online tool to play around with ECDSA public keys, message signature verification?
Here are a few I'd recommend:


https://bitcore.io/playground/#/address (private key - public key)
https://brainwalletx.github.io/#generator (signing and verifying but bitcoin specific)


Or play with pybitcointools at the command line (not online):

$ git clone https://github.com/vbuterin/pybitcointools.git
$ cd pybitcointools
$ python
&gt;&gt;&gt; from bitcoin import *
&gt;&gt;&gt; sk = random_key()  # Generate a private key
&gt;&gt;&gt; vk = privtopub(sk) # Generate a public key
&gt;&gt;&gt; msg = 'hello world' # Create a simple message
&gt;&gt;&gt; sig = ecdsa_sign(msg, sk) # Sign the message using your private key
&gt;&gt;&gt; print sig
GxXGAt...2L/eJk=
&gt;&gt;&gt; print ecdsa_verify(msg, sig, vk) # Use sig and public key to verify
True
&gt;&gt;&gt; msg = 'hello mars' # Change the message
&gt;&gt;&gt; print ecdsa_verify(msg, sig, vk) # Changing the msg invalidates sig
False


http://showterm.io/203b168061b0156c4d1dd

HD Wallet stuff:


https://iancoleman.github.io/bip39/
http://bip32.org/


If you're looking to learn more about ECDSA suggest checking out: https://jeremykun.com/2014/02/08/introducing-elliptic-curves/