The signmessagewithprivkey RPC signs a message with a given private key.
signmessagewithprivkey "privkey" "message"
*bitcoin-cli help signmessagewithprivkey
signmessagewithprivkey "privkey" "message" Sign a message with the private key of an address Arguments: 1. "privkey" (string, required) The private key to sign the message with. 2. "message" (string, required) The message to create a signature of. Result: "signature" (string) The signature of the message encoded in base 64 Examples: Create the signature > bitcoin-cli signmessagewithprivkey "privkey" "my message" Verify the signature > bitcoin-cli verifymessage "1D1ZrZNe3JUo7ZycKEYQQiQAWd9y54F4XX" "signature" "my message" As json rpc > curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "signmessagewithprivkey", "params": ["privkey", "my message"] }' -H 'content-type: text/plain;' http://127.0.0.1:8332/