Network Security Cryptography
This paper tries to present an insight into cryptography, the ways of implementing it, its uses and implications. Cryptography, the art and science of secret codes has been existent right from the advent of human civilization; it has been used to transmit messages safely and secretively across groups of people so that their adversaries did not get to know their secrets. As civilizations progressed more and more complex forms of cryptography came into being, they were now not only symbolic representations in an unrecognizable form but complex mathematical transforms carried out on the messages. In the present day world cryptography plays a major role in safe transmission of data across the Internet, and other means of communications.
In this paper we have dealt with examples of how different crypto algorithms are implemented, and have tried to cite some of the most used crypto algorithms, like DES- the data encryption standard, RSA, IDEA, RC#4, etc. We have also dealt with some of the applications of these algorithms like link encryption, pretty good privacy, public key cryptography, PEM etc. We have also cited some methods of code-breaking or cryptanalysis like the mathematical attack, the brute force attack and the power analysis.
Cryptography
If you want something to stay a secret, don't tell anyone, don't write it down. If you do have to send it to someone else, hide it in another message so that only the right person will understand. Many creative methods of hiding messages have been invented over the centuries. Cryptography can be defined as the art and science of secret codes. It is a collection of techniques that transform data in ways that are difficult to mimic or reverse by some one who does not know the secret. These techniques involve marking transforming and reformatting the messages to protect them from disclosure, change or both. Cryptography in the computer age basically involves the translation of the original message into a new and unintelligible one by a mathematical algorithm using a specific "key". People mean different things when they talk about cryptography. Children play with toy ciphers and secret languages. However, these have little to do with real security and strong encryption. Strong encryption is the kind of encryption that can be used to protect information of real value against organized criminals, multinational corporations, and major governments. Strong encryption used to be only military business; however, in the information society it has become one of the central tools for aintaining privacy and confidentiality.
Why do we need cryptography?
The art of long distance communication has been mastered by civilizations many centuries ago. The transmission of secret political or confidential information was a problem ever since. To solve this
problem to some extent, secret codes were developed by groups of people who had to carry out such kind of secretive communications. These codes were designed to transform words into code words using some basic guide lines known only to their members. Now messages could be sent or received with a reduced danger of hacking or forgery as the code breaker would have to struggle really hard to break the code.
As time progressed and radio, microwave and internet communication developed, more complex and safer codes started to evolve. The traditional use of cryptography was to make messages unreadable to the enemy during wartime. However the introduction of the computing age changed this perspective dramatically. Through the use of computers, a whole new use for information hiding was evolved. Around the early 1970's the private sector began to feel the need for cryptographic methods to protect their data. This could include 'sensitive information' (corporate secrets), password files or personal records.
Needfor Cryptography
Some day to day examples
Encryption technology is used nowadays in almost any of the digital communication systems. For example, the most common one is the satellite T.V or the cable T.V. All the signals are available in the air, but the programs can be viewed only by those subscribers who have made the payment. This is done by a simple password security system. The subscriber gets an authenticated password on payment and can use it only for the time he has paid up after which it gets lapsed. Another common application of the encryption technique is the ATM card. Here also the transaction is done only on the acceptance of a secure and authenticated password. The mobile phones and for that matter even the internet connections are based on small scale cryptographic techniques.
Crypto algorithm
The crypto algorithm specifies the mathematical transformation that is performed on data to encrypt or decrypt it. A crypto algorithm is a procedure that takes the plain text data and transforms it in to cipher text in a reversible way. A good algorithm produces cipher text that yields very few clues about either the key or the plain text that produced it. Some algorithm are for stream ciphers which encrypt a digital data stream bit by bit .The best known algorithm are for the block ciphers which transform data in fixed sized blocks one at a time.
• Stream ciphers
Stream cipher algorithm is designed to accept a crypto key and a stream of plain text to produce a stream of cipher text.
• Block cipher
Block ciphers are designed to take data blocks of a specific size, put them with a key of a particular size and yield a block of cipher text of a certain size. Block ciphers are
analyzed and tested for their ability to encrypt data blocks of their given block size. A reasonable cipher should generate a cipher text that has as few noticeable properties as possible. A statistical analysis of cipher text generated by the block cipher algorithm should find that individual data bits as well as patterns of bits appear completely random. Non random patterns are the first thing for which a code breaker looks as they usually provide the entering wedge needed to crack a code.
