I've got a very simple but for me a difficult question. I analyzed the public key cryptography and understand the process. But I didn't understand why is only the receiver able to decrypt to message.

Please give an advice. when the sender encrypts the message with the public key why isn't any man in the middle not able to decrypt when he knows the public key.

Thanks a lot before

  • $\begingroup$ Side note: "PKI infrastructure" is redundant; PKI stands for "public key infrastructure." Also, PKI is somewhat separate from how only a receiver can view a message; public-key cryptography is quite possible without a PKI (e.g. giving public keys out in person). $\endgroup$
    – cpast
    Jan 15, 2015 at 2:55

1 Answer 1


The whole point of public key cryptography is that there is a secret key, which, if the system is to be any good, cannot be easily computed from the public key, but is known to the legitimate owner of the public (i.e. the one who generated and published it). And decryption uses the secret key, not the public one. Encryption only uses the public key.

But the point is that in order to know (or believe) the public key you are using is actually the one that the intended recipient has the private key for, some extra mechanisms (like signed certificates and certificate authorities, or some trusted third party, etc.) are needed. This is what is called PKI for short, in essence.

  • $\begingroup$ Thanks very much for good answer! This system is very interesting, but can you give me a real example (or a reference) to check/verify it? $\endgroup$ Jan 14, 2015 at 21:41
  • 1
    $\begingroup$ Have a look at the SSL certificates your browser gets when connecting to a site over https. Any browser will show you the complete certificate. In order to really understand those you have to read up on PKCS , X509 standards, ASN1 encoding etc. It's a big area. $\endgroup$ Jan 14, 2015 at 21:44

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