More complicated ciphers require a combination of experience, experimentation and the occasional shot-in-the-dark guess. The most difficult ciphers are short, continuous blocks of characters. If the cryptographer's message includes word breaks, spaces between each enciphered word, it makes deciphering much easier. The cryptanalyst looks for groups of repeated ciphers, analyze where those groups of letters fall within the context of words and make guesses at what those letters might mean. If the cryptanalyst has a clue about the message's content, he might look for certain words. A cryptanalyst intercepting a message from a Navy captain to command might look for terms referring to weather patterns or sea conditions. If he guesses that "hyuwna" means "stormy," he might be able to crack the rest of the cipher.
Christopher Furlong/ Getty Images
Breaking the code carved into the ceiling of the Rosslyn Chapel in Scotland reveals a series of musical passages.
Many polyalphabetic ciphers rely on key words, which makes the message vulnerable. If the cryptanalyst correctly guesses the right key word, he can quickly decipher the entire message. It's important for cryptographers to change key words frequently and to use uncommon or nonsense key words. Remembering a nonsense key word can be challenging, and if you make your cipher system so difficult that your recipient can't decipher the message quickly, your communication system fails.
Cryptanalysts take advantage of any opportunity to solve a cipher. If the cryptographer used a ciphering device, a savvy cryptanalyst will try to get the same device or make one based on his theories of the cryptographer's methodology. During World War II, Polish cryptanalysts obtained an Enigma Machine and were close to figuring out Germany's ciphering system when it became too dangerous to continue. The Polish exchanged their information and technology with the Allies, who created their own Enigma Machines and deciphered many of Germany's coded messages.
Modern high-level encryption methods rely on mathematical processes that are relatively simple to create, but extremely difficult to decipher. Public-key encryption is a good example. It uses two keys -- one for encoding a message and another for decoding. The encoding key is the public key, available to whomever wants to communicate with the holder of the secret key. The secret key decodes messages encrypted by the public key and vice versa. For more information on public-key encryption, see How Encryption Works.
The complex algorithms cryptographers use ensure secrecy for now. That will change if quantum computing becomes a reality. Quantum computers could find the factors of a large number much faster than a classic computer. If engineers build a reliable quantum computer, practically every encrypted message on the Internet will be vulnerable. To learn more about how cryptographers plan to deal with problem, read How Quantum Encryption Works.
In the next section, we'll look at some codes and ciphers that remain unsolved, much to cryptanalysts' chagrin.