Nethermind Changes
The Merge brings changes the JSON-RPC API. Such as the Engine API, JWT authentication, additional RPC ports, and additional block tags.

Engine API

The Engine API adds new endpoints that allow the execution layer client to receive messages from the consensus layer client.
Port 8551 is the default port that Engine API communicates with the CL on.
JWT Authentication is used on all ports with the Engine API Module enabled.
execution-apis/specification.md at main · ethereum/execution-apis
GitHub

JWT Secrets

JSON Web Token authentication was added to the JSON-RPC API for security reasons to ensure that nothing interferes with the communication between the Nethermind and the Consensus client. This requires you to create a .txt file containing a hexadecimal “secret” that will be passed to each client.
Nethermind will create it's own jwtsecret file if you do not specify a location or pass the wrong location.
JWT.IO
To create this “Secret File” use the following command.
openssl rand -hex 32 | tr -d "\n" > "/tmp/jwtsecret"
where "/tmp/jwtsecret" will be the file path and name when created.
If you do not want to install OpenSSL, you may use a random hex generator website. All you need is a 64 character hex string saved to a .txt file.
We strongly recommend you use OpenSSL to generate the secret locally because it is more secure
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Configuration Module

The merge also adds new configuration options such as:
  • JsonRpc.AdditionalRpcUrls
  • JsonRpc.JwtSecretFile
  • New Merge module

Block Tags

Some requests on the JSON-RPC API require a block tag for additional context. Previously there were three block tags earliest, latest, and pending. The Merge adds an additional two tags finalized and safe.
Block Tag
Description
earliest
The lowest numbered block the client has available
safe
The most recent crypto-economically secure block, cannot be re-orged outside of manual intervention driven by community coordination
finalized
The most recent crypto-economically secure block, cannot be re-orged outside of manual intervention driven by community coordination
latest
The most recent block in the canonical chain observed by the client, this block may be re-orged out of the canonical chain even under healthy/normal conditions
pending
A sample next block built by the client on top of latest and containing the set of transactions usually taken from local mempool