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What is Cryptography?
Cryptography is the encryption of messages or files, to ensure that only a sender and recipient are able to read them. It can be done using pen and paper, email, website connections, or even audio/video. In fact it can be used (and is used) anywhere that privacy is a concern.
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Pen and Paper
As children we've all used the simple forms of cryptography to pass notes. Substitution, where you would equate a letter of the alphabet with either another one, a number, or symbols. You would tell your recipient what the "key" is, and they were able to decode the messages. Now, you see this in "Cryptograms" in the Puzzles section of newspapers.
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Encrypting email messages entails the recipient having an encryption/decryption key that they share with the sender. The sender uses their "public key" (the encryption key) to encrypt the email message. The recipient uses their "private key" (the decryption key) to decode the message and read it. This also works with sending files via email or other means.
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If you've visited a site where the address starts with https:// or it has a padlock icon, then you've used Website-based encryption. Website encryption is not meant to hide the contents of the site. Instead it is meant to ensure that no one except for the authorized viewer can see the information transmitted between them and the site. That sounds contradictory, but it isn't. If two people visit the same site at the same time, they will potentially see the exact same data. The encryption prevents a third person from intercepting the transmissions (such as the passing of credit card information between a purchaser and the site).
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A little story will cover the audio portion of this article. I listen to shortwave radio on occasion. On a few nights, I've heard a transmission consisting of numbers repeated over and over. Given the length of the string of numbers, and different things that I've read about "number messages" I've realized that these were encrypted messages. Numbers are not the only method though. Code words such as "Is an Administrator Available?" may not mean anything to the average person, but to people who are trained in the circumstances behind that, it means "I need help, but cannot explicitly say it." (This is paraphrased from a college's guidelines for an intercom system).
Video includes hiding messages inside of video files. Likewise pictures can have messages encoded inside of them. Typically you see this in movies and television, but it does exist. The art of encrypting messages inside of pictures, audio, or video is called Steganography, and it has been around since Ancient Greece.
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The next time you visit your favorite online store, or log into your email or social networking site, remember that the transmission between your computer and their server is using cryptography. And if you hear static in an otherwise clear audio or video, there's a chance that it is an encrypted message as well.