Cipher modes
The term cipher mode refers to a set of techniques used to apply to a block cipher to a data stream. Several modes have been developed to disguise repeated plaintext blocks and improve security of the block cipher. Each mode defines a method of combining the plaintext , crypto key, and encrypted cipher text in a special way to generate the stream of cipher text actually transmitted to the recipient In theory there could be countless different ways of combining and feeding back the inputs and outputs of a cipher. In practice, four basic modes are used.
• Electronic Code Book (ECB)
It is the simplest of all the modes .The cipher is simply applied to the plaintext block by block it is the most efficient mode. It can be speedup by using parallel hardware and unlike other modes, does not require an extra data word for seeding a feed back loop. However a block of padding may be needed to guarantee that full blocks are provided for encryption and decryption. ECB has security problems in the sense that repeated plain text blocks yield repeated cipher text blocks.
• Cipher Block Chaining (CBC)
This mode hides patterns in the plaintext block by systematically combining each plaintext block with a cipher text block before actually encrypting it the two blocks are combined bit by bit using the
exclusive or operation. In order to guarantee that there is always some random looking cipher text to apply to the actual plaintext the process is started with a block of random bits called the initialization vector. Two messages will never yield the same cipher text even if the plain texts are identical as long as the initialization vector is different. In most applications the initialization vector is added at the beginning of message in plain text. A shortcoming of CBC is that encrypted messages may be as many as two blocks longer than the same message in ECB mode. One of the blocks is added to transmit the initialization vector to the recipient. Proper decryption depends on the initialization vector to start the feedback process. The other block is added as padding so that a full block is always encrypted or decrypted.
• CFB - Cipher Feedback mode
CFB is similar to CBC in that, it feeds the cipher text block back to the block cipher. However it is different because the block cipher doesn’t directly encrypt the plain text. Instead it is used to generate a constantly varying key that encrypts the plain text with a vernam cipher. In other words blocks of cipher text are exclusive or’ed with successive blocks of data generated by the block cipher to generate the cipher text. This mode is also called the cipher text auto key (CTAK).The advantage with this method is that it is not limited to the cipher block size. This mode can be adapted to work with smaller blocks down to bits. Like CBC however it needs an initialization vector to be sent for decryption.
• OFB - Output Feedback
It is similar to CFB but it is simpler. It uses the block cipher all by itself to generate the vernam keys. The key stream doesn’t depend on the data stream at all. Here the block cipher has nothing to do with processing the message .It is only used to generate the keys. This mode is also called auto key mode. The advantage is that like CFB length of the plain text need not have to fit into block boundaries, also each block requires the initialization vector and doesn’t depend on the data stream, so the decryption key stream can be prepared and kept at the receivers end with the knowledge of the key and the initialization vector.
Crypto Algorithms
1. DES
This is a widely used algorithm. It was developed by IBM (previously Lucifer) and was adopted as an official Federal Information Processing Standard (FIPS PUB 46) in 1976.This algorithm uses a 64 bit key (including 8 parity bits +56 key bits), converting 64 bit blocks of plaintext in to 64 bit blocks of code (block cipher method).This is done by putting the original text through a series of permutations and substitutions. The results are then merged with the original plain text using an XOR operation. This encryption sequence is repeated 16 times using a different arrangement of the key bits each time.
2. One time pads
A one-time pad is a very simple yet completely unbreakable symmetric cipher. That is it uses the same key for encryption as for decryption. As with all symmetric ciphers, the sender must transmit the key to the recipient via some secure channel, otherwise the recipient won't be able to decrypt the cipher ext. The key for a one-time pad cipher is a string of random bits, usually generated by a cryptographically strong pseudo-random number generator (CSPRNG).With a one-time pad, there are as many bits in the key as in the plaintext. This is the primary drawback of a one-time pad, but it is also the source of its perfect security. It is essential that no portion of the key should ever be used for another encryption (therefore the name "one-time pad"), otherwise cryptanalysis can break the cipher. The algorithm is very simple for example an ex-or operation between the plain text and the key, the same ex-or operation would also give back the plain text.
