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Assuming you have a list of possible recipients' public keys, and a ciphertext encrypted with one of those public keys, would it be possible to determine which public key?

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    ... for what PKE scheme? – user49075 May 19 '15 at 2:49
  • @RickyDemer I was hoping for an overview, and whether or not the scheme matters. However I was specifically wondering about PGP+RSA – Shelvacu May 19 '15 at 5:39
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    PGP may specifically disclose recipient information by referencing key ID; however, this is unrelated to public key algorithms per se. It is part of the format specification. See also stackoverflow.com/a/18505637 and related RFC. – Archimedix May 19 '15 at 8:25
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It depends on a lot of things.

Let's consider a message encrypted by OpenPGP, the most used encryption standard. By default, it includes in plain text a field containing the key ID the message has been encrypted to. However, note that the OpenPGP standard allows for this field to be empty i.e. containing a key ID = 0x0. So the correct answer to your question is "It depends, if you have a ciphertext encrypted with OpenPGP it might be possible to find out the recipients."

  • +1 do you know if S/MIME uses the same structure? – Mike Ounsworth May 19 '15 at 14:41
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The only way you could match a encrypted text to the key-pair it is used with would be to own the private key and successfully decipher the message. Apart from this, there is no telltale sign as to which public key has been used to, as any message could have been used for encryption.

Though, the protocol could tell you. For example, the OpenPGP protocol uses packet to identify the keys a message has to be used with. Or usually, the message is send to the recipient through mail, so the key has to be the key for this person.

  • "The only way you could match a encrypted text to the key-pair it is used with would be to own the private key and successfully decipher the message." This is not correct; by default, an OpenPGP encrypted packet contains the key IDs it has been encrypted to. – dr01 May 19 '15 at 14:32
  • @dr01 thus the 2nd paragraph. OP did not specifically asked for OpenPGP encryption, so the only information he has might just be the encrypted payload. From this, you cannot know which key was used. – M'vy May 19 '15 at 14:41

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