In the blockchain, there is nothing encrypted, but still, asymmetric cryptography is heavily used. Each wallet contains at least a pair of keys, a private one used to sign transactions (and to claim ownership of UTXO), and a public key, generated from the private one, and used to receive funds.

Just the holder of the private key can produce the numerical signature of a transaction from its fingerprint. But anyone can verify the signature with the public key and the fingerprint.

## Private Key

A blockchain private key is nothing but a random number in the range `[0, 2^256]`. And here, random is paramount. Basic pseudo random generators are not enough. Remember, this random number is the only required proof to claim ownership (and to spend) associated funds. Obviously, Hardware Random Number Generator where entropy is obtained on some physical phenomena are better than Pseudo-random Number Generator, but even them might be good enough. They key here is that the generated number should be unpredictable1.

## Elliptic Curve

An elliptic curve is a plane algebraic curve. In other words, is the set of points on the Euclidean plane whose coordinates are zeros of a polynomial in two variables.

The standard used in bitcoin is `secp256k1`, and choose a particular polynomial, `y2 = x3 + 7`. When evaluated in the real plane, you get a smooth curve symmetrical with respect to the x-axis. However, `secp256k1` is defined over the field of prime order p2. That means you can kiss your smooth curve goodbye, now you are dealing with what seems to be a pattern of dots scattered through two dimensions. But that’s just a matter of representation, remember the definition, we just need a set. The mathematical properties we can find are associated with the object and not with the representation.

For a pedagogical purpose, let’s reduce the prime order of the field to 11. We end up with a pattern like this one.

### Addition

One of the properties of the elliptic curve is that if we draw a line between two points in the curve, the line will intersect the curve in exactly an additional point. This operation is called addition. If P1 and P2 have the same x coordinate, and different y coordinate, then the line joining them will be parallel to the y axis, and P3 is defined as a point at infinity. Now, if either P1 or P2 are defined as points at infinity, then we have P1+P2=P2 (iff P1 is the point at infinity), or P1+P2=P1 (when P2 is a point at infinity). That’s the reason a point at infinity is sometimes called zero over additions.

### Multiplication

Now that we have defined addition, we can extend the definition to multiplication pretty easily. From classic arithmetic, we know that to multiple a value V by K means to add V to itself K times. Then, let’s take a point P on the elliptic curve, and add P to itself K times and we have ‘k*P’.

To visualize the process, we can go back to the smooth representation of the curve over the real plane. To add a point to itself is equivalent to draw a tangent line to that point, find where the tangent intersects the curve, and then, reflect that point over the x-axis. The reflection point is the result of the multiplication. By performing the operation once, we will be adding P1 to itself, meaning, the result is equivalent to 2*P.

Back to our field of prime order 11, we can multiply (6,5) * 2, and the result is going to be (3, 1)

## Public Key

We have a number chosen randomly in the range [0, 2^256], and we have defined such number our private key. We have also defined the multiplication operation over elliptic curves. And the standard defines a point in the elliptic curve called a Generator Point. We can then compute

``````K = k * G
``````

Where k is the the private key, G is the generator point, and K is defined as the Public Key.

Because G is provided by the standard, given k, will always result in the same value for K, and the computation is pretty straightforward. But to find k given K and G is unfeasible (basically, you have to try every possible value, which on a field so vast as the one defined by the standard is unfeasible). Thus, we have defined a function with a wonderful property, is fairly easy to calculate in one direction, and almost impossible in the other one. A trap door.

1. Unpredictability is the property found in systems that prevent attackers to guess its output, even knowing previous results.

2. p = 2^256-2^32-2^9-2^7-2^6-2^4-1 = 1.15792089×10^77