Understanding public key cryptography example Frankly, I wasn't sure whether this belongs in crypto.stackexchange.com or here, so forgive me. I've been going through a book where in one of the chapters the author tries to illustrate public key cryptography by imagining a &quot;four-step business plan for Murder Incorporated&quot; in a &quot;world of strong privacy&quot;.


Arrange for mystery billboards on major highways. Each contains a
single long number and the message “write this down.” Display ads with
the same message appear in major newspapers.

Put a full-page ad in the New York Times, apparently written in
gibberish.

Arrange a multiple assassination with high-profile targets, such as
film stars or major sports figures – perhaps a bomb at the Academy
Awards.

Send a message to all major media outlets telling them that the number
on all of those bulletin boards is a public key. If they use it to
decrypt the New York Times ad they will get a description of the
assassination, published the day before it happened.



I don't quite understand the mechanics of how the outlets are able to decrypt the ad and get a description of the assassination. Is the ad encrypted with his private key and since both public and the private keys are mathematically related, whatever is encrypted with the private key can be decrypted with the public key (visible on the billboard) and vice versa?
Interesting premise.
Q : &quot;Is the ad encrypted with his private key ...&quot;
A : no, I don't believe so
Edit - To clarify, I entertained the possibility that the author meant to say &quot;private key&quot; in item #4, but once you consider the subsequent paragraph in the source (excerpt below), I don't think the ciphertext that describes the crime in advance is encrypted with public-key cryptography. I reckon the author's protagonist is using the same bytes that are in the public key as the key for a symmetric cipher. (In this case, KDF could potentially be HKDF, PBKDF2, Argon2, scrypt, or even a keyed hash-digest.) eg.
broker_pk = PUBLIC_KEY( secret_key )
ciphDetailsOfCrime = E( KDF(broker_pk) , EvilPlanToMakeCelebsEvenMoreFamous )

eph_cli_pk = PUBLIC_KEY( ephemeral_client_secret_key )
kRequestForQuote = DH_or_ECDH( broker_pk , ephemeral_client_secret_key )
ciphRFQ = E( kRequestForQuote , DetailsOfNewTarget )
msg = eph_cli_pk || ciphRFQ

(This is a contrived example that shouldn't be used, especially if conspiring to commit murder and/ or acts of terrorism.) My original answer continues ...
It may be that the author meant to say &quot;private key&quot; in list item #4, however, I suspect the author is thinking about using the public key, posted on billboards, as a symmetric key (or password to be used in some key derivation function - aka. KDF) to decrypt the prior description of the heinously foul act of murdering film celebrities en masse ...
The paragraph that follows is:

&quot;You have now made sure that everyone in the world has, or can get, your public key – and knows that it belongs to an organization willing and able to kill people. Once you have taken steps to tell people how to post messages where you can read them, everyone in the world will know how to send you messages that nobody else can read and how to identify messages that can only have come from you. You are now in business as a middleman selling the services of hit men. Actual assassinations still have to take place in realspace, so being a hit man still has risks. But the problem of locating a hit man – when you are not yourself a regular participant in illegal markets – has been solved.&quot;