Ciphertext = plaintext (+) key
Plaintext = ciphertext (+) key
However the security of the one time pad is dependant upon the randomness of the generated key. The code is supposed to be safe even from brute force attack, running the text through all possible keys, because equal number of probably correct plaintext messages would be generated.
3. Triple DES
Triple encoding makes DES encoded text even more secure. It is equivalent to having a 112 bit key length. However, triple DES is significantly slower than commercial alternatives with similar key lengths.
4. Rivest Cipher #4
Rc4 is a symmetrical stream cipher developed by Ron Rivest. It has a key whose size can be varied according to the level of security required, generally it can be used with a 128 bit key length. This algorithm is fairly immune to differential crypt analysis but since it is used with short key lengths it is vulnerable to brute force cracking.
5. Idea
Idea is an algorithm which appeared in 1990. It was developed at the Swiss federal institute of technology. Its security is based not on hiding the algorithm but on keeping a secret key. Its key is 128 bit long key which makes it more attractive than DES, and it can be used with the usual block ciphers. This algorithm is publicly available and easy to implement. It is suitable for e-commerce, it can be exported and used world wide. Of late none of the cryptanalysis techniques have worked against IDEA. A brute force attack (with its 128 bit key length) would take trying 1 billion keys/sec for over a billion yrs would still not find the key.
6. Skip Jack
It is a block encryption algorithm developed by NSA (National security agency –USA). It encrypts 64 bit blocks using an 80 bit key. The usual block ciphers can be used to implement it to encrypt streams of data. It is provided in prepackaged encryption chipsets and in the Fortezza crypto card, a pc card containing a crypto processor and storage for keying material. The disadvantage of skipjack is that it is very less publicly known {reportedly to keep NSA’s design techniques secret}.It is fairly resistant to differential cryptanalysis and other short cut attacks. NSA’s skipjack is being promoted to protect military communications in the Defense Messaging System (DMS) which reflects a measure of confidence that skipjack is secure.
7. RSA public key algorithm
The best known and most popular embodiment of the public key idea is the RSA also named after its inventors Ronald Rivest, Adi Shamir and Leonard Adleman. The high level of security the RSA algorithm offers derives from the difficulty of decomposing large integers in to prime factors. Two primes which when multiplied by one another give the original number. Prime factoring of very large numbers is an important field in the number theory .One of the drawbacks with RSA algorithm compared with symmetrical methods is that encrypting and decrypting messages takes much more computing power .The fastest RSA chip now in existence can only manage a through put of 600 k bits when using 512 b it primes. Comparable DES hardware implementations are anything from 1000 to 10000 times faster. At present Des software implementation can encrypt around 100 times faster than the RSA algorithm. Crypt analysis can be done by factorizing the key in to two primes, estimates for factoring a 512 bit key show that computer system running at a million operations a sec (1 MIPS) and using current algorithms would take 420000 years to find the prime factors involved.
8. AES
The AES is a new algorithm that has now replaced DES as the new standard in the NIST. The Advanced Encryption Standard (AES) provides a better combination of safety and speed than DES. Using 128-bit secret keys, AES offers higher security against brute-force attack than the old 56-bit
DES keys, and AES can use larger 192-bit and 256-bit keys, if necessary. AES is a block cipher, and encrypts data in fixed-size blocks, but each AES cycle encrypts 128 bits-twice the size of DES blocks. While DES was designed for hardware, AES runs efficiently in a broad range of environments, from programmable gate arrays, to smart cards, to desktop computer software and browsers. In 2000, NIST selected Rijndael, an encryption algorithm developed by two Belgian cryptographers, as the new AES. There are a few products that already use the Rijndael algorithm, notably Unix's NetBSD open-source version. Rijndael has also appeared as an option in several desktop file-encryption programs. The AES is expected to be the FIPS (Federal information processing standards) quite soon.
