you are taking your wonderful photo trek into the wilderness consider a monopod. It offers stability as well as making a good walking stick.

All of these items are considered photo add-ons. None of them are required to make a great photo, but when the opportunity arises it’s good to know you have the right tools at your disposal. Of course there are hundred of other photo gadgets and gizmos that you can get, but I advise sticking with the basics. Far more important than anything you can add to the outside, is the vision you have on the inside. Learn the elements of design. Practice leading lines, balance, repetition, framing and the rule of thirds . . . these are the ingredients of a great photo. All the rest is just frosting on the cake.

I take a lot of pictures so obviously the first thing I loved about it is that it had no film. I also got two large memory cards from my father along with the camera and I never run out of film. I no longer have to pay for developing photos that I don’t even like. Actually, I don’t develop photos at all. And now that I’m able to see them on the spot, I can delete the ones that turn out wrong and free up space for new ones. I take hundreds of pictures and this helps me improve my technique and get better results. I can still keep the things I liked about traditional photography as my camera lets me select film speed and worry about shutter speed/aperture combinations. This means that I can still use my photographic skills, which is a very good thing. I also love that I don’t have to wait for days to see how the pictures turned out like.

Another thing I love about digital photos is the fact that you can edit them later. I haven’t purchased Photoshop yet, though I plan to buy the Elements program soon. For now I just use Picasa, which is free, and my PhotoDeluxe application which came with the scanner. Even with these basic programs I can still change the photos to a great extent. I sometimes have problems with the exposure because I don’t always notice backlighting. Luckily, poor exposure can be easily corrected afterwards, and the same thing you can do with red eyes or excessive tint.

You can also adjust contrast, sharpen focus and warm up colors. Cropping is another tool I use very often. It helps me remove parts of the background or excess sky. I can even transform a landscape photo into a close-up if I remove a large part of it. You also have a lot of special effects. You can transform your photo into an oil painting, add filters, neon, glow – just set your imagination free.

When I’m all set, I can send those photos I really like to a processing service. With Picasa you have an option to send your photos to a processor over the Internet and you will get the printed photos into your mail. Or you can upload the pictures to a website, use them in a blog or email them to your friends. Having so much control over your photos is just great. If you haven’t already switched to digital photography, you should think about it. It is a real blessing.

Ad hoc networks are a new paradigm of wireless communication for mobile hosts (which we call nodes). In an ad hoc network, there is no fixed infrastructure such as base stations or mobile switching centers. Mobile nodes that are within each other’s radio range communicate directly via wireless links, while those that are far apart rely on other nodes to relay messages as routers. Node mobility in an ad hoc network causes frequent changes of the network topology. Military tactical operations are still the main

Application of ad hoc networks today. For example, military units (e.g., soldiers, tanks, or planes), equipped with wireless communication devices, could form an ad hoc network when they roam in a battlefield. Ad hoc networks can also be used for emergency, law enforcement, and rescue missions. Since an ad hoc network can be deployed rapidly with relatively low cost, it becomes an attractive option for commercial uses such as sensor networks or virtual classrooms.

1.1 Security goals

Security is an important issue for ad hoc networks, especially for those security-sensitive applications. To secure an ad hoc network, we consider the following attributes: availability, confidentiality, integrity, authentication, and non-repudiation.

Availability ensures the survivability of network services despite denial of service attacks. A denial of service attack could be launched at any layer of an ad hoc network. On the physical and media access control layers, an adversary could employ jamming to interfere with communication on physical channels. On the network layer, an adversary could disrupt the routing protocol and disconnect the network. On the higher layers, an adversary could bring down high-level services. One such target is the key management service, an essential service for any security framework.

Confidentiality ensures that certain information is never disclosed to unauthorized entities. Network transmission of sensitive information, such as strategic or tactical military information, requires confidentiality. Leakage of such information to enemies could have devastating consequences. Routing information must also remain confidential in certain cases, because the information might be valuable for enemies to identify and to locate their targets in a battlefield.

Integrity guarantees that a message being transferred is never corrupted. A message could be corrupted because of benign failures, such as radio propagation impairment, or because of malicious attacks on the network.

Authentication enables a node to ensure the identity of the peer node it is communicating with. Without authentication, an adversary could masquerade a node, thus gaining unauthorized access to resource and sensitive information and interfering with the operation of other nodes.

Finally, non-repudiation ensures that the origin of a message cannot deny having sent the message. No repudiation is useful for detection and isolation of compromised nodes. When a node A receives an erroneous message from a node B, non-repudiation allows A to accuse B using this message and to convince other nodes that B is compromised.

There are other security goals (e.g., authorization) that are of concern to certain applications, but we will not pursue these issues in this paper.

1.2 Challenges

The salient features of ad hoc networks posses both challenges and opportunities in achieving these security goals.

First, use of wireless links renders an ad hoc network susceptible to link attacks ranging from passive eavesdropping to active impersonation, message replay, and message distortion. Eavesdropping might give an adversary access to secret information, violating confidentiality. Active attacks might allow the adversary to delete messages, to inject erroneous messages, to modify messages, and to impersonate a node, thus violating availability, integrity, authentication, and non-repudiation.

Secondly, nodes, roaming in a hostile environment (e.g., a battlefield) with relatively poor physical protection, have non-negligible probability of being compromised. Therefore, we should not only consider malicious attacks from outside a network, but also take into account the attacks launched from within the network by compromised nodes. Therefore, to achieve high survivability, ad hoc networks should have a 2 distributed architecture with no central entities. Introducing any central entity into our security solution could lead to significant vulnerability; that is, if this centralized entity is compromised, then the entire network is subverted.

Thirdly, an ad hoc network is dynamic because of frequent changes in both its topology and its membership (i.e., nodes frequently join and leave the network). Trust relationship among nodes also changes, for example, when certain nodes are detected as being compromised. Unlike other wireless mobile networks, such as mobile IP [21, 48, 34], nodes in an ad hoc network may dynamically become affiliated with administrative domains. Any security solution with a static configuration would not suffice. It is desirable for our security mechanisms to adapt on-the-fly to these changes.

Finally, an ad hoc network may consist of hundreds or even thousands of nodes. Security mechanisms should be scalable to handle such a large network.

1.3 Routing Protocol and Threats

Routing protocols for ad hoc networks are still under active research. There is no single standard routing protocol. Therefore, we aim to capture the common security threats and to provide guidelines to secure routing protocols. In most routing protocols, routers exchange information on the topology of the network in order to establish routes between nodes. Such information could become a target for malicious adversaries who intend to bring the network down. There are two sources of threats to routing protocols. The first comes from external attackers. By injecting erroneous routing information, replaying old routing information, or distorting routing information, an attacker could successfully partition a network or introduce excessive traffic load into the network by causing retransmission and inefficient routing.

The second and also the more severe kind of threats come from compromised nodes, which might advertise incorrect routing information to other nodes. Detection of such incorrect information is difficult: merely requiring routing information to be signed by each node would not work, because compromised nodes are able to generate valid signatures using their private keys.

To defend against the first kind of threats, nodes can protect routing information in the same way they protect data traffic, i.e., through the use of cryptographic schemes such as digital signature. However, this defense is ineffective against attacks from compromised servers. Worse yet, as we have argued, we cannot neglect the possibility of nodes being compromised in an ad hoc network. Detection of compromised nodes through routing information is also difficult in an ad hoc network because of its dynamically changing topology: when a piece of routing information is found invalid, the information could be generated by a compromised node, or, it could have become invalid as a result of topology changes. It is difficult to distinguish between the two cases.

On the other hand, we can exploit certain properties of ad hoc networks to achieve secure routing. Note that routing protocols for ad hoc networks must handle outdated routing information to accommodate the dynamically changing topology. False routing information generated by compromised nodes could, to some extent, be considered outdated information. As long as there are sufficiently many correct nodes, the routing protocol should be able to find routes that go around these compromised nodes. Such capability of the routing protocols usually relies on the inherent redundancies — multiple, possibly disjoint, routes between nodes — in ad hoc networks.

2. Key Management Service

We employ cryptographic schemes, such as digital signatures, to protect both routing information and data traffic. Use of such schemes usually requires a key management service. We adopt a public key infrastructure because of its superiority in distributing keys and in achieving integrity and non-repudiation. Efficient secret key schemes are used to secure further communication after nodes authenticate each other and establish a shared secret session key. In a public key infrastructure, each node has a public/private key pair. Public keys can be distributed to other nodes, while private keys should be kept confidential to individual nodes. There is a trusted entity called Certification Authority (CA) [11, 47, and 26] for key management. The CA has a public/private key pair, with its public key known to every node, and signs certificates binding public keys to nodes. The trusted CA has to stay on-line to reflect the current bindings, because the bindings could change over time: a public key should be revoked if the owner node is no longer trusted or is out of the network; a node may refresh its key pair periodically to reduce the chance of a successful brute-force attack on its private key. It is problematic to establish a key management service using a single CA in ad hoc networks. The CA, responsible for the security of the entire network, is a vulnerable point of the network: if the CA is unavailable, nodes cannot get the current public keys of other nodes or to establish secure communication with others. If the CA is compromised and leaks its private key to an adversary, the adversary can then sign any erroneous certificate using this private key to impersonate any node or to revoke any certificate.

A standard approach to improve availability of a service is replication. But a naive replication of the CA makes the service more vulnerable: compromise of any single replica, which possesses the service private key, could lead to collapse of the entire system. To solve this problem, we distribute the trust to a set of nodes by letting these nodes share the key management responsibility.

3. Push! Photo: Informal Photo Sharing in Ad-Hoc Networks

As mobile camera phones become ubiquitous the practice of photography changes. Camera phone pictures are usually taken with sharing in mind. Meanwhile, publicly sharing photographs online has become increasingly popular with websites such as Flickr. Push! Photo is a mobile photo sharing application where photos can be made public and immediately accessed by anyone nearby. The application also automatically searches for photos on nearby devices to find interesting and relevant photos. Push! Photo shows how it is possible to share digital photos just as easy as paper photos.

Shoot!

Publicize!

Discover!

Enjoy!

3.1 THE PUSH! PHOTO PROTOTYPE

The current prototype of Push! Photo allows photos to be made public, and users can browse their own photo collection as well as those of others nearby. When devices are in proximity of one another, they will automatically start to search each other’s public photo collections for

Photographs relevant to one self. These photos are shown as a multi-picture slideshow, which is extended as new photos are found. To browse photos from an event shown in a particular photo the user can click on that picture in the slideshow. The application will then download all photos from nearby devices taken at that event. In this way, if a user spots an interesting picture in the slideshow, she can easily find more photos from the same occasion. To decide

Whether two photos are from the same event, information about whom else was around and the time of shooting is used. The application implements a discovery service to find other devices when they are within Wi Fi-range. Thus the application is always aware of who else (using Push!Photo) is around at a particular time. As a photograph is taken, the resulting picture is tagged with this information together with the time and the identity of the photographer. The current prototype is an application running on

Pocket PCs with WiFi-cards and external SD-cameras

3.2 RELATED WORK

In previous work with Push! Music [2] music files were replaced with so called media agents which were enabled to autonomously copy themselves between devices over a wireless ad hoc network. The media agents try to find their

way to potential listeners as users meet, and as a song is copied it automatically enters the play list. In this way the users discover new music while passively listening. Other projects have looked at mobile photo sharing. Davis et al. in MM2 uses the notion of co-presence to simplify the decision of with whom to share [1]. Photos are then uploaded automatically to a central web server where the sharing recipients can access the photos. Kohno and Rekimoto instead use GPS information and time stamps to decide if pictures are from the same event or not [4]. This is used to let users easily browse each others photos when standing in a group to serve as a topic of discussion. The system also let users drag and drop pictures between your own and other’s devices. As a contrast, Push! Photo aims to look into how mobile sharing can be simplified by allowing seamless sharing, and using context and tagging to automatically find interesting and relevant photographs

4 Conclusions

In this paper, we have analyzed the security threats an ad hoc network faces and presented the security objectives that need to be achieved. On one hand, the security-sensitive applications of ad hoc networks require high degree of security; on the other hand, ad hoc networks are inherently vulnerable to security attacks. Therefore, security mechanisms are indispensable for ad hoc networks. The idiosyncrasy of ad hoc networks poses both challenges and opportunities for these mechanisms. This paper focuses on how to secure routing and how to establish a secure key management service in an ad hoc networking environment. These two issues are essential to achieving our security goals. Besides the standard security mechanisms, we take advantage of the redundancies in ad hoc network topology and use diversity coding on multiple routes to tolerate both benign and Byzantine failures. To build a highly available and highly secure key management service, we propose to use threshold cryptography to distribute trust among a set of servers. Furthermore, our key management service employs share refreshing to achieve proactive security and to adapt to changes in the network in a scalable way. Finally, by relaxing the consistency requirement on the servers, our service does not rely on synchrony assumptions. Such assumptions could lead to vulnerability. A prototype of the key management service has been implemented, which shows its feasibility. The paper represents the first step of our research to analyze the security threats, to understand the security requirements for ad hoc networks, and to identify existing techniques, as well as to propose new mechanisms to secure ad hoc networks. More work needs to be done to deploy these security mechanisms in

an ad hoc network and to investigate the impact of these security mechanisms on the network performance.

5 Acknowledgments

I would like to thank my friends for their invaluable contributions to this work. I am also grateful to my family and the anonymous reviewers for their comments and suggestions that helped to improve the quality of the paper.

I am grateful to Almighty for His blessings upon me.

6 References

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