ICMP floods

A smurf attack is one particular variant of a flooding DoS attack on the public Internet. It relies on misconfigured network devices that allow packets to be sent to all computer hosts on a particular network via the broadcast address of the network, rather than a specific machine. The network then serves as a smurf amplifier. In such an attack, the perpetrators will send large numbers of IP packets with the source address faked to appear to be the address of the victim. To combat Denial of Service attacks on the Internet, services like the Smurf Amplifier Registry have given network service providers the ability to identify misconfigured networks and to take appropriate action such as filtering.
Ping flood is based on sending the victim an overwhelming number of ping packets, usually using the "ping -f" command. It is very simple to launch, the primary requirement being access to greater bandwidth than the victim.
SYN flood sends a flood of TCP/SYN packets, often with a forged sender address. Each of these packets is handled like a connection request, causing the server to spawn a half-open connection, by sending back a TCP/SYN-ACK packet, and waiting for an TCP/ACK packet in response from the sender address. However, because the sender address is forged, the response never comes. These half-open connections saturate the number of available connections the server is able to make, keeping it from responding to legitimate requests until after the attack ends.

Application level floods

On IRC, IRC floods are a common electronic warfare weapon.
Various DoS-causing exploits such as buffer overflow can cause server-running software to get confused and fill the disk space or consume all available memory or CPU time.
Other kinds of DoS rely primarily on brute force, flooding the target with an overwhelming flux of packets, oversaturating its connection bandwidth or depleting the target's system resources. Bandwidth-saturating floods rely on the attacker having higher bandwidth available than the victim; a common way of achieving this today is via Distributed Denial of Service, employing a botnet. Other floods may use specific packet types or connection requests to saturate finite resources by, for example, occupying the maximum number of open connections or filling the victim's disk space with logs.
A "banana attack" is another particular type of DoS. It involves redirecting outgoing messages from the client back onto the client, preventing outside access, as well as flooding the client with the sent packets.
An attacker with access to a victim's computer may slow it until it is unusable or crash it by using a fork bomb.
A 'pulsing zombie' is a term referring to a special denial-of-service attack. A network is subjected to hostile pinging by different attacker computers over an extended amount of time. This results in a degraded quality of service and increased workload for the network's resources. This type of attack is more difficult to detect than traditional denial-of-service attacks due to their surreptitious nature.

Nuke

A Nuke is an old denial-of-service attack against computer networks consisting of fragmented or otherwise invalid ICMP packets to the target, achieved by using a modified ping utility to repeatedly send this corrupt data, thus slowing down the affected computer until it comes to a complete stop.
In online gaming, nuking is used by spamming another user, or all other users, with random repeated messages in quick succession. Such techniques are also seen in instant messaging programs as repeatedly sending text can be assigned to a macro or AppleScript. Modern operating systems are usually resistant to these nuke attacks, and online games now have third party "Flood control."
A specific example of a nuke attack that gained some prominence is the WinNuke, which exploited the vulnerability in the NetBIOS handler in Windows 95. A string of out-of-band data was sent to TCP port 139 of the victim machine, causing it to lock up and display a Blue Screen of Death.

