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owner: main key that controls everything and can change all keys including owner key.

active: "coin" transactions such as spending Steem, powering up, etc. Can't change the owner key.

posting: social actions -- posting and voting. Can't spend coins or change keys.

memo: used to encrypt and decrypt private messages

Thank you. Is the owner key also used for mining?

I don't know which keys are needed to mine in an existing account, although it probably includes the signing key which isn't listed there but is yet another key that is part of the witness data attached to an account. When you mine without having already created an account you just give one WIF key and it is inserted in all the slots when the account is created.

What and where is the WIF key ????

I have a feeling this video will answer your question.
Sorry, it's 2 years too late.
Still, it may help others who come to this post.

Hi, I would also like to add that if you go and generate the pdf document with your keys. Do not copy paste them directly from the pdf generated because it comes out gibberish. I learned that the hard way - 4 hours trying to figure out what was wrong when I was trying to log in. Put it in a word document/text/onenote/evernote and then tag each key accordingly. From here you can copy paste the code to the field to log you in.

DO NOT FORGET TO HAVE A HARD COPY STORED SECURELY OFFLINE.

Thanks for posting this, but I'm still confused. I think I need a diagram!

Did I hear correctly that it's okay to give out your PRIVATE posting key??? That doesn't seem so private.

Yes, your private posting key is okay to give out. That is for posting only. Your active key is for posting and money transfers.

How do I take my owner key off line?

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