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An Overview of Cryptography. Does increased security provide comfort to paranoid people? Or does security provide some very basic protections that we are naive to believe that we don't need? During this time when the Internet provides essential communication between literally billions of people and is used as a tool for commerce, social interaction, and the exchange of an increasing amount of personal information, security has become a tremendously important issue for every user to deal with.
One essential aspect for secure communications is that of cryptography. But it is important to note that while cryptography is necessary for secure communications, it is not by itself sufficient.
The reader is advised, then, that the topics covered here only describe the first of many steps necessary for better security in any number of situations. The first is to define some of the terms and concepts behind basic cryptographic methods, and to offer a way to compare the myriad cryptographic schemes in use today. The second is to provide some real examples of cryptography in use today. Such mention is for example purposes only and, unless explicitly stated otherwise, should not be taken as a recommendation or endorsement by the author. Cryptography is the science of secret writing is an ancient art; the first documented use of cryptography in writing dates back to circa 1. B. C. Some experts argue that cryptography appeared spontaneously sometime after writing was invented, with applications ranging from diplomatic missives to war- time battle plans.
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It is no surprise, then, that new forms of cryptography came soon after the widespread development of computer communications. In data and telecommunications, cryptography is necessary when communicating over any untrusted medium, which includes just about any network, particularly the Internet. Plaintext is encrypted into ciphertext, which will in turn (usually) be decrypted into usable plaintext. The encryption and decryption is based upon the type of cryptography scheme being employed and some form of key. For those who like formulas, this process is sometimes written as. C = Ek(P)P = Dk(C). P = plaintext, C = ciphertext, E = the encryption method, D = the decryption method, and k = the key.
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If there is a third and fourth party to the communication, they will be referred to as Carol and Dave, respectively. A malicious party is referred to as Mallory, an eavesdropper as Eve, and a trusted third party as Trent.
Cryptology is the term referring to the broad study of secret writing, and encompasses both cryptography and cryptanalysis. For purposes of this paper, they will be categorized based on the number of keys that are employed for encryption and decryption, and further defined by their application and use. The three types of algorithms that will be discussed are (Figure 1). Secret Key Cryptography (SKC): Uses a single key for both encryption and decryption; also called symmetric encryption. Primarily used for privacy and confidentiality.
Public Key Cryptography (PKC): Uses one key for encryption and another for decryption; also called asymmetric encryption. Primarily used for authentication, non- repudiation, and key exchange. Hash Functions: Uses a mathematical transformation to irreversibly . Primarily used for message integrity. FIGURE 1: Three types of cryptography: secret key, public key,and hash function.
As shown in Figure 1. A, the sender uses the key to encrypt the plaintext and sends the ciphertext to the receiver. The receiver applies the same key to decrypt the message and recover the plaintext.
Because a single key is used for both functions, secret key cryptography is also called symmetric encryption. The biggest difficulty with this approach, of course, is the distribution of the key (more on that later in the discussion of public key cryptography). Stream ciphers come in several flavors but two are worth mentioning here (Figure 2). Self- synchronizing stream ciphers calculate each bit in the keystream as a function of the previous n bits in the keystream. It is termed . One problem is error propagation; a garbled bit in transmission will result in n garbled bits at the receiving side. Synchronous stream ciphers generate the keystream in a fashion independent of the message stream but by using the same keystream generation function at sender and receiver. While stream ciphers do not propagate transmission errors, they are, by their nature, periodic so that the keystream will eventually repeat.
In general, the same plaintext block will always encrypt to the same ciphertext when using the same key in a block cipher whereas the same plaintext will encrypt to different ciphertext in a stream cipher. The most common construct for block encryption algorithms is the Feistel cipher, named for cryptographer Horst Feistel (IBM). As shown in Figure 3, a Feistel cipher combines elements of substitution, permutation (transposition), and key expansion; these features create a large amount of .
One advantage of the Feistel design is that the encryption and decryption stages are similar, sometimes identical, requiring only a reversal of the key operation, thus dramatically reducing the size of the code (software) or circuitry (hardware) developed to implement the cipher. One of Feistel's early papers describing this operation is . Two identical plaintext blocks, then, will always generate the same ciphertext block.
ECB is susceptible to a variety of brute- force attacks (because of the fact that the same plaintext block will always encrypt to the same ciphertext), as well as deletion and insertion attacks. I addition, a single bit error in the transmission of the ciphertext results in an error in the entire block of decrypted plaintext. Cipher Block Chaining (CBC) mode adds a feedback mechanism to the encryption scheme; the plaintext is exclusively- ORed (XORed) with the previous ciphertext block prior to encryption so that two identical plaintext blocks will encrypt differently. While CBC protects against many brute- force, deletion, and insertion attacks, a single bit error in the ciphertext yields an entire block error in the decrypted plaintext block and a bit error in the next decrypted plaintext block. Cipher Feedback (CFB) mode is a block cipher implementation as a self- synchronizing stream cipher. CFB mode allows data to be encrypted in units smaller than the block size, which might be useful in some applications such as encrypting interactive terminal input. If we were using one- byte CFB mode, for example, each incoming character is placed into a shift register the same size as the block, encrypted, and the block transmitted.
At the receiving side, the ciphertext is decrypted and the extra bits in the block (i. CFB mode generates a keystream based upon the previous ciphertext (the initial key comes from an Initialization Vector . In this mode, a single bit error in the ciphertext affects both this block and the following one. Output Feedback (OFB) mode is a block cipher implementation conceptually similar to a synchronous stream cipher. OFB prevents the same plaintext block from generating the same ciphertext block by using an internal feedback mechanism that generates the keystream independently of both the plaintext and ciphertext bitstreams. In OFB, a single bit error in ciphertext yields a single bit error in the decrypted plaintext.
Counter (CTR) mode is a relatively modern addition to block ciphers. Like CFB and OFB, CTR mode operates on the blocks as in a stream cipher; like ECB, CTR mode operates on the blocks independently. Unlike ECB, however, CTR uses different key inputs to different blocks so that two identical blocks of plaintext will not reuslt in the same ciphertext. Boudoir Photography Guide Excellence Pdf To Jpg there. Finally, each block of ciphertext has specific location within the encrypted message.