Merge PR cosmos#3968: Documentation on how to sign Cosmos transactions

docs/clients/service-providers.md

@@ -121,3 +121,69 @@ mechanism for instance.
 In order to generate an unsigned transaction (example with
 [coin transfer](https://cosmos.network/rpc/#/ICS20/post_bank_accounts__address__transfers)),
 you need to use the field `generate_only` in the body of `base_req`.
+
+## Cosmos SDK Transaction Signing
+
+Cosmos SDK transaction signing is a fairly simple process.
+
+Every Cosmos SDK transaction has a canonical JSON representation. The `gaiacli`
+and Stargate REST interfaces provide canonical JSON representations of transactions
+and their "broadcast" functions will provide compact Amino (a protobuf-like wire format)
+encoding translations.
+
+Things to know when signing messages:
+
+The format is as follows
+
+```json
+{
+  "account_number": XXX,
+  "chain_id": XXX,
+  "fee": XXX,
+  "sequence": XXX,
+  "memo": XXX,
+  "msgs": XXX
+}
+```
+
+The signer must supply `"chain_id"`, `"account number"` and `"sequence number"`.
+
+The `"fee"`, `"msgs"` and `"memo"` fields will be supplied by the transaction
+composer interface.
+
+The `"account_number"` and `"sequence"` fields can be queried directly from the
+blockchain or cached locally. Getting these numbers wrong, along with the chainID,
+is a common cause of invalid signature error. You can load the mempool of a full
+node or validator with a sequence of uncommitted transactions with incrementing
+sequence numbers and it will mostly do the correct thing.  
+
+Before signing, all keys are lexicographically sorted and all white space is
+removed from the JSON output.
+
+The signature encoding is the 64-byte concatenation of ECDSArands (i.e. `r || s`),
+where `s` is lexicographically less than its inverse in order to prevent malleability.
+This is like Ethereum, but without the extra byte for PubKey recovery, since
+Tendermint assumes the PubKey is always provided anyway.
+
+Signatures and public key examples in a signed transaction:
+
+``` json
+{
+  "type": "auth/StdTx",
+  "value": {
+    "msg": [...],
+    "signatures": [
+      {
+        "pub_key": {
+          "type": "tendermint/PubKeySecp256k1",
+          "value": XXX
+        },
+        "signature": XXX
+      }
+    ],
+  }
+}
+```
+
+Once signatures are properly generated, insert the JSON into into the generated
+transaction and then use the broadcast endpoint.