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5: Encryption Algorithms in Java
5: Encryption Algorithms
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Encryption basics Symmetric and asymmetric encryption Cracking encryption with attacks
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ncryption involves scrambling information, or plaintext, and converting it into another format ciphertext essentially turning ordered data into seemingly random gibberish By encrypting information, you can keep data information out of the hands of other people, which can be useful for sending coded messages for military use, sending credit card information over the Internet to online shopping Web sites, or just hiding your personal documents from the prying eyes of family members, co-workers, or strangers The simplest form of encryption is substitution cipher, which basically replaces each letter with a specific symbol, such as another letter A substitution cipher simply replaces one letter with another letter from the alphabet a fixed distance away, such as replacing the letter A with the letter Z, the letter B with the letter A, the letter C with the letter B, and so on In this case, each letter gets replaced by the previous letter in the alphabet, like this: I AM HOT Replacing the letter I with the letter H, the letter A with the letter Z, and so on creates the following ciphertext: H ZL GNS This information may be scrambled, but after someone discovers that each letter in the ciphertext actually represents the next letter in the alphabet, this simple substitution cipher can be cracked easily When an encryption method can be broken easily, it s weak encryption If an encryption method can t be broken easily, it s strong encryption The key to deciphering the substitution cipher is recognizing both the method it s using (replacing one letter with another) and the specific way it implements that method (replacing each letter with the previous letter in the alphabet) A slightly more-complicated substitution cipher might replace
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Book IV: Algorithms
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each letter with the third letter from the current letter So the letter A would be replaced by the letter D, the letter B by the letter E, and so on In this case, the method is the same, but the implementation is slightly different while being only marginally harder to decipher Although substitution ciphers are easy to implement, they re also easy to break After you know to replace a letter in the ciphertext by another letter that s shifted by a specific distance in the alphabet (such as the third letter), you can easily break the code One way to avoid this problem is to use a onetime pad, which consists of a series of random numbers that tell how far to shift the next letter in a message So a one-time pad might contain three random numbers, like this: 2 7 3 The first number (2) tells the algorithm to shift the first letter of the text by two letters So if the first three letters of the message are SAM, the first letter, S, would get replaced by the second letter from S in the alphabet, which is U The second number (7) tells the algorithm to shift the second letter by seven letters So the letter A gets replaced by the seventh letter down, which is H Finally, the third number (3) tells the algorithm to shift the third letter by the third letter down, so the letter M gets replaced by the letter P Now the entire message SAM gets encrypted as the ciphertext UHP The one-time pad gets its name because the random series of numbers are used only once Now it s virtually impossible for anyone to discover how the letters are substituted because the replacement letters are picked at random The only way to decipher this ciphertext is to get a copy of the one-time pad Of course, the one-time pad has its drawbacks To work, both parties need a copy of the same one-time pad If you could transfer a copy of the one-time pad securely, you might as well transfer the message you re delivering instead Also, one-time pads can be used only once If they re used more than once, someone can eventually guess the random pattern of letters Even worse is that a one-time pad must specify how far to shift each letter in a message If you re encrypting a message consisting of 1,000 letters, you need a one-time pad to specify how to shift all 1,000 letters If you re encrypting a message consisting of 10,000 letters, you need a one-time pad that specifies how to shift all 10,000 letters Given these problems, one-time pads are generally impractical for normal use A slight variation of the one-time pad is the use of a password A password acts like a one-time pad; instead of defining how to alter each individual character
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