There are different kinds of bits. One kind is the number of 1's and 0's. As Dany said a ECDSA private key as used in bitcoin has 256 different of these.
Another kind is a measurement for the amount of information a message contains (this message may or may not be encoded in bits). This measurement is called entropy and is given in bits as well.
There is also the kind of bits used by DeathAndTaxes which is a comparisson between algorithms. What it means is that ECDSA is as strong as a 128bit symmetric encryption scheme. Another common example is RSA, where a 1024 bit key is as strong as ~70 bits. This number can decrease over time as better ways are found to calculate the private key from the public key.
Yes but I was asking for the entropy NOT the keysize, hence the title of the thread:
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I`m a bit confused about the maximum entropy that bitcoin provides.
Alright so private key = 128 bit entropy, finally that is settled
No.
Ok so then how much entropy security does a spent vs unspent address provide?