Bitcoin Forum

Is there a way that i can know only the first letter of the private key after the "5" , is it easy like getting an address starts with certain letter or it's just impossible.
Edit: i just know the publickey point

No. That would imply that you were able to derive the private key from the public key, which is currently impossible.

I see , so can i know if the private key is even or odd from the publickey point :),

No.

There is currently no known mathematical process for determining anything about the private key from the publickey point.

I have wondered about this also. 

We could determine whether the private key is odd or even from the public key point only if:

There exists a definition of an "even point" and an "odd point" such that for (X, Y)' = (X, Y) + G

(X, Y)' is odd iff (X, Y) is even and (X, Y)' is even iff (X, Y) is odd

So there would have to be a property of the points on the curve (such as the sign of Y for example) that toggles with each new addition of the point G.

If such a property could be found then it would be possible to determine one bit of the private key from the public key.

Assume for example the sign of Y behaved this way then the private key would be odd if the sign of Y was negative and the private key would be even if the sign of Y was positive.

If you can find such property i can figure out the private key from the public key not only one bit , and because of that i am sure it's too hard to figure it out but not impossible.