Encryption and decryption ask questions

For example , for example, I use DES encrypted string "abc" get "ddd",
It is not someone just call the DES decryption method provided , you can put "ddd" decrypt "abc"?

If so , then the market would not be a popular encryption algorithm useless ? Are we going to write their own algorithms out ?
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you have to know the key . . .
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key how did you solve . . . .
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seems now more than two-way encrypted with it, too generate a pair of public and private keys , and CA certification certificate. . Specifically check for yourself
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asymmetric encryption is currently used by many other letters you want to get to know each other 's public key , and then after each received his private key solution
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know, thank