|...xt xmj ywnji yt gwjfp nsyt ymj kfymjw gjfw'x htruzyjw, gzy ny bfx ytt mfwi. Jxud ixu jhyut je rhuqa ydje jxu cejxuh ruqh'i secfkjuh, rkj jxqj mqi jee uqio...|
The following characters were rudely traced, in a red tint, between the death's-head and the goat:The story goes on to tell how the hero figures out which symbol is which letter, based on how common different letters are in English (hint: E is most common). Read the story to find out what the hidden message is.
"But," said I, returning him the slip, "I am as much in the dark as ever."
A second application of public key encryption is to encode something using your own private key. Then it would not be secret, because anyone who knows your public key can decode it, but that's not the point. The point is that whoever reads it knows that only you could have sent it, because only you know the secret key. In this way, you can "digitally sign" a message so people know it's from you. (This is called "signature verification".) Of course, these two applications can be combined, so you digitally sign (with your private key) and then encode your message to Bob (using Bob's public key). Then nobody else can read it, but Bob can decode it using his private key, and check your digital signature using your public key. Yes, it all sounds complicated, but modern software automates most of it...
This type of encryption (coding) has become popular for electronic communications. For example, if I want to buy a "PKE Rules!" sweatshirt from the online site "Encryption Unlimited" (names changed to protect the innocent), then my computer sends its own public key to the online site, and then the online site sends back its public key, at which point the two computers can send digitally signed encrypted messages to each other that only the other can read, such as credit card numbers... This results in pretty secure transactions; even if someone tries to read the messages going back and forth, they can't figure them out because they don't have either private key.
I developed a simple DOS demonstration program called PKE to show how this works. This program makes it easy to create pairs of keys, share the public key with your friends, and encode and decode short messages using one key that can only be decoded using the other key. (Warning: this is a "toy" version of encryption, which any real cryptographer could crack easily because the keys are so small; for serious encryption, see below for real software. But it might be good enough to confuse your parents...) This program can also encode ASCII text files.
To install PKE, create a new directory for it and download the compressed program to it, then run pkecomp.exe to uncompress it. See the Help File for more details on using the program, or the How It Works File for the mathematics behind the method, or the About PKE File for general information. For those who are really interested, the source code is also available.
RSA has an extensive site including an excellent "Frequently Asked Questions"
RSA Laboratories - Cryptography FAQ
I first got interested in cryptography as a child from reading
"The Gold Bug" by E. A. Poe (a related site is The Edgar Allan Poe Cryptographic Challenge)
Here are some pages about cryptography in general:
Cryptography (from Trinity College, Hartford, CT)
Cryptography (by M. Saberi)
NOTE: All software made available through this page and subsidiary pages is supplied on an "as is" basis, with no warrantees of any kind. The author bears no responsibility for any consequences of using this software.
David Canright -- DCanright@NPS.edu