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Unformatted text preview: an verify the hash value. You can create a MAC out of a hash function or a block encryption algorithm; there are also dedicated MACs. 2.5 Communications Using Public-Key Cryptography
Think of a symmetric algorithm as a safe. The key is the combination. Someone with the combination can open the safe, put a document inside, and close it again. Someone else with the combination can open the safe and take the document out. Anyone without the combination is forced to learn safecracking. In 1976, Whitfield Diffie and Martin Hellman changed that paradigm of cryptography forever . (The NSA has claimed knowledge of the concept as early as 1966, but has offered no proof.) They described public-key cryptography. They used two different keys—one public and the other private. It is computationally hard to deduce the private key from the public key. Anyone with the public key can encrypt a message but not decrypt it. Only the person with the private key can decrypt the message. It is as if someone turned the cryptographic safe into a mailbox. Putting mail in the mailbox is analogous to encrypting with the public key; anyone can do it. Just open the slot and drop it in. Getting mail out of a mailbox is analogous to decrypting with the private key. Generally it’s hard; you need welding torches. However, if you have the secret (the physical key to the mailbox), it’s easy to get mail out of a mailbox. Mathematically, the process is based on the trap-door one-way functions previously discussed. Encryption is the easy direction. Instructions for encryption are the public key; anyone can encrypt a message. Decryption is the hard direction. It’s made hard enough that people with Cray computers and thousands (even millions) of years couldn’t decrypt the message without the secret. The secret, or trapdoor, is the private key. With that secret, decryption is as easy as encryption. This is how Alice can send a message to Bob using public-key cryptography: (1) Alice and Bob agree on a public-key...
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- Fall '10