You cannot because Bitcoin Core does not store such information. Bitcoin Core does not store all information for all addresses, it only stores information for its own addresses and the lower level information needed for verifying transactions (addresses are a higher level abstraction). There are no commands that allow you to get the balances or transactions ...
signmessage and verifymessage only work with legacy type addresses. They do not work with segwit addresses. There is ongoing work to introduce a new message signing standard that will work regardless of address type.
Some discussion about why they only work on legacy addresses can be found in this issue: https://github.com/bitcoin/bitcoin/issues/10542
It means you either have no headers, no block, or both known to be (*) in common with this peer.
From the help:
$ bitcoin-cli help getpeerinfo
"synced_headers" : n, (numeric) The last header we have in common with this peer
"synced_blocks" : n, (numeric) The last block we have in common with this peer
There are currently no specifications for interoperable text/file formats between lightning network node implementations. The lightning BOLT specifications deal with standardization of the network protocols so that different lightning implementations can talk to one another, but all other details which are not part of this protocol are left up to the ...
The 'wallet import format' (WIF) is a standardized way of encoding a private key so that other wallets can recognize it. You could use it to import the private key to your node's wallet, and then sign and send the transaction from there, but I think thats different than what you want to do: keep the private keys on the mobile, but broadcast signed ...
You can download a database dump from blockchair.com at https://gz.blockchair.com/bitcoin/addresses/
The database is updated daily and is contains a balance of satoshis for every address on the network.
While you may be able to import the key into another wallet, it is better to create a new wallet with a new key and transfer all the bitcoin out of the old wallet and into the new one by making a regular bitcoin transaction on the blockchain. This eliminates all sorts of possible complications, including the security risk of disclosing the keys from your old ...
In fact, there are few CLI utilies that allow you this:
sx : command line Bitcoin to empower the sysadmin
cat key| sx pubkey
pybitcointools : SImple, common-sense Bitcoin-themed Python ECC library
pybtctool privtopub key
To the best of my knowledge there is no standard CLI utility.
However, there are some tools which seem to be generally accepted, like bitcoin-bash-tools
Alternatively, you can use some script library, such as pycoin (written in Python), or bitcoinjs.
Personally I like Bitcore libraries (Bitcore is not Bitcoin Core, despite their names are similar). They ...