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At 45:43 in this video (from the Princeton Coursera Bitcoin and CryptoCurrency Course), they talk about GoofyCoin and show the image below.

My question is, why does it say "signed by pk"... isn't the actual signing done using the "sk", and the verification done using the "pk"?

screencap from lecture video about goofycoin, containing the phrase "signed by pk(goofy)"

Update: I think I figured out what's going on here, and am leaving this here for future reference. signed by pk(goofy) means "signed by the identity of goofy" because as mentioned in an earlier video, pk = identity

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    $\begingroup$ The signature is associated / bundled with the public key? Either way, associating with the public key may be easier, as it's now trivial which key has to be used for verification. $\endgroup$
    – SEJPM
    Commented Oct 7, 2016 at 18:56
  • $\begingroup$ I think I figured out what's going on here, and am leaving this for mostly my own future reference. signed by pk(goofy) means "signed by the identity of goofy" because as mentioned in an earlier video, pk = identity. $\endgroup$ Commented Dec 5, 2016 at 4:58

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My question is, why does it say "signed by pk"... isn't the actual signing done using the "sk", and the verification done using the "pk"?

Yes. The actual signing is done by the "sk".

However, the signature is usually bundled with the public key which can be used to verify it. Thus it makes sense to more strongly associated the public key with the signature. Additionally it makes it trivial from a formal standpoint which public key to use for verification.

Furthermore, the mapping of public-key to private-key is 1:1 most of the time, making the choice of which key to use for indication here a matter of preference.

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  • $\begingroup$ What would be better wording instead of calling it "signed by pk"? $\endgroup$ Commented Oct 8, 2016 at 20:14
  • $\begingroup$ @user1883050 maybe (my personal preference) "signed, verifieable with pk" or just (short) "verifieabe with pk" or (what I'd use in texts) "signed with the private key associated with pk". $\endgroup$
    – SEJPM
    Commented Oct 9, 2016 at 12:18

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