Internet cryptography techniques (Applications of the crypto algorithms)
• Point-to-point link encryption
• IP link encryption
• A virtual private network(VPN) constructed with IP security protocol routers
• A VPN constructed with IPSEC firewalls
• Public key algorithm with Pretty Good Privacy(PGP)
• E-mail with privacy enhanced mail (PEM)
• Water marking.
• Point-to-point link encryption
This produces a fully isolated connection between a pair of computers by applying crypto to the data link. It yields the highest security by being the most restrictive in physical and electronic access. It is not necessarily an internet solution since it doesn’t need to use TCP/IP software. It is the simplest design, but the most expensive to implement and extend.
• IP link encryption
This produces a highly secure extensible TCP/IP network by applying crypto to the data link and by restricting physical access to hosts on the network. This architecture blocks communication with untrusted hosts and sites. Sites use point to point interconnections and apply encryption to all traffic on those interconnections.
• VPN construction with IP security
This is a virtual private newt that uses the internet to carry traffic between trusted sites. Crypto is applied at the internet layer using IPSEC. This approach uses encrypting routers and doesn’t provide the sites with access to untrusted internet sites.
• VPN construction with IPSEC firewalls
This is a different approach to the VPN that uses encrypting firewalls instead of encrypting routers. Crypto is still applied at the internet layer using IPSEC (IP security protocol).The firewalls encrypt all traffic between trusted sites and also provide control access to untrusted hosts. Strong firewall access control is necessary to reduce the risk of attacks on the crypto mechanisms as well as attacks on hosts within the trusted sites.
Digital signature
Digital signatures can be used to check the authenticity of the author of the message using the above mentioned technique. In 1991the national institute of standards and technology (NIST) decided on a standard for digital signatures, DSS (digital signature standard). DSS proposes an algorithm for digital signatures (DSA, digital signatures algorithm) although this is not based on the RSA but on the public key implementation of the “discrete logarithm problem” (what value must the exponent x assume to satisfy y= g ^x mod p where p is a prime. While the problem underlying this method is just as hard to solve as RSA’s prime factor decomposition, many people have claimed that DSA’s security is not perfect. After massive criticism its length was finally increased from 512 to 1024 bits. DSS is expected to become an official standard for US Govt. bureaus in not too distant future.
• PEM
PEM is the standard for encrypting messages on the internet’s mail service. It uses both RSA public key method and the symmetrical DES method to send a file in encrypted form, it is first encrypted using a randomly generated DES key generated using a DES algorithm. The DES key itself is then encoded with the recipient’s public key on the RSA system and sent along with the DES encoded file. The advantage of this is that only a small part of the message, the DES key has to be encoded using the time consuming RSA algorithm. The contents of the message itself are encrypted much faster using the DES algorithm alone.
• Message Digests
There is one more important encryption technique worth mentioning and that is the one way function. It is basically a non reversible quick encryption. In other words encrypting is easy but decrypting is not. While encryption could take a few seconds, decryption could take 100s or 1000s or millions of years even for the most powerful computers. These are used basically to test the integrity of a document or file by generating a digital fingerprint using special hash functions on the document. Assume that you have a document to send someone or store for the future and you need a way to prove at sometime that the document has been altered. You run a one way function which produces a fixed length value called a hash (also called a message digest). The hash is a unique signature of a document that you can keep and send with the document. The recipient can run the same one way function to produce a hash that should match the one you sent with the document. If the hashes don’t match the document has been altered or corrupted.
• Water marking
A watermark is that which is actually imperceptibly added to the cover-signal in order to convey the hidden data. It is used to protect the copy rights of the author on the internet. A water mark is a hidden file consisting of either a picture or data that gets copied with the document when ever it is downloaded from the web, and because of this the article cannot be unauthorizedly copied or distributed.
Latest crypto techniques
The policy about regulating technology ends up being obsolete by technological innovations. Trying to regulate confidentiality by regulating encryption closes one door and leaves two open steganography and winnowing.
