Multi Protocol Commitments - MPC
Last updated
Last updated
Multi Protocol Commitments address the following important requirements:
How the tagged mpc::Commitment hash, committed in Bitcoin Blockchain according to Opret or Tapret schemes, is constructed.
How state changes associated with more than one contract can be stored in a single commitment.
The preceding points are addressed through an ordered merkelization of the multiple contracts (actually their transition bundles IDs) in an MPC Tree whose properties will be addressed in depth in this section. Eventually, the root of the tree (mpc::Root) is hashed once more to get the mpc:Commitment which is finally committed in an output of the witness transaction using the appropriate Deterministic Bitcoin Commitment construction.
The commitment of the MPC tree - which goes either into Opret or into Tapret commitments - is the mpc::Commitment constructed in BIP-341 fashion as follows:
mpc::Commitment = SHA-256(SHA-256(mpc_tag) || SHA-256(mpc_tag) || depth || cofactor || mpc::Root )
Where:
mpc_tag = urn:ubideco:mpc:commitment#2024-01-31 follows RGB tagging conventions.
depth is the depth of the tree as a single byte
cofactor is the value used to obtain distinct positions for the contracts in the tree as a 16-bit Little Endian unsigned integer (see MPC Tree Construction)
mpc::Root is the root of the MPC tree whose construction is explained in the following paragraphs.
In order to construct the MPC tree we must deterministically find a unique leaf position for each contract, thus:
By setting C the number of contracts and i = {0,1,..,C-1} and by having a ContractId(i) = c_i to be included in the MPC, we can construct a tree with w leaves with w > C (corresponding to a depth d such that 2^d = w), so that each contract identifier c_i representing a different contract is placed in a unique position pos(c_i) determined as a modulus operation detailed below.
In essence, the construction of a suitable tree of width w that hosts each contract c_i in a unique position represents a kind of mining process. The greater the number of contract C, the greater should be the number of leaves w. Assuming a random distribution of pos(c_i), as per Birthday Paradox, we have ~50% probability of a collision occurring in a tree with w ~ C^2.
In order to avoid too large MPC trees and assuming that the occurrence of collisions is a random process, an additional optimization has been introduced.
The actual formula for determining the leaf position of the contract is:
Where cofactor is a number that allows to increase the chance of deterministically obtaining distinct values of pos(c_i) with a given w.
The tree construction process starts from the smallest tree such that w > C and performing a certain number of cofactor attempts; if none of them can produce C distinct positions, d is incremented by one and a new series of cofactor trials is attempted. In particular, cofactor is chosen starting from 0 and trying every number up to w/2: larger values cannot succeed since otherwise it would have been possible to build a tree of depth d-1.
Once C distinct positions pos(c_i) with i = 0,...,C-1 are found, the corresponding leaves are populated through a tagged hash constructed in the following way:
tH_MPC_LEAF(c_i) = SHA-256(SHA-256(merkle_tag) || SHA-256(merkle_tag) || 0x10 || c_i || BundleId(c_i))
Where:
merkle_tag = urn:ubideco:merkle:node#2024-01-31 is chosen according to RGB conventions on Merkle Tree tagging commitments.
0x10 is the integer identifier of contract leaves.
c_i is the 32-byte contract_id which is derived from the hash of the Genesis of the contract itself.
BundleId(c_i) is the 32-byte hash that is calculated from the data of the Transition Bundle which groups all the State Transitions of the contract c_i.
For the remaining w - C uninhabited leaves, a dummy value must be committed. To do that, each leaf in position j != pos(c_i) is populated in the following way:
tH_MPC_LEAF(j) = SHA-256(SHA-256(merkle_tag) || SHA-256(merkle_tag) || 0x11 || entropy || j )
Where:
merkle_tag = urn:ubideco:merkle:node#2024-01-31 is chosen according to RGB conventions on Merkle Tree tagging commitments.
0x11 is the integer identifier of entropy leaves.
entropy is a 64-byte random value chosen by the user constructing the tree.
j is the position of the current leaf as a 32-bit Little Endian unsigned integer.
After generating the base of the MPC tree having w leaves, merkelization is performed following the rule of commit_verify crate detailed here.
The hash for non-leaf nodes in the tree is computed as:
tH_MPC_BRANCH(tH1 || tH2) = SHA-256(SHA-256(merkle_tag) || SHA-256(merkle_tag) || b || d || w || tH1 || tH2)
Where:
merkle_tag = urn:ubideco:merkle:node#2024-01-31 is chosen according to RGB conventions on Merkle Tree tagging commitments.
b is the branching of the tree merkelization scheme, i.e. the number of children the current node has, encoded as a 8-bit unsigned integer. If the tree is complete, this is always 0x02.
d is the node depth within the tree (i.e. the length of the path to the root), encoded as an 8-bit unsigned integer.
w is the tree width, encoded as a 256-bit Little Endian unsigned integer.
tH1 and tH2 are respectively the hash of the left and the right child, calculated according to the appropriate formula depending on their role in the tree (contract leaf, entropy leaf or merkle node).
The following diagram shows the construction of an example MPC tree, where:
C = 3 number of contracts to place.
As an example: pos(c_0) = 7, pos(c_1) = 4, pos(c_2) = 2.
BUNDLE_i = BundleId(c_i).
d depends on the position of the node within the tree; for example, tH_MPC_BRANCH(tHA || tHB) has d=2.
w=8 for every node in the tree.
From a verifier's perspective, in order to prove the presence of client-side validated data related to some contract c_i collected in BUNDLE_i, only a Merkle Proof pointing at it inside the tree is needed. Because of this, different verifiers of different contracts don't need to have the full view of the Merkle Tree as the builder does, and this, together with the dummy entropy leaves, provides a high degree of privacy. Using the example tree in the diagram above, a verifier of, say, the contract c_2 will receive the following Merkle Proof from the tree builder:
So the Merkle Proof provided to verify the existence and uniqueness of contract commitment in the tree is: tH_MPC_LEAF(D), tH_MPC_BRANCH(tHA || tHB) and tH_MPC_BRANCH(tHEF || tHGH). These are enough to recompute the tree root and, together with pos(c_2) and cofacor, reproduce the MPC commitment to be compared with the one included in the anchor.
