What is Cryptanalysis?
Cryptanalysis is the study of taking encrypted data, and trying to unencrypt it without use of the key. The other side of cryptography, cryptanalysis is used to break codes by finding weaknesses within it. In addition to being used by hackers with bad intentions, cryptanalysis is also often used by the military. Cryptanalysis is also appropriately used by designers of encryption systems to find, and subsequently correct, any weaknesses that may exist in the system under design. There are several types of attacks that a cryptanalyst may use to break a code, depending on how much information they have. A ciphertext-only attack is one where the cryptanalyst has a piece of ciphertext (text that has already been encrypted), with no plaintext (unencrypted text). This is probably the most difficult type of cryptanalysis, and calls for a bit of guesswork. In a known-plaintext attack, the cryptanalyst has both a piece of ciphertext and the corresponding piece of plaintext. Other types of attack
Cryptanalysis is the flip-side of cryptography: it is the science of cracking codes, decoding secrets, violating authentication schemes, and in general, breaking cryptographic protocols. In order to design a robust encryption algorithm or cryptographic protocol, one should use cryptanalysis to find and correct any weaknesses. This is precisely the reason why the most trusted encryption algorithms are ones that have been made available to public scrutiny. For example, DES (see Section 3.2) has been exposed to public scrutiny for years, and has therefore been well-trusted, while Skipjack (see Question 3.6.7) was secret for a long time and is less well-trusted. It is a basic tenet of cryptology that the security of an algorithm should not rely on its secrecy. Inevitably, the algorithm will be discovered and its weaknesses (if any) will be exploited. The various techniques in cryptanalysis attempting to compromise cryptosystems are referred to as attacks. Some attacks are general, whereas
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