Distributed attack

A distributed denial of service attack (DDoS) occurs when multiple compromised systems flood the bandwidth or resources of a targeted system, usually one or more web servers. These systems are compromised by attackers using a variety of methods.
Malware can carry DDoS attack mechanisms; one of the more well known examples of this was MyDoom. Its DoS mechanism was triggered on a specific date and time. This type of DDoS involved hardcoding the target IP address prior to release of the malware and no further interaction was necessary to launch the attack.
A system may also be compromised with a trojan, allowing the attacker to download a zombie agent (or the trojan may contain one). Attackers can also break into systems using automated tools that exploit flaws in programs that listen for connections from remote hosts. This scenario primarily concerns systems acting as servers on the web.
Stacheldraht is a classic example of a DDoS tool. It utilizes a layered structure where the attacker uses a client program to connect to handlers, which are compromised systems that issue commands to the zombie agents, which in turn facilitate the DDoS attack. Agents are compromised via the handlers by the attacker, using automated routines to exploit vulnerabilities in programs that accept remote connections running on the targeted remote hosts. Each handler can control up to a thousand agents.[4]
These collections of compromised systems are known as botnets. DDoS tools like stacheldraht still use classic DoS attack methods centered around IP spoofing and amplification like smurf attacks and fraggle attacks (these are also known as bandwidth consumption attacks). SYN floods (also known as resource starvation attacks) may also be used. Newer tools can use DNS servers for DoS purposes. (see next section)
Unlike MyDoom's DDoS mechanism, botnets can be turned against any IP address. Script kiddies use them to deny the availability of well known websites to legitimate users.[5] More sophisticated attackers use DDoS tools for the purposes of extortion — even against their business rivals.[6]
It is important to note the difference between a DDoS and DoS attack. If an attacker mounts a smurf attack from a single host it would be classified as a DoS attack. In fact, any attack against availability would be classed as a Denial of Service attack (e.g. using High-energy radio-frequency weapons to render computer equipment inoperable, would be a DoS attack, albeit an exotic one.)[7]. On the other hand, if an attacker uses a thousand zombie systems to simultaneously launch smurf attacks against a remote host, this would be classified as a DDoS attack.
The major advantages to an attacker of using a distributed denial-of-service attack are that multiple machines can generate more attack traffic than one machine, multiple attack machines are harder to turn off than one attack machine, and that the behavior of each attack machine can be stealthier, making it harder to track down and shut down. These attacker advantages cause challenges for defense mechanisms. For example, merely purchasing more incoming bandwidth than the current volume of the attack might not help, because the attacker might be able to simply add more attack machines.
Although most DDoS attacks are malicious in nature, the same technique can be used to aid the Internet community. Internet fraud schemes, such as Nigerian 419 scams or phishing, commonly involve fraudulent websites that either impersonate a real website for purposes of stealing the victim's identity, or lend credibility to a scammer's fictional business venture to lure the victim into a false sense of confidence. Scam baiters, who combat these scams by posing as victims for the purpose of wasting the scammer's time and money and obtaining information that can be used by authorities, will forward sites they encounter during the course of their conversations to groups that specialize in site-killing.[citation needed] The group will first try to have a site taken down by informing the host of said site that the site is being used fraudulently. In the case where that approach fails, the group will organize a "takedown" of the site by encouraging its members to visit the site en masse and continually refresh its content (an intentional form of the Slashdot effect sometimes referred to as flash mobbing, although that term is technically reserved for real-world gatherings). Alternately, some groups have special web pages that link to images hosted by these fake sites and show the images to visitors (usually members or supporters of the site-killing group) while constantly reloading them, which is known as intentional bandwidth hogging.[citation needed] The purpose, similar to malicious DoS attacks, is to (a.) rapidly consume all of the website's allocated monthly bandwidth, after which requests for the site's content are refused, (b.) draw the attention of the site's host, who when faced with the constant onslaught on the entire hosting network's resources, will usually remove the site, and/or (c.) take up all available connections and maximum throughput of the host so that would-be victims cannot access the site.

Reflected attack

A distributed reflected denial of service attack (DRDoS) involves sending forged requests of some type to a very large number of computers that will reply to the requests. Using Internet protocol spoofing, the source address is set to that of the targeted victim, which means all the replies will go to (and flood) the target.
ICMP Echo Request attacks (described above) can be considered one form of reflected attack, as the flooding host(s) send Echo Requests to the broadcast addresses of mis-configured networks, thereby enticing many hosts to send Echo Reply packets to the victim. Some early DDoS programs implemented a distributed form of this attack.
Many services can be exploited to act as reflectors, some harder to block than others.[8] DNS amplification attacks involve a new mechanism that increased the amplification effect, using a much larger list of DNS servers than seen earlier.

