@owstack/bch-lib/docs/publickey.md

A Javascript library to build bitcoin cash and blockchain-based applications.

# Public Key
Represents a bitcoin cash public key and is needed to be able to receive bitcoin cash , as is usually represented as a bitcoin cash [Address](address.md). See the official [Bitcoin Wiki](https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addresses).

A PublicKey in Bch Lib is an immutable object and can be instantiated from a [Point](crypto.md), string, [PrivateKey](privatekey.md), Buffer or a [BN](crypto.md).

## Instantiate a Public Key
Here is how to instantiate a public key:

```javascript

var privateKey = new PrivateKey();

// from a private key
var publicKey = new PublicKey(privateKey);

// from a der hex encoded string
var publicKey2 = new PublicKey('02a1633cafcc01ebfb6d78e39f687a1f0995c62fc95f51ead10a02ee0be551b5dc');
```

## Validating a Public Key
A public key point should be on the [secp256k1](https://en.bitcoin.it/wiki/Secp256k1) curve, instantiating a new PublicKey will validate this and will throw an error if it's invalid. To check that a public key is valid:

```javascript
if (PublicKey.isValid('02a1633cafcc01ebfb6d78e39f687a1f0995c62fc95f51ead10a02ee0be551b5dc')){
  // valid public key
}
```

## Compressed vs Uncompressed
It's important to note that there are two possible ways to represent a public key. The standard is _compressed_ and includes the X value and parity (as represented above in the documentation). There is also a longer version that is _uncompressed_ which includes both X and Y values. Using this encoding will generate a different bitcoin cash address, so be careful when selecting the encoding. Uncompressed public keys start with 0x04; compressed public keys begin with 0x03 or 0x02 depending on whether they're greater or less than the midpoint of the curve. These prefix bytes are all used in official secp256k1 documentation.

Example:

```javascript
> var bchLib = require('@owstack/bch-lib');

// compressed public key starting with 0x03 (greater than midpoint of curve)
> var compressedPK = bchLib.PublicKey('030589ee559348bd6a7325994f9c8eff12bd'+
    '5d73cc683142bd0dd1a17abc99b0dc');
> compressedPK.compressed;
true
> compressedPK.toAddress().toString();
'1KbUJ4x8epz6QqxkmZbTc4f79JbWWz6g37'
// compressed public key starting with 0x02 (smaller than midpoint of curve)
> var compressedPK2 = new bchLib.PublicKey('02a1633cafcc01ebfb6d78e39f687a1f'+
    '0995c62fc95f51ead10a02ee0be551b5dc');
> compressedPK2.compressed;
true
> compressedPK.toAddress().toString();
'1KbUJ4x8epz6QqxkmZbTc4f79JbWWz6g37'
// uncompressed public key, starting with 0x04. Contains both X and Y encoded
> var uncompressed = bchLib.PublicKey('0479BE667EF9DCBBAC55A06295CE870B07