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I am currently reading about X.509 certificates from RFC 5280 and there is a thing in section 3.1 that I cannot understand. It says:

Users of a public key require confidence that the associated private key is owned by the correct remote subject (person or system) with which an encryption or digital signature mechanism will be used. This confidence is obtained through the use of public key certificates, which are data structures that bind public key values to subjects.

I do not understand, why the confidence about private key association is ensured by binding public key to subjects.

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  • Without a way of proving that the server presenting the certificate has the private key associated with the public key in the certificate, an attacker could simply steal a server's certificate, and use it to MITM the server. See security.stackexchange.com/questions/105376/… for more info.
    – mti2935
    Mar 13, 2022 at 13:19

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The whole purpose of certificate is namely to confirm that a particular key pair belongs to a particular subject (email address, host, person, company).

One of the use cases is digital signature. A signature is created using private key of the signer. You validate the signature using the public key. From the certificate you know that this public key belongs to the particular subject. Thus, you are sure that the signature (done by the private key) was really created by this subject.

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    The signer of the certificate is the CA, am I right?
    – retne
    Mar 13, 2022 at 13:35
  • @retne "signing CA" not CA itself, to be precise.
    – elsadek
    Mar 13, 2022 at 14:28
  • @retne: Correct. Certificate issued by particular CA is signed by this CA.
    – mentallurg
    Mar 13, 2022 at 14:49

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