DoS (Denial of Service)

A denial-of-service attack (DoS attack) is an attempt to make a computer resource unavailable to its intended users. Although the means to, motives for and targets of a DoS attack may vary, it generally comprises the concerted, malevolent efforts of a person or persons to prevent an Internet site or service from functioning efficiently or at all, temporarily or indefinitely.
Perpetrators of DoS attacks typically — but not exclusively — target sites or services hosted on high-profile web servers; a pair of DNS Backbone DDoS Attacks, on October 22, 2002 and February 6, 2007, targeted DNS root servers, in an apparent attempt to "disable the Internet" itself by taking away an option of addressing Internet servers by their human-friendly names.
One common method of attack involves saturating the target (victim) machine with external communications requests, such that it cannot respond to legitimate traffic, or responds so slowly as to be rendered effectively unavailable. In general terms, DoS attacks are implemented by:
forcing the targeted computer(s) to reset, or consume its resources such that it can no longer provide its intended service; and/or,
obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.
Denial-of-service attacks are considered violations of the IAB's Internet proper use policy. They also commonly constitute violations of the laws of individual nations.

Methods of attack

A "denial-of-service" attack is characterized by an explicit attempt by attackers to prevent legitimate users of a service from using that service. Examples include:
-flooding a network, thereby preventing legitimate network traffic;
-disrupting a server by sending more requests than it can possibly handle, thereby preventing access to a service;
-preventing a particular individual from accessing a service;
-disrupting service to a specific system or person.

Clones denying entrance into a Yahoo! chatroom.
Attacks can be directed at any network device, including attacks on routing devices and Web, electronic mail, or Domain Name System servers.

A DoS attack can be perpetrated in a number of ways. There are five basic types of attack:
-consumption of computational resources, such as bandwidth, disk space, or CPU time;
-disruption of configuration information, such as routing information;
-disruption of state information, such as unsolicited resetting of TCP sessions;
-disruption of physical network components.
-obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

A DoS attack may include execution of malware intended to:
-max out the CPU's usage, preventing any work from occurring;
-trigger errors in the microcode of the machine;
-trigger errors in the sequencing of instructions, so as to force the computer into an unstable state or lock-up;
-exploits errors in the operating system to cause resource starvation and/or thrashing, i.e. to use up all available facilities so no real work can be accomplished;
-crash the operating system itself.

Spoofing attack

Man-in-the-middle attack and internet protocol spoofing

An example from cryptography is the man-in-the-middle attack, in which an attacker spoofs Alice into believing they're Bob, and spoofs Bob into believing they're Alice, thus gaining access to all messages in both directions without the trouble of any cryptanalytic effort.
The attacker must monitor the packets sent from Alice to Bob and then guess the sequence number of the packets. Then the attacker knocks out Alice with a SYN attack and injects his own packets, claiming to have the address of Alice. Alice's firewall can defend against some spoof attacks when it has been configured with knowledge of all the IP addresses connected to each of its interfaces. It can then detect a spoofed packet if it arrives at an interface that is not known to be connected to the IP address.
Many carelessly designed protocols are subject to spoof attacks, including many of those used on the Internet. See Internet protocol spoofing

URL spoofing and phishing

Another kind of spoofing is "webpage spoofing," also known as phishing. In this attack, a legitimate web page such as a bank's site is reproduced in "look and feel" on another server under control of the attacker. The intent is to fool the users into thinking that they are connected to a trusted site, for instance to harvest user names and passwords.
This attack is often performed with the aid of URL spoofing, which exploits web browser bugs in order to display incorrect URLs in the browsers location bar; or with DNS cache poisoning in order to direct the user away from the legitimate site and to the fake one. Once the user puts in their password, the attack-code reports a password error, then redirects the user back to the legitimate site.

Another kind of spoofing is "webpage spoofing," also known as phishing. In this attack, a legitimate web page such as a bank's site is reproduced in "look and feel" on another server under control of the attacker. The intent is to fool the users into thinking that they are connected to a trusted site, for instance to harvest user names and passwords.
This attack is often performed with the aid of URL spoofing, which exploits web browser bugs in order to display incorrect URLs in the browsers location bar; or with DNS cache poisoning in order to direct the user away from the legitimate site and to the fake one. Once the user puts in their password, the attack-code reports a password error, then redirects the user back to the legitimate site.

Referer spoofing

Some websites, especially pornographic paysites, allow access to their materials only from certain approved (login-) pages. This is enforced by checking the Referer header of the HTTP request. This referer header however can be changed (known as "Referer spoofing" or "Ref-tar spoofing"), allowing users to gain unauthorized access to the materials.

