1

AFAIK MLSAG is being used for one input because of a privacy problem. Why not just use bLSAG, since in the MRL005 it says that MLSAG is better once we have > 1 sets of keys.

2

MLSAG allows us to include more than just the set of keys. We can thus use both keys and commitments.

Quoting the paper:

An MLSAG is essentially similar to the LSAG’s described in [8], but rather than having a ring signature on a set of n keys, instead, an MLSAG is a ring signature on a set of n key-vectors.

Currently we use RCTTypeSimple, which creates a signature for each input ring. Each ring contains public keys and commitments. This means we are creating a signature on a matrix whereby each column represents an input that has two rows. The first row contains the key, the second the commitment. A "matrix" is another way of saying "set of n key-vectors".

  • Yes, but afaik mlsag is being used for one input at a time due to the shared index bug. In that ring, we prove that we know one of the keys that sign it. I imagine it is like a mxn matrix, where each row of keys is assigned to an input and we need to prove that we know the secret keys to an entire column. – WeCanBeFriends Apr 28 at 9:43
  • Yes this is correct, there is one sig per input ring, but note they also link the key images - MLSAG incorporates both keys and Pedersen commitments. – jtgrassie Apr 28 at 11:57
  • Not sure I understand. The MLSAG is used to sign each input individually, how would the keyImages be linked? For n inputs, we would have n signatures each with their own keyImages afaiu – WeCanBeFriends Apr 28 at 12:38
  • From your quote, an MLSAG signature on 1 key vector is the same as a signature of n keys, where n is the size of the key vector – WeCanBeFriends Apr 28 at 12:46
  • If you look at how mlsag is used in Monero, you'll see that both keys and commitments are used. Not just keys or just commitments. The quote doesn't state 1 key vector, it says "on a set of n key-vectors". Also recall how you link the key image, the zero commitment. – jtgrassie Apr 28 at 13:03

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