Begin multisig draft

0029-multisig-addresses.md

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+# HIP 29: Multisig transactions
+
+- Author(s): @xandkar, Helium Team; @Vagabond, Helium Team.
+- Start Date: 2021-04-19
+- Category: economic, technical
+- Original HIP PR:
+- Tracking Issue:
+
+# Summary
+[summary]: #summary
+
+Add  a new type of public key and a new type of signature, which are composites
+of the scalar public keys and signatures combined with a specification for the
+minimal subset of signatures required to make a transaction.
+
+# Motivation
+[motivation]: #motivation
+
+New ways and interesting ways to organize asset ownership, such as:
+- joint account
+- backup in case of key loss
+- additional security via multi-factor verification
+- ...
+
+# Stakeholders
+[stakeholders]: #stakeholders
+
+
+# Detailed Explanation
+[detailed-explanation]: #detailed-explanation
+
+Currently an address is a single public key owned my a single private key.
+The single owner is required to sign a transaction involving the address.
+
+We propose:
+
+1. 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. signature scheme composed of `N` public keys and `M` corresponding signatures;
+3. algorithm for verification of the above composite signature, given the above
+   composite public key.
+
+### format
+Roughly, the schemes are:
+1. Multi-pub-key: `| net-type | key-type | m | n | multihash-digest-of-n-pub-keys |`;
+2. Multi-signature: `| pub-key[0]..pub-key[n-1] | isig[1]..isig[m] |`.
+
+More-precisely, given a sequence N public keys: `pub-key[0]..pub-key[n-1]`,
+a `multihash-digest-of-n-pub-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-pub-key
+
+      net type   key type   M        N        multihash-digest-of-N-pub-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 signature to be valid if all of the following is true:
+1. we have at least M scalar signatures within the composite multi-signature;
+2. _all_ of the provided signatures _uniquely_ point to a public key within 0..N-1;
+3. at least M of the scalar signatures pass verification.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+
+# Rationale and Alternatives
+[alternatives]: #rationale-and-alternatives
+
+
+# Unresolved Questions
+[unresolved]: #unresolved-questions
+
+
+# Deployment Impact
+[deployment-impact]: #deployment-impact
+
+
+# Success Metrics
+[success-metrics]: #success-metrics