add api spec for the signer (#133)

* add api spec for the signer

* fix typo

Author: KonradStaniec

covenant-signer/docs/signing_api_spec.md

@@ -0,0 +1,97 @@
+# Covenant Signer API specification
+
+Covenant signer must support following APIs to correctly work with covenant emulator.
+
+## Endpoints
+
+### `POST /v1/sign-transactions`
+
+This endpoint is used to request signatures for a set of transactions.
+
+#### Request Body
+
+The request body is a JSON object with the following structure:
+
+```json
+{
+  "staking_tx_hex": "string",
+  "slashing_tx_hex": "string",
+  "unbonding_tx_hex": "string",
+  "slash_unbonding_tx_hex": "string",
+  "staking_output_idx": "number",
+  "slashing_script_hex": "string",
+  "unbonding_script_hex": "string",
+  "unbonding_slashing_script_hex": "string",
+  "fp_enc_keys": ["string"],
+  "stake_expansion": {
+    "previous_active_stake_tx_hex": "string",
+    "other_funding_output_hex": "string",
+    "previous_staking_output_idx": "number",
+    "previous_active_stake_unbonding_script_hex": "string"
+  }
+}
+```
+
+#### `SignTransactionsRequest` description
+
+| Field | Type | Description |
+| --- | --- | --- |
+| `staking_tx_hex` | `string` | The hexadecimal representation of the staking transaction. |
+| `slashing_tx_hex` | `string` | The hexadecimal representation of the slashing transaction. |
+| `unbonding_tx_hex` | `string` | The hexadecimal representation of the unbonding transaction. |
+| `slash_unbonding_tx_hex` | `string` | The hexadecimal representation of the slash unbonding transaction. |
+| `staking_output_idx` | `number` | The index of the staking output in the staking transaction. |
+| `slashing_script_hex` | `string` | The hexadecimal representation of the slashing script. |
+| `unbonding_script_hex` | `string` | The hexadecimal representation of the unbonding script. |
+| `unbonding_slashing_script_hex` | `string` | The hexadecimal representation of the unbonding slashing script. |
+| `fp_enc_keys` | `[]string` | An array of hexadecimal representations of the finality provider encryption keys. |
+| `stake_expansion` | `StakeExpansionRequest` | An optional object containing information about the stake expansion. |
+
+#### `StakeExpansionRequest` description
+
+| Field | Type | Description |
+| --- | --- | --- |
+| `previous_active_stake_tx_hex` | `string` | The hexadecimal representation of the previous active stake transaction. |
+| `other_funding_output_hex` | `string` | The hexadecimal representation of the other funding output. |
+| `previous_staking_output_idx` | `number` | The index of the previous staking output in the previous active stake transaction. |
+| `previous_active_stake_unbonding_script_hex` | `string` | The hexadecimal representation of the unbonding script of the previous active stake transaction. |
+
+#### Response Body
+
+The response body is a JSON object with the following structure:
+
+```json
+{
+  "slashing_transactions_signatures": ["string"],
+  "unbonding_transaction_signature": "string",
+  "slash_unbonding_transactions_signatures": ["string"],
+  "stake_expansion_transaction_signature": "string"
+}
+```
+
+#### `SignTransactionsResponse` description
+
+| Field | Type | Description |
+| --- | --- | --- |
+| `slashing_transactions_signatures` | `[]string` | An array of hexadecimal representations of the adaptor signatures for the slashing transactions. |
+| `unbonding_transaction_signature` | `string` | The hexadecimal representation of the signature for the unbonding transaction. |
+| `slash_unbonding_transactions_signatures` | `[]string` | An array of hexadecimal representations of the adaptor signatures for the slash unbonding transactions. |
+| `stake_expansion_transaction_signature` | `string` | The hexadecimal representation of the signature for the stake expansion transaction. field will only be filled if `stake_expansion` object in request was not nil|
+
+### `GET /v1/public-key`
+
+This endpoint is used to retrieve the public key of the signer.
+
+#### Response Body
+
+```json
+{
+  "public_key": "string"
+}
+```
+
+#### Response body description
+
+| Field | Type | Description |
+| --- | --- | --- |
+| `public_key` | `string` | A hexadecimal representation of covenant signer 33byte public key. |