Add notes on transaction signing

Add notes on transaction signing with Dogecoin 1.21 experimental.

docs/signing_notes_dogecoin_core_1_21_experimental.md

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+# Signing Notes
+
+Writing up my notes on the transaction creation and signing process so they're
+readily available to refer to later, and because they may be useful to others.
+
+All of this is tested on an experimental branch of Dogecoin 1.21; the theory is
+generally applicable
+but other tools will have different interfaces.
+
+## Pre-requisites
+
+In order to sign the output transaction, the PSBT needs to include the details
+of the input transaction(s). As nodes do not track transactions unless they're
+relevant to an address the node is watching, the address the transaction is to
+needs to be watched.
+
+This only needs to be done once, as the transaction creator can add these to
+the PSBT and then circulate the expanded PSBT.
+
+If this is done before the address receives any funds, you can avoid rescanning,
+but as the address now exists the node needs to rescan the blockchain, which
+takes around an hour.
+
+I ran the following to add the address:
+
+```shell
+$ dogecoin-cli importaddress 9xEP9voiNLw7Y7DS87M8QRqKM43r6r5KM5 "corefunds" true
+```
+
+The following will then return a non-empty list:
+
+```shell
+$ dogecoin-cli listunspent 1 9999999 "[\"9xEP9voiNLw7Y7DS87M8QRqKM43r6r5KM5\"]"
+```
+
+## Creating the transaction
+
+Creating the transaction involves defining its inputs, and the amounts to send.
+This generates a very bare PSBT which isn't very useful until it's processed
+by a node with a wallet.
+
+The following consumes input #1 (the second) of transaction
+169a73cf972fbc1ff9d480474e5de7db03d63c243f4b57d25cdd09ba56ff2738, which was 10 Doge,
+and sends 9.9 Doge back to 9xEP9voiNLw7Y7DS87M8QRqKM43r6r5KM5. The remaining 0.1 Doge
+is burnt as a (high) transaction fee.
+
+```shell
+dogecoin-cli "[{\"txid\":\"169a73cf972fbc1ff9d480474e5de7db03d63c243f4b57d25cdd09ba56ff2738\",\"vout\":1}]" "[{\"9xEP9voiNLw7Y7DS87M8QRqKM43r6r5KM5\":9.9}]"
+```
+
+This creates the following PSBT, which you'll see as inputs to later examples:
+
+```
+cHNidP8BAFMCAAAAATgn/1a6Cd1c0ldLPyQ81gPb511OR4DU+R+8L5fPc5oWAQAAAAD/////AYAzAjsAAAAAF6kUP5EutQN3mspXxZVmYkhcTKkTMK2HAAAAAAAAAA==
+```
+
+I can decode the resulting PSBT to check its contents:
+
+```shell
+$ dogecoin-cli decodepsbt cHNidP8BAFMCAAAAATgn/1a6Cd1c0ldLPyQ81gPb511OR4DU+R+8L5fPc5oWAQAAAAD/////AYAzAjsAAAAAF6kUP5EutQN3mspXxZVmYkhcTKkTMK2HAAAAAAAAAA==
+```
+
+## Wallet Processing the Transaction
+
+The next step is to add the required details to the PSBT can be signed.
+
+```
+$ dogecoin-cli  -datadir=/data/dogecoin walletprocesspsbt cHNidP8BAFMCAAAAATgn/1a6Cd1c0ldLPyQ81gPb511OR4DU+R+8L5fPc5oWAQAAAAD/////AYAzAjsAAAAAF6kUP5EutQN3mspXxZVmYkhcTKkTMK2HAAAAAAAAAA==
+```
+
+If this has worked correctly, decoding the resulting PSBT should show it
+includes the transaction inputs (i.e. pipe it to `| jq .inputs`).
+
+If this is done on a wallet with an available signing key, by default
+it will add a signature to the PSBT too. The process I follow involves
+a node which does not have the key, and then moving the PSBT to an isolated
+machine for signing.
+
+## Signing the Transaction
+
+Signing the transaction is identical to wallet processing, except performed
+on a node with one of the signing keys present. It will then update the
+PSBT with the signature from that signing key.
+
+Before signing, it's important to `decodepsbt` the PSBT to check what is
+being signed. This produces an output such as that below (note this is
+trimmed to focus on just the "tx" and "fee"):
+
+```
+$ dogecoin-cli decodepsbt <psbt after wallet processing>
+{
+  "tx": {
+    "txid": "e80a06fb189acef79e98ffd86ceea78ff7504e434f4aea0a86c50373327ddd98",
+    "hash": "e80a06fb189acef79e98ffd86ceea78ff7504e434f4aea0a86c50373327ddd98",
+    "version": 2,
+    "size": 83,
+    "vsize": 83,
+    "weight": 332,
+    "locktime": 0,
+    "vin": [
+      {
+        "txid": "169a73cf972fbc1ff9d480474e5de7db03d63c243f4b57d25cdd09ba56ff2738",
+        "vout": 1,
+        "scriptSig": {
+          "asm": "",
+          "hex": ""
+        },
+        "sequence": 4294967295
+      }
+    ],
+    "vout": [
+      {
+        "value": 9.90000000,
+        "n": 0,
+        "scriptPubKey": {
+          "asm": "OP_HASH160 3f912eb503779aca57c5956662485c4ca91330ad OP_EQUAL",
+          "hex": "a9143f912eb503779aca57c5956662485c4ca91330ad87",
+          "reqSigs": 1,
+          "type": "scripthash",
+          "addresses": [
+            "9xEP9voiNLw7Y7DS87M8QRqKM43r6r5KM5"
+          ]
+        }
+      }
+    ]
+  },
+  ...
+  "fee": 0.10000000
+}
+```
+
+The "tx" is the transaction being built. Within it, "vin" is the inputs (what
+is being spent) and "vout" is the outputs. Here you can see it has a single
+output of 9.9 Dogecoins being sent to `9xEP9voiNLw7Y7DS87M8QRqKM43r6r5KM5`.
+The fee is then calculated based on the total input value, minus the total
+output value, and in this case is 0.1 Dogecoins.
+
+## Finalizing the Transaction
+
+Once you have enough signatures, you can assemble the transaction. Note
+"can" doesn not mean "should" - getting a signature from every signer
+is definitely preferable. That said, conventionally only sufficient
+signatures to meet the threshold are relayed, so the other signatures
+should be kept for audit purposes elsewhere.
+
+Depending how the signing has happened, if each person has passed the
+PSBT along to the next in turn, you should be able to call `finalizepsbt`
+directly. Alternatively, if they were signed independently then the
+signatures need assembling, `combinepsbt` can put the PSBTs together
+and return the assembled set ready to pass to `finalizepsbt`.
+
+`finalizepsbt` returns a fully signed transaction, but does not relay it.
+Before relaying, it's a good idea to `decoderawtransaction` the transaction
+to verify it is correct.
+
+## Relaying the Transaction
+
+Once the finalized transaction is verified, it can be relayed with
+`sendrawtransaction`, which will relay it to the P2P network for mining.