Question about transaction signing vs. message signing

[bitvinda]

I've been playing with coinb.in recently to try to better understand how transactions are put together. It's been massively eye-opening so far, but I've reached to a point now where I'm stuck and wondering if what I'm trying to do is even possible.

I've created a transaction in coinb.in using the public address and inputs of an address from my mycelium wallet. I don't have direct access to the private key and I don't really want to put the HD seed into a key generator to try to find the private key for the address I'm looking for. Is there any way to sign the raw transaction from the mycelium wallet as is? Maybe through using the 'sign message' functionality or some variant of it?

[achow101]

It is not possible. Although transaction signing and message signing are similar, it is what happens to the data afterwards and how the data is treated that makes the difference and makes those two operations incompatible.

[Monnt]

Although they're both signing something, they're signing different things different ways for different uses. Sure, you have to use your private key to sign the message, but i'm not so sure you have access to that, as you've said. Maybe try finding the private key from the seed offline?