Update CIP-1853 usecase derivation from non-hardened to hardened

CIP-1853/CIP-1853.md

@@ -22,12 +22,12 @@ License: CC-BY-4.0
 Using `1853'` as the purpose field, we define the following derivation path structure for stake pool cold keys.
 
 ```
-m / purpose' / coin_type' / usecase / cold_key_index'
+m / purpose' / coin_type' / usecase' / cold_key_index'
 ```
 
-Example: `m / 1853' / 1815' / 0 / 0'`
+Example: `m / 1853' / 1815' / 0' / 0'`
 
-Here the `usecase` is currently fixed to `0`.
+Here the `usecase` is currently fixed to `0'`.
 
 Given that stake pool cold keys are cryptographically the same as wallet keys already covered in CIP-1852, the master node and subsequent child keys derivation **MUST** be implemented in the same way as specified for wallets in CIP-1852.
 
@@ -49,9 +49,9 @@ Similarly as we have the `chain` path component in CIP-1852 paths for different
 
 Each stake pool is supposed to be managed separately so there is currently no incentive to connect them via a parent public key.
 
-### Non-hardened derivation at `usecase`
+### Hardened derivation at `usecase`
 
-We chose non-hardened derivation at the usecase index for the sake of future-proofness. It may be convenient to simultaneously derive non-hardened child public keys across different usecases. Since the the derivation at `usecase` index is non-hardened, it would be enough for the host machine to request just the `m / 1853' / 1815'` extended public key from the hardware wallet and derive the respectice child public cold keys chains locally.
+We chose hardened derivation at the usecase index as there is no incentive to mix the stake pool cold keys with other potential usecases and if there was such incentive, it would most likely be more appropriate to create a separate usecase/purpose for that.
 
 ## Copyright