HIP 29: M-of-N Multisig (#154)

Co-authored-by: Abhay Kumar <abhay@helium.com>
Co-authored-by: Andrew Thompson <andrew@hijacked.us>
Co-authored-by: Jamie Wilkinson <jamie@jamiedubs.com>

0029-multisignature-keys.md

@@ -0,0 +1,136 @@
+# HIP 29: Multi-signature Keys
+
+- Author(s): [@xandkar](http://github.com/xandkar), [@Vagabond](http://github.com/Vagabond), Helium Systems, Inc. team
+- Start Date: 2021-04-19
+- Category: Technical
+- Original HIP PR: [#154](https://github.com/helium/HIP/pull/154)
+- Tracking Issue: [#157](https://github.com/helium/HIP/issues/157)
+- Status: Discussion
+
+# Summary
+[summary]: #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
+[motivation]: #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
+[stakeholders]: #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
+[detailed-explanation]: #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
+[drawbacks]: #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
+[alternatives]: #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
+[unresolved]: #unresolved-questions
+
+# Deployment Impact
+[deployment-impact]: #deployment-impact
+
+We don't believe there is any deployment impact as this approach is additive
+to existing key types.
+
+# Success Metrics
+[success-metrics]: #success-metrics
+
+This will be a successful feature if we see applications of multi-signature
+keys on chain.

README.md

@@ -39,6 +39,7 @@ If you have questions or feedback, please ask in [#hip-open-discussion in the co
 | 26 | [Payment Notes](https://github.com/helium/HIP/blob/master/0026-payment-notes.md) | [In Discussion](https://github.com/helium/HIP/issues/125) |
 | 27 | [Support CBRS 5G](https://github.com/helium/HIP/blob/master/0027-cbrs-5g-support.md) | [Approved](https://github.com/helium/HIP/pull/133) |
 | 28 | [Consensus Reward Adjustments](https://github.com/helium/HIP/blob/master/0028-consensus-reward-adjustments.md) | [In Discussion](https://github.com/helium/HIP/issues/140) |
+| 29 | [Multi-signature Keys](https://github.com/helium/HIP/blob/master/0029-multisignature-keys.md) | [In Discussion](https://github.com/helium/HIP/issues/157) |
 
 
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