![]() Related attacks include relative length analysis and similar plaintext analysis. We saw in this chapter how weak ciphers are subject to frequency analysis attacks that use language characteristics to decipher the message. Information is sometimes concealed using weak or reversible algorithms. Using easily broken passwords or passphrases as secret keys in symmetric algorithms can have unpleasant effects, and improperly stored private and secret keys can negate the security provided by encryption altogether. Man-in-the-middle attacks could cripple the security of a Diffie-Hellman key exchange, and even DES-encrypted LANMAN password hashes can be broken quite easily. Cracking passwords is the most widely used application of brute force programs such as L0phtcrack and John the Ripper are used exclusively for this purpose.Įven secure algorithms can be implemented insecurely, or in ways not intended by the algorithm's developers. Fundamental differences between public key and symmetric cryptography include public key cryptography's reliance on the factoring problem for extremely large integers.īrute force is an effective method of breaking most forms of cryptography, provided you have the time to wait for keyspace exhaustion, which could take anywhere from several minutes to billions of years. After Diffie-Hellman, the RSA algorithm was developed, heralding a new era of public key cryptography systems such as PGP. The early development of asymmetric cryptography was begun in the mid-1970s by Diffie and Hellman, who developed the Diffie-Hellman key exchange algorithm as a means of securely exchanging information over a public network. From the European perspective, we saw how IDEA came to be developed in the early 1990s and examined its advantages over DES. Since DES is showing its age, we looked at how NIST managed the development of AES as a replacement, and how Rijndael was selected from five finalists to become the AES algorithm. The most common symmetric algorithms in use today include DES, AES, and IDEA. More modern branches of cryptography are symmetric and asymmetric cryptography, which are also known as secret key and public key cryptography, respectively. This chapter looked into the meaning of cryptography and some of its origins, including the Caesar Cipher. In Hack Proofing Your Network (Second Edition), 2002 Summary The primary purpose of discussing it here is as an introduction to ciphers.įurther topics of interest and places to find information involving substitution ciphers are the chi-square statistic, Edgar Allan Poe, Sherlock Holmes, Benford’s Law, Google, and Wikipedia. So, though this particular cipher is a child’s game, it is useful in that it is an underlying principle of cryptography and should be well understood before continuing. Forensic techniques incorporate such tools to uncover accounting fraud. Where numerical information is encrypted, tools such as Benford’s Law can be used to elicit patterns of numbers that should be occurring. Ultimately, substitution ciphers are vulnerable to either word-pattern analysis, letter-frequency analysis, or some combination of both. Ciphers with small unicity distances are weaker than those with large ones. ![]() The amount of ciphertext needed to successfully break a cipher is called unicity distance. However, this would only be true for very short and very obscure messages that could be code words to decrypt other messages or could simply be sent to misdirect the opponent. It stands to reason, then, that the formulation of the cipher, where a substitution that is based partially on frequency similarities and with a whole lot of obfuscation so that when messages are decrypted they have ambiguous or multiple meanings, would be desirable for simple ciphers. They must take into consideration spacing and word lengths when considering whether or not a string matches a word. The cryptograms that use formatting (every word becomes the same length) are considerably more difficult for basic online decryption programs to crack. ![]()
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