A very short primer on attacks and methods.
“I may mention here that radio-aerograms are seldom if ever used in war time, or for the
transmission of secret dispatches at any time, for as often as one nation discovers a new cipher, or invents a new instrument for wireless purposes its neighbours bend every effort until they are able to
intercept and translate the messages. For so long a time has this gone on that practically every possibility of wireless communication has been exhausted and no nation dares transmit dispatches of importance in this way.”
Edgar Rice Burroughs, The Gods of Mars.
John Carter of Mars aside, there are plenty of practically unbreakable ciphers, but there are even more ways to sound like a complete idiot when you write about them.
Some vocabulary first. A cryptosystem is a function that takes one set of symbols to another in a uniquely reversible manner. The key determines the specific way that this happens. Plaintext refers to what you want to hide and ciphertext to the stuff that's hidden. An attack is an approach to finding the key. A good modern cryptosystem will withstand an attack where both the plaintext and ciphertext are known.
First and foremost, the security of a cryptosystem lies entirely in the key, and not at all in secrecy about the system itself. That James Bond film, “From Russia with Love,” where he and a suitably buxom cipher clerk steal a cipher machine. Forget it. They were amateurs. They could have pulled out and copied the rotors, returned the machine and been in the British embassy in time for tea or martinis with no one the wiser.
A “key search” attacks a known cryptosystem by searching for a key that works. Build the fastest computer you can and try to search every possible key. If you do this for a lot of messages, you'll hit the key often enough to understand the communications. The “bombe” was at Bletchley Park was a fast key searching machine. It's also why we use big keys, like 1024 bits. No use in making it too easy for the NSA, is there?
A side attack uses some other information to break the key. These are seriously cool, and you want your hacker to use one on her adversary. A classic example monitors power use. Computers use different amounts of power and time for different arithmetic operations. Ciphering with your 1024 bit key uses a lot of arithmetic. So you monitor your target by metering its power use. Researchers have used the sound of the machines' fan, the brightness of a power LED with a telescope and many other tricks to limit the number of possible keys to a search-able number. You can even buy special purpose boards that will do this with “sealed” devices.
(Any opinions offered in this posting are those of the author and not the Science Fiction Romance Brigade.)
Coarse mouthed, hard drinking and bound only by the laws of physics, the space pirate, “Cynthia the Invincible” is on the run with from the alien Cataxi. She is stranded in Earth's dim prehistory when a lucky shot on her ship destroys the jump unit. Marooned in 1810 she is forced to hide while the ship tries to repair itself. Since she enjoys playing the computer game "Jane Austen World" she hides in Regency England. She finds that real life in the Regency is different from the game. Especially when it comes to love.
Living in the Regency poses many novel challenges for her. Mundane things like eating real food rather than the ship's synthetics or learning to ride a horse with a mind of her own pose obstacles that she must learn to overcome. Others are subtle, such as the withdrawal from the drugs that allow her to conquer the long times required for interstellar travel, even with a jump drive. Even the process of learning to live with other people rather than alone in a small ship can cause problems. Especially when she faces the moral quandary of whether to use her advanced survival technology to save a friend's life. Even worse, one of the less pleasant alien species is trying to alter Earth's history to eliminate the competition.
These problems would sort themselves out, given time. Does she have the time?
Amelia Grace Treader is an author of (mostly) historical romances, with the occasional science fiction romance thrown in for good measure. Based near Atlanta, she writes a unique combination of romance and action. She enjoys reading history, science fiction, and historical romance. Her day job has something to do with computers and making life difficult for college students. While a child of the American South, She's also an Anglophile and not unfamiliar with the south of England. They're more alike than you know - There's even a Krispy Kreme doughnut shop in the Oracle in Reading, and they're just as good as in Kennesaw.
Despite the descriptive name of romance as “bodice rippers,” Amelia tends to write more in the sweet style of "bodice unbuttoners" where the romance is there but not explicit. After all, a good quality bodice was expensive, and only a cad or puppy would damage it. Besides that, tearing a hole in a space suit could have unfortunate consequences.