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If public key encrypts only private can decrypt and vice versa?

why we say "If public key encrypts only private can decrypt and vice versa" from the following?  thanks

Public key cryptography
–Asymmetric – two inversely related keys
•Private key
•Public key
–If public key encrypts only private can decrypt and vice versa
–Each party has both a public and a private key
–Either the public key or the private key can be used to encrypt a message
–Encrypted with public key and private key
•Proves identity while maintaining security
•RSA public key algorithm www.rsasecurity.com 
1 Solution
public key is derived from the private key, therefore, the only person that can decrypt something that has been encrypted using a public key is the person who owns the corresponding private key.

The same is in the reverse, if you encrypt using the private key, anyone with the public key can decrypt it.

Both have practical benefits.

There two ways of encrypting your data. One is to use a asymetric algorithm. the other one is to use a symetric algorithm. When you use a symetric algorithm, data is encrypted using a shared key. If subject A sends encrypted data to subject B, both subjects must share the same key. The problem arises when to share the key .. How can one be sure the key is transmited securely? How can one be sure, the key doesn't get into "bad" hands. ...

The public key encryption, uses a asymetric algorithm ... This means that pair keys are used and they exlude each other. If you encrypt data with one key, ONLY the other key can decrypt data. That's why, one of the keys is public the other one is private. The private key is never shared. The public key SHOULD BE SHARED. Therefore, there is no security issue here as the key is actually supposed to be PUBLIC.

One of the applications of the public key encryption, besides the encryption itself, is the use of digital signatures (by the way, the digital certificate is just a way to publish your public key and to ensure the public key is realy yours - this is where CA (certificate authorities) come in place - it's the same as someone is validtaing your passport). When you digitally sign a document, you apply the private key to it. Therefore, if you send the document to me, I must have your public key. As you can imagine, I can get your public key cause it's PUBLIC. Then I know, the message came indeed from you.

!!! All this info is put here in a simple way ... It might not look like, but it is!!!!

If you need further explanation, let me know.


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