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I'm looking at the multisignature scheme provided by MRL005 Section 4.4, and some things are unclear to me. I've attached a picture below of the expression for which I'm seeking some clarification.

Monero Multisignature Snippet

Above, we have:

  1. A generated key image (Ij)
  2. A hash function (H)
  3. A shared public key (Pe)
  4. And a signer-specific public key (Pj)

No ambiguity there.

The actual operations required to calculate a given key image are the core of my questions.

Question 1: For the term (Pe | Pj)

  • Am I correct to interpret this as an inner product of the shared, and signer-specific public keys?

Question 2: For the term (Pe | Pj)

  • How would one actually calculate this value, given the two public keys? A detailed example would be preferred here.

Question 3: For the term H()

  • Which hash function is used here? Sha512, as in EdDSA? Keccak? Or something else?
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  • Keep in mind MRL0005 is out of date. You can find more current resources on multisig in MRL0009 and Zero to Monero 2 ch. 9.
    – koe
    Commented Aug 19, 2020 at 1:40

1 Answer 1

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1 & 2 - Pe | Pj is concatenation of the keys.

3 - Keccak, although as @knaccc comments, H() is not simply a hash function, it's hash-to-point, so should read Hp().

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    I think that there is a typo in MRL005, and that H() is actually Hp(). Otherwise the result of multiplication would not result in an EC point key image. I failed to figure out the nature of their entire multisig scheme from their brief description.
    – knaccc
    Commented Aug 19, 2020 at 1:00
  • @knaccc indeed, it should read Hp() not H(). The MRL005 note (which is all it seems to be) is quite different to much of the multisig code.
    – jtgrassie
    Commented Aug 19, 2020 at 1:08
  • This makes a lot more sense now. Thanks to you both. I don't have enough rep to upvote
    – justlearning
    Commented Aug 19, 2020 at 3:39

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