• Steganography
An encrypted message looks garbage like, and alert people that there is something to hide. But what if the message is totally innocuous looking? This is an old trick that started centuries ago with writing in ink that is invisible until the paper has been heated. The microdot, a piece of film containing a very highly reduced image of the secret message and embedded in the punctuation marks of a normal document, was invented during World War II.. For example if you used the least significant bit of each pixel in a bitmap image to encode a message the impact on the appearance of the image would not be noticeable. This is known as steganography, or covered writing. A 480 pixel wide by 100 pixel high image - smaller than many WWW home page banners, could theoretically contain a message of more than 5,000 characters. The encoding is quite easy with a computer - and no complicated mathematics at
all. And of course the same principles apply to audio and video files as well. The image can be used simply as a carrier, with the message being first encrypted.
• Winnowing and Chaffing
Just as the name suggests the above technique believes in adding chaff (garbage data) to the wheat (message) before sending the message and then winnowing or removing the chaff from the wheat at the receiver. Since winnowing does not use encryption it is not affected by the regulations on crypto products. The message is first broken into packets and then each packet is mac’ed using a mac program such as (HMAC-SHA1). This is very similar to running the program through a hash function. Then chaff is added (chaffing) to the packets of mac’ed data before it is sent. At the receiving end only those packets are accepted that produce the same mac (showing that no changes have been made) and then the chaff is removed, this is called winnowing.
Cryptanalysis
There are many kinds of cryptanalytic techniques:
1) Differential cryptanalysis.
2) Linear cryptanalysis.
3) Brute force cracking
4) Power analysis
5) Timing analysis, etc.
Cryptographers have traditionally analyzed the security of ciphers by modeling crypto algorithms as ideal mathematical objects. A modern cipher is conventionally a black box that accepts plaintext inputs and provides cipher text outputs. Inside this box, the algorithm maps the inputs to the outputs using a predefined function that depends on the value of a secret key. The black box is described mathematically and formal analysis is used to examine the systems security. In a modern cipher an algorithms security rests solely on the concealment of the secret key. Thus attack strategies often reduce to methods that can expose the value of the secret key. Unfortunately hardware implementations of the algorithm can leak information about the secret key, which the adversaries can use.
Mathematical attacks
Techniques such as differential and linear cryptanalysis introduced in early 1990s are representative of traditional mathematical attacks. Differential and linear cryptanalysis attacks work by exploiting statistical properties of crypto algorithms to uncover potential weaknesses. These attacks are not dependent on a particular implementation of the algorithm but on the algorithm itself, therefore they can be broadly applied. Traditional attacks however require the acquisition and manipulation of large amounts of data. Attacks that exploit weaknesses in a particular implementation are an attractive alternative and are often more likely to succeed in practice.
Implementation attacks
The realities of a physical implementation can be extremely difficult to control and often result in unintended leakage of side channel information like power dissipation, timing information, faulty outputs etc. The leaked information is often correlated to the secret key, thus enemies monitoring the information may be able to learn the secret key and breach the security of the crypto system. Algorithms such as DES, RSA which are now being implemented in smart cards also are under a considerable threat. Smart cards are often used to store crypto keys and execute crypto algorithms. Data on the card is also stored using cryptographic techniques.
Power consumption is one of the potential side channel information; generally supplied by an external source it can be directly observed. All calculations performed by the smartcard operate on logical 0s or 1s. Current technological constraints result in differential power consumptions when manipulating a logic one or a logic zero, based on a spectral analysis of the power curve or based on the timing between the one and the zero the secret code can be cracked by the adversaries.
Countermeasures
Many countermeasures are being worked out to prevent implementation attacks such as power analysis or timing analysis. These attacks are normally based on the assumption that the operations being attacked are occurring at fixed intervals of time. If the operations are randomly shifted in time then statistical analyisis of side channel information becomes very difficult. Another side of the coin is that the hardware implementations must be carefully designed so that they do not leak any side channel information. Hard ware design methodologies are often difficult to design, analyse and test, hence software methods of introducing delay or data masking are the only easy methods to overcome this problem.
Conclusion
The internet has brought with it an unparalleled rate of new technology adoption. The commercially established, the industry and the armed forces would need an assortment of cryptographic products and other mechanisms to provide privacy, authentication, message integrity and trust to achieve their missions. These mechanisms demand procedures, policies and law. However, cryptography is not an end unto itself but the enabler of safe business and communication. Good cryptography and policies are therefore as essential for the future of internet based communications as the applications that utilize them.
