ADR: Fix malleability problems in Secp256k1 signatures

Previously you could not assume that your transaction hash would
appear on chain.

docs/architecture/adr-014-secp-malleability.md

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+# ADR 014: Secp256k1 Signature Malleability
+
+## Context
+
+Secp256k1 has two layers of malleability.
+The signer has a random nonce, and thus can produce many different valid signatures.
+This ADR is not concerned with that.
+The second layer of malleability basically allows one who is given a signature
+to produce exactly one more valid signature for the same message from the same public key.
+(They don't even have to know the message!)
+The math behind this will be explained in the subsequent section.
+
+Note that in many downstream applications, signatures will appear in a transaction, and therefore in the tx hash.
+This means that if someone broadcasts a transaction with secp256k1 signature, the signature can be altered into the other form by anyone in the p2p network.
+Thus the tx hash will change, and this altered tx hash may be committed instead.
+This breaks the assumption that you can broadcast a valid transaction and just wait for its hash to be included on chain.
+You may not even know to increment your sequence number for example.
+Removing this second layer of signature malleability concerns could ease downstream development.
+
+### ECDSA context
+
+Secp256k1 is ECDSA over a particular curve.
+The signature is of the form `(r, s)`, where `s` is an elliptic curve group element.
+However `(r, -s)` is also another valid solution.
+Note that anyone can negate a group element, and therefore can get this second signature.
+
+## Decision
+
+We can just distinguish a canonical form for the ECDSA signatures. 
+Then we require that all ECDSA signatures be in the canonical form between the two.
+
+The canonical form is rather easy to define and check. 
+It would just be the smaller of the two y coordinates for the given x coordinate, defined lexicographically.
+Example of other systems using this: https://github.com/zkcrypto/pairing/tree/master/src/bls12_381#serialization.
+
+## Proposed Implementation
+
+Fork https://github.com/btcsuite/btcd, and just update the [parse sig method](https://github.com/btcsuite/btcd/blob/master/btcec/signature.go#195) and serialize functions to enforce our canonical form.
+
+## Status
+
+Proposed.
+
+## Consequences
+
+### Positive
+* Lets us maintain the ability to expect a tx hash to appear in the blockchain.
+
+### Negative
+* More work in all future implementations (Though this is a very simple check)
+* Requires us to maintain another fork
+
+### Neutral