File|Sign Message fails for Segwit addresses (3, bc1) #15256

<!-- Describe the issue -->

Creating a labelled bitcoin address on the Receive tab (32hCAqWGcA85Ge9kMV2YpFoRyAuChvF6oN) and selecting it from File|Sign Message results in an error when the "Sign Message" button is pressed:

"The entered address does not refer to a key. Please check the address and try again."

<!--- What behavior did you expect? -->

That the message would be signed and a Signature created.

<!--- What was the actual behavior (provide screenshots if the issue is GUI-related)? -->

The error message as described.

<!--- How reliably can you reproduce the issue, what are the steps to do so? -->

Everytime I use an address that doesn't start with 1.

<!-- What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)? -->

v0.17.1 (64bit)

<!-- What type of machine are you observing the error on (OS/CPU and disk type)? -->

Windows 10

Duplicate of #10542, see that issue for discussion.

Thanks!

Cryptographic signatures and private keys. The discussion seems to see little or no value. A signature proves possession of the private key. It may be the application requires nothing else, certainly nothing to do with Bitcoin or balances.