Caller ID spoofing

In public telephone networks, it has for a long while been possible to find out who is calling you by looking at the Caller ID information that is transmitted with the call. There are technologies that transmit this information on landlines, on cellphones and also with VoIP. Unfortunately, there are now technologies (especially associated with VoIP) that allow callers to lie about their identity, and present false names and numbers, which could of course be used as a tool to defraud or harass. Because there are services and gateways that interconnect VoIP with other public phone networks, these false Caller IDs can be transmitted to any phone on the planet, which makes the whole Caller ID information now next to useless. Due to the distributed geographic nature of the Internet, VoIP calls can be generated in a different country to the receiver, which means that it is very difficult to have a legal framework to control those who would use fake Caller IDs as part of a scam

E-mail address spoofing

The sender information shown in e-mails (the "From" field) can be spoofed easily. This technique is commonly used by Spammers to hide the origin of their e-mails and leads to problems such as misdirected bounces (i.e. e-mail spam backscatter).

Login spoofing

The user is presented with an ordinary looking login prompt for username and password, which is actually a malicious program under the control of the attacker.

IP Spoofing

In computer networking, the term IP (Internet Protocol) address spoofing refers to the creation of IP packets with a forged (spoofed) source IP address with the purpose of concealing the identity of the sender or impersonating another computing system.

How spoofing works

The basic protocol for sending data over the Internet and many other computer networks is the Internet Protocol ("IP"). The header of each IP packet contains, among other things, the numerical source and destination address of the packet. The source address is normally the address that the packet was sent from. By forging the header so it contains a different address, an attacker can make it appear that the packet was sent by a different machine. The machine that receives spoofed packets will send response back to the forged source address, which means that this technique is mainly used when the attacker does not care about response or the attacker has some way of guessing the response.
In certain cases, it might be possible for the attacker to see or redirect the response to his own machine. The most usual case is when the attacker is spoofing an address on the same LAN or WAN.

Uses of spoofing

IP spoofing is most frequently used in denial-of-service attacks. In such attacks, the goal is to flood the victim with overwhelming amounts of traffic, and the attacker does not care about receiving responses to his attack packets. Packets with spoofed addresses are thus suitable for such attacks. They have additional advantages for this purpose - they are more difficult to filter since each spoofed packet appears to come from a different address, and they hide the true source of the attack. Denial of service attacks that use spoofing typically randomly choose addresses from the entire IP address space, though more sophisticated spoofing mechanisms might avoid unroutable addresses or unused portions of the IP address space. The proliferation of large botnets makes spoofing less important in denial of service attacks, but attackers typically have spoofing available as a tool, if they want to use it, so defenses against denial-of-service attacks that rely on the validity of the source IP address in attack packets might have trouble with spoofed packets. Backscatter, a technique used to observe denial-of-service attack activity in the Internet, relies on attackers' use of IP spoofing for its effectiveness.
IP spoofing can also be a method of attack used by network intruders to defeat network security measures, such as authentication based on IP addresses. This method of attack on a remote system can be extremely difficult, as it involves modifying thousands of packets at a time. This type of attack is most effective where trust relationships exist between machines. For example, it is common on some corporate networks to have internal systems trust each other, so that a user can log in without a username or password provided they are connecting from another machine on the internal network (and so must already be logged in). By spoofing a connection from a trusted machine, an attacker may be able to access the target machine without authenticating.

Defense against spoofing

Packet filtering is one defense against IP spoofing attacks. The gateway to a network usually performs ingress filtering, which is blocking of packets from outside the network with a source address inside the network. This prevents an outside attacker spoofing the address of an internal machine. Ideally the gateway would also perform egress filtering on outgoing packets, which is blocking of packets from inside the network with a source address that is not inside. This prevents an attacker within the network performing filtering from launching IP spoofing attacks against external machines.
It is also recommended to design network protocols and services so that they do not rely on the IP source address for authentication.

Upper layers

Some upper layer protocols provide their own defense against IP spoofing. For example, Transmission Control Protocol (TCP) uses sequence numbers negotiated with the remote machine to ensure that arriving packets are part of an established connection. Since the attacker normally can't see any reply packets, he has to guess the sequence number in order to hijack the connection. The poor implementation in many older operating systems and network devices, however, means that TCP sequence numbers can be predicted.

