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Since you passed posting as third argument, it should have worked. Are you sure you weren't passing public posting key?

100% sure, I use my keys correctly all the time. It only works with master as far as I can tell.

In fact I just logged out and logged back in with the posting key I was using just to make sure.

Probably a bug then. Steem api works with posting key.

apparently I have to use .fromString():

var key = dsteem.PrivateKey.fromString('posting-key-goes-here')

This is your public posting key: STM7KtxFtRj4jXcvmvbifcAzh1rXnBWJiGuTeZAyWnKBKBoZerT1G

yeah all private keys start with a 5. master password starts with a P

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