When someone claims that the private key corresponding to the public key has been stolen, and sends to the CA a statement of this fact signed with respect to public key pk.

Why doesn't the CA need to check his identity in this case, and why is there no concern
that the person who has stolen his key can forge signatures with respect to pk?

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    $\begingroup$ If someone signs a revocation request with his private key, then this is sufficient. In case of a stolen (copied) key, It doesn't matter if the attacker or the honest owner of the key makes this request. $\endgroup$ – user27950 Nov 28 '16 at 18:21
  • $\begingroup$ It doesn't matter! how? if the attacker succeeded to get the private key. Then the owner sends a revocation request. $\endgroup$ – han Nov 28 '16 at 18:32
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    $\begingroup$ If the owner does no longer have the private key, say because his smart card has been stolen, then he must use some other means for revocation. For instance, to revoke the German ID card, you need a special "revocation password". $\endgroup$ – user27950 Nov 28 '16 at 18:35
  • $\begingroup$ So, why CA doesn't need to check his identity? and why there is no concern of forging signature. $\endgroup$ – han Nov 28 '16 at 18:40

There are two cases to distinguish: The attacker issues the revokation or the owner issues the revokation.

  • Attacker revoking the cert. In this case the attacker has the private key and uses this to issue a revokation. He thus effectively takes any use out he could get out of theh certificate and the certificate is revoked as it should be in such a situation.
    "Thank you attacker, you just did my job without me having noticed the breach!"
  • Owner revoking the cert. The owner wants to prevent missuse of the cert and thus revokes it. This stops the attacker. Everything as it's supposed to be.
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  • $\begingroup$ As I user, and I want to revoke the certificate, what should I do? should I sign it with my private key and that's enough, so CA doesn't need to check my identity, right? $\endgroup$ – han Nov 28 '16 at 18:51
  • $\begingroup$ @han sign a statement that you want to revoke certificate X with the private key associated with that cert. (That's what my answer is based on) $\endgroup$ – SEJPM Nov 28 '16 at 18:52
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    $\begingroup$ @han Right, you need to sign it as otherwise anybody could revoke the certificate. It doesn't matter if the attacker or if the person that owns the private key signs the revocation request. If the owner wants to revoke then everything is fine. If the attacker wants to revoke then that is OK as well; the user can hardly argue that the private key was not leaked as the attacker could never have signed the revocation message. $\endgroup$ – Maarten Bodewes Nov 28 '16 at 23:18

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