Pretty Good Privacy (PGP)

How PGP encryption worksPGP encryption uses public-key cryptography and includes a system which binds the public keys to a user name. Encryption/decryptionPGP message encryption normally uses both asymmetric key encryption and symmetric key encryption algorithms.Commonly, when encrypting a message, the sender uses the public key half of the recipient's key pair to encrypt a symmetric cipher session key. That session key is used, in turn, to encrypt the plaintext of the message. There are several other operational modes (eg, symmetric key operation only), but these are less commonly used.The recipient of a PGP-encrypted message decrypts the session key using his private key (the session key was encrypted by the sender using his public key). Next, he decrypts the ciphertext of the message using the session key.Use of two ciphers in this way was chosen, despite higher complication, in part because of the very considerable difference in operating speed between asymmetric key and symmetric key ciphers (the difference is often a factor of 1000 or more). This approach also makes it easily possible to send the same encrypted message to two or more recipients.The entire encryption and decryption operations are completely automated in current PGP desktop versions. Many PGP users' public keys are available to all from the many PGP key servers around the world, most of which coordinate their records so as to act as mirror sites for each other.Digital signaturesA similar strategy is used to detect whether a message has been altered since it was completed (the message integrity property), and whether it was actually sent by the person/entity claimed to be the sender (a digital signature). In PGP, it is used by default in conjunction with encryption, but can be applied to plaintext as well. The sender uses PGP to create a digital signature for the message with either the RSA or DSA signature algorithms. To do so, PGP computes a hash (also called a message digest) from the plaintext, and then creates the digital signature from that hash using the sender's private key.The message recipient uses the sender's public key and the digital signature to recover the original message digest. He compares this message digest with the message digest he computed himself from the (recovered) plaintext. If the signature matches the received plaintext's message digest, it must be presumed (to a very high degree of confidence) that the message received has not been tampered with, either deliberately or accidentally. As well, since it was properly signed, it is very likely (to a very high degree of confidence) that the claimed sender actually did send it.Web of trustBoth when encrypting messages and when verifying signatures, it is critical that the public key one uses to send messages to someone or some entity actually does 'belong' to the intended recipient. Simply downloading a public key from somewhere is not overwhelming assurance of that association; deliberate (or accidental) spoofing is possible. PGP has, from its first versions, always included provisions for distributing a user's public keys in an 'identity certificate' which is so constructed cryptographically that any tampering (or accidental garble) is readily detectable. But merely making a certificate effectively impossible to modify undetectably is also insufficient. It can prevent corruption only after the certificate has been created, not before. Users must also ensure by some means that the public key in a certificate actually does belong to the person/entity claiming it. From its first release, PGP products have included an internal certificate 'vetting scheme' to assist with this; it has been called a web of trust. A given public key (or more specifically, information binding a user name to a key) may be digitally signed by a third party user to attest to the association between someone (actually a user name) and the key. There are several levels of confidence which can be included in such signatures. Although many programs read and write this information, few (if any) include this level of certification when calculating whether to trust a key.The web of trust protocol was first described by Zimmermann in the manual for PGP version 2.0:As time goes on, you will accumulate keys from other people that you may want to designate as trusted introducers. Everyone else will each choose their own trusted introducers. And everyone will gradually accumulate and distribute with their key a collection of certifying signatures from other people, with the expectation that anyone receiving it will trust at least one or two of the signatures. This will cause the emergence of a decentralized fault-tolerant web of confidence for all public keys.The web of trust mechanism has advantages over a centrally managed PKI scheme, but has not been universally used. Users have been willing to accept certificates and check their validity manually, or to simply accept them. The underlying problem has found no satisfactory solution.CertificatesIn the (more recent) OpenPGP specification, trust signatures can be used to support creation of certificate authorities. A trust signature indicates both that the key belongs to its claimed owner and that the owner of the key is trustworthy to sign other keys at one level below their own. A level 0 signature is comparable to a web of trust signature, since only the validity of the key is certified. A level 1 signature is similar to the trust one has in a certificate authority because a key signed to level 1 is able to issue an unlimited number of level 0 signatures. A level 2 signature is highly analogous to the trust assumption users must rely on whenever they use the default certificate authority list (like those included in web browsers); it allows the owner of the key to make other keys certificate authorities.PGP versions have always included a way to cancel ('revoke') identity certificates. A lost or compromised private key will require this if communication security is to be retained by that user. This is, more or less, equivalent to the certificate revocation lists of centralized PKI schemes. Recent PGP versions have also supported certificate expiration dates.The problem of correctly identifying a public key as belonging to a particular user is not unique to PGP. All public key / private key cryptosystems have the same problem, if in slightly different guise, and no fully satisfactory solution is known. PGP's original scheme, at least, leaves the decision whether or not to use its endorsement/vetting system to the user, while most other PKI schemes do not, requiring instead that every certificate attested to by a central certificate authority be accepted as correct.Security qualityTo the best of publicly available information, there is no known method which will allow a person or group to break PGP encryption by cryptographic, or computational means. Early versions of PGP have been found to have theoretical vulnerabilities and so current versions are recommended. Indeed, in 1996, cryptographer Bruce Schneier characterized an early version as being "the closest you're likely to get to military-grade encryption."[1] In contrast to security systems/protocols like SSL which only protect data in transit over a network, PGP encryption can also be used to protect data in long-term data storage such as disk files.The cryptographic security of PGP encryption depends on the assumption that the algorithms used are unbreakable by direct cryptanalysis with current equipment and techniques. For instance, in the original version, the RSA algorithm was used to encrypt session keys; RSA's security depends upon the one-way function nature of mathematical integer factoring. New, now unknown, integer factorization techniques might, therefore, make breaking RSA easier than now, or perhaps even trivially easy. However, it is generally presumed by informed observers that this is an intractable problem, and likely to remain so. Likewise, the secret key algorithm used in PGP version 2 was IDEA, which might, at some future time, be found to have a previously unsuspected cryptanalytic flaw. Specific instances of current PGP, or IDEA, insecurities -— if they exist -— are not publicly known. As current versions of PGP have added additional encryption algorithms, the degree of their cryptographic vulnerability varies with the algorithm used. In practice, each of the algorithms in current use is not publicly known to have cryptanalytic weaknesses.

