| | This is a spin-off from the "Bitcoin" topic in Banter. Bitcoins are encrypted, but can be verified. How do they do that?
Today, encryption is embedded in many apps. We do not even think about it. Any zipped or compressed file is encrypted, really. Any website with https in the URL is supposed to be on a secure socket server.
The game of code-and-cipher is as old as writing. (Indeed, writing is a kind of code for clay tokens which were codes for material goods. We think in symbols. and abstractions.) In our time, the Federal government developed a Data Encryption Algorithm for non-secure transmissions such as banking (versus the military, for instance). It was their admission that the DEA was not strong; and it was not. Ronald Rivest, Adi Shamir, and Leonard Adelman announced their RSA Cryptosystem a few years later. With RSA the key to decrypt was not the reverse of the encryption.
With ciphers, if A=Z, B=Y, C=X, etc.... if I turn ED THOMPSON into VW GSLNKHLM, you just reverse the process. (Plus, of course with any long-enough text, letter frequencies in every language will let you tease out the assignments, no matter how many twists and turns they put into it.) A cipher is an ordered transformation. You do not need a key, only the algorithm, the formula.
A code requires a key. For a "dictionary code" take a book lke Atlas Shrugged. You can send strings of page-number-word-number but without the book, the encoded string cannot be broken. (With Atlas, in fact, I have two copies, an old gold-cover paperback 95-cents in great condition, and a newer printing $2.95 that I actually use. The pages are the same. 5050102 4031029 9520102 2810104. Knowing that two communicators are Objectivists might tell a government cryptohacker where to find a key - and you can see that I got a little lazy - but we could be using the Tenth Britannica for all the difference it makes to the process. With so many books in digital format the process can be programmatically automated. This is called a "one time pad" and many such communications from World War II remain unbroken. The war is over, so that removes some incentive, but, even so, without the codebook, messages encrypted via a one-time pad are secure.
I do not understand the Euler Totient Function upon which the RSA is built, but what goes in is not what comes out. It is not an inverse process. So, you can publish your public key for encryption to you, but keep secret your decryption key. A hacker named Phil Zimmerman took the same Euler Totient Function and created a free system called PGP: Pretty Good Privacy ((Wikipedia). RSA sued and lost -- no one can own a mathematical function: they exist in the objective world to be discovered as rational truths. Now PGP is a commercial product (still free at the intro level) from Symantec here.
In 1979, I wrote a book for Loompanics on cryptography, The Code Book: All About Unbreakable Codes and How to Use Them. It got bad reviews from the cryptography community and deservedly so. But we persevered; and the Third Edition (1984) stands well on its own merits. Written for the Apple II, TRS-80, and IBM-PC, the computer programs are only instructional cores now. Technology has moved very far ahead.
"They built the first computers to crack German ice. Codebreakers. So, there was ice before there was computers, you wanna think of it like that." -- The Finn in William Gibson's Count Zero.
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