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HIP 29: Multi-signature Keys

  • Author(s): @xandkar, @Vagabond, Helium Systems, Inc. team
  • Start Date: 2021-04-19
  • Category: Technical
  • Original HIP PR: #154
  • Tracking Issue: #157
  • Status: Deployed (2021-05-04)

Summary

Hotspot owners, HNT holders, and other Helium participants, have different ways to organize assets. This HIP proposes a multi-signature key which can be used to authorize a transaction on the Helium blockchain. This key is a composite of the scalar keys which can be combined to generate a minimal subset of signatures required to make a valid transaction.

Motivation

We have already seen that Hotspot owners are already engaging in interesting ways to own their Hotspots off-chain. This HIP proposes a way to allow for the following:

  • Dividing up the responsibility of various assets (Hotspots, Validators, OUIs, HST, HNT, etc.) across multiple participants. This enables joint accounts and shared custody.
  • Making it substantially more difficult for a compromise of an entire wallet due to a single key. This improves security via multi-owner verification.
  • Creating a M of N backup where a loss of a single key does not cause a loss of the entire wallet.

Multi-signature keys are seen in many other blockchains and enable security applications not listed above.

Stakeholders

This change should not affect any current asset owners as it is a purely additive change to existing key types.

We believe that this feature will enable new ownership organization of assets like Hotspots, Validators, OUIs, HNT, HST, etc.

Detailed Explanation

Currently, a wallet address is derived from a single public key owned by a single private key. This private key is required to sign any transaction involving the address.

We propose:

  1. an address scheme composed of a multihash digest of N public keys and a specification of the number of signatures required to approve a transaction: M such that 0 < M <= N;
  2. a signature scheme composed of N public keys and M corresponding signatures;
  3. conditions for verification of the above composite signature, given the above composite public key;
  4. an implementation of all of the above.

Format

Roughly, the schemes are:

  1. Multi-signature public key: | net-type | key-type | m | n | multihash-digest-of-n-public-keys |;
  2. Multi-signature: | public-key[0]..public-key[n-1] | isig[1]..isig[m] |.

More precisely, given a sequence of n public keys: public-key[0]..public-key[n-1], a multihash-digest-of-n-public-keys is its multihash digest and isig[1]..isig[m] is a sequence of triples: i | len | sig where i is the index into the preceding sequence of public keys, len is the length of the following signature (in bytes) and sig is the signature.

Illustration:

Multi-signature public key
  net-type   key-type   m        n        multihash-digest-of-n-public-keys
+----------+----------+--------+--------+-----------------------------------+
| 4 bits   | 4 bits   | 8 bits | 8 bits | variable length                   |
+----------+----------+--------+--------+-----------------------------------+
Multi-signature
  pub keys          isigs
+-----------------+------------------+
| variable length | variable length  |
+-----------------+------------------+

Due to variable lengths of the components, parsing of the multi-signature requires knowledge of:

  1. N from the corresponding multi-pub-key;
  2. length of each scalar key type (type itself is encoded, but the length corresponding to type is not, so must be known in advance).

Verification

After parsing the scalar components of the multi-pub-key and multi-signature, we consider the multi-signature to be valid if all of the following conditions are true:

  1. we have at least M scalar signatures within the composite multi-signature;
  2. all of the provided scalar signature triples uniquely point to a public key within 0..N-1 range (i.e. any isig containing an-already-seen index - invalidates the multi-signature);
  3. at least M of the scalar signatures pass verification.

Drawbacks

We don't believe there are any drawbacks to this approach. In fact, it is an in-place upgrade to the existing supported key types.

Rationale and Alternatives

An alternative that was considered, but rejected, was to have separate on-chain multisignature transactions as we do for the vars_v1 transaction. This approach to multisignature keys makes that approach unnecessary. We do not, at this time, intend to update vars_v1 but may do so at a later date.

Unresolved Questions

Deployment Impact

We don't believe there is any deployment impact as this approach is additive to existing key types.

Success Metrics

This will be a successful feature if we see applications of multi-signature keys on chain.