Cryptography

Cryptography is, traditionally, the study of ways to convert information from its normal, comprehensible form into an obscured guise, unreadable without special knowledge — the practice of encryption. In the past, cryptography helped ensure secrecy in important communications, such as those of spies, military leaders, and diplomats. In recent decades, the field of cryptography has expanded its remit. Examples include schemes like digital signatures and digital cash, digital rights management for intellectual property protection, and securing electronic commerce. Cryptography is now often built into the infrastructure for computing and telecommunications; users may not even be aware of its presence.In cryptology, RSA is an algorithm for public-key cryptography. It was the first algorithm known to be suitable for signing as well as encryption, and one of the first great advances in public key cryptography. RSA is widely used in electronic commerce protocols, and is believed to be secure given sufficiently long keys and the use of up-to-date implementations.Padding schemesWhen used in practice, RSA is generally combined with some padding scheme. The goal of the padding scheme is to prevent an number of attacks that potentially work against RSA without padding:• When encrypting with low encryption exponents (e.g., e = 3) and small values of the m, (i.e. m is less than n1/e) the result of me is strictly less than the modulus n. In this case, ciphertexts can be easily decrypted by taking the eth root of the ciphertext over the integers. • Because RSA encryption is a deterministic encryption algorithm – i.e., has no random component – an attacker can successfully launch a chosen plaintext attack against the cryptosystem, by encrypting likely plaintexts under the public key and test if they are equal to the ciphertext. A cryptosystem is called semantically secure if an attacker cannot distinguish two encryptions from each other even if the attacker knows (or has chosen) the corresponding plaintexts. As described abouve, RSA without padding is not semantically secure. • RSA has the property that the product of to ciphertexts is equal to the encryption of the product of the respective plaintexts. That is Because of this multiplicatvive property a chosen-ciphertext attack is possible. E.g. an attacker, who wants to know the decryption of a ciphertext c=me mod n may ask the holder of the secret key to decrypt an unsuspiciously looking ciphertext cremod n for some value r chosen by the attacker. Because of the multiplicative property this is the encryption of mrmod n. Hence, if the attacker is successful with the attack, he will learn mrmod n from which he can derive the message m by multiplying mr with the modular inverse of r modulo n. To avoid these problems, practical RSA implementations typically embed some form of structured, randomized padding into the value m before encrypting it. This padding ensures that m does not fall into the range of insecure plaintexts, and that a given message, once padded, will encrypt to one of a large number of different possible ciphertexts.Standards such as PKCS have been carefully designed to securely pad messages prior to RSA encryption. Because these schemes pad the plaintext m with some number of additional bits, the size of the un-padded message M must be somewhat smaller. RSA padding schemes must be carefully designed so as to prevent sophisticated attacks which may be facilitated by a predictable message structure. Early versions of the PKCS standard (i.e. PKCS #1 up to version 1.5) used a construction that turned RSA into a semantically secure encryption scheme. This version was later found vulnerable to a practical adaptive chosen ciphertext attack. Later versions of the standard include Optimal Asymmetric Encryption Padding (OAEP), which prevents these attacks. The PKCS standard also incorporates processing schemes designed to provide additional security for RSA signatures, e.g., the Probabilistic Signature Scheme for RSA (RSA-PSS).Signing messagesSuppose Alice uses Bob's public key to send him an encrypted message. In the message, she can claim to be Alice but Bob has no way of verifying that the message was actually from Alice since anyone can use Bob's public key to send him encrypted messages. So, in order to verify the origin of a message, RSA can also be used to sign a message.Suppose Alice wishes to send a signed message to Bob. She produces a hash value of the message, raises it to the power of d mod n (as she does when decrypting a message), and attaches it as a "signature" to the message. When Bob receives the signed message, he raises the signature to the power of e mod n (as he does when encrypting a message), and compares the resulting hash value with the message's actual hash value. If the two agree, he knows that the author of the message was in possession of Alice's secret key, and that the message has not been tampered with since.Note that secure padding schemes such as RSA-PSS are as essential for the security of message signing as they are for message encryption, and that the same key should never be used for both encryption and signing purposes.

Symmetric Key Cryptography

In symmetric key cryptography, both parties must possess a secret key which they must exchange prior to using any encryption. Distribution of secret keys has been problematic until recently, because it involved face-to-face meeting, use of a trusted courier, or sending the key through an existing encryption channel. The first two are often impractical and always unsafe, while the third depends on the security of a previous key exchange.In public key cryptography, the key distribution of public keys is done through public key servers. When a person creates a key-pair, he keeps one key private and the other, public-key, is uploaded to a server where it can be accessed by anyone to send the user a private, encrypted, message. Disclosure of these public keys is not only not a problem, but is actively encouraged. The private keys are never transmitted, and can therefore be physically secured.Secure Sockets Layer (SSL) uses Diffie-Hellman key exchange if the client does not have a public-private key pair and a published certificate in the Public Key Infrastructure, and Public Key Cryptography if the user does have both the keys and the credential.In secret sharing a secret (password, key, trade secret,...) is used as a seed to generate a number of distinct secrets, and the pieces are distributed so that some subset of the recipients can jointly authenticate themselves and use the secret information without learning what it is. Secret sharing is also called secret splitting, key splitting, and split knowledge.We want to share N secrets among M people so that any M < N of them (M of N) can regenerate the original information, but no smaller group up to M − 1 can do so. There are mathematical problems of this type, such as the number of points needed to identify a polynomial of a certain degree (used in Shamir's scheme), or the number of intersecting hyperplanes needed to specify a point (used in Blakley's scheme). We can hand out data specifying any number of points on the curve, or hyperplanes through the point, without altering the number needed to solve the problem and, in our application, access the protected resource.Key distribution is an important issue in wireless sensor network (WSN) design. There are many key distribution schemes in the literature that are designed to maintain an easy and at the same time secure communication among sensor nodes. The most accepted method of key distribution is WSNs is key predistribution, where secret keys are placed in sensor nodes before deployment. When the nodes are deployed over the target area, the secret keys are used to create the network. For more info see: key distribution in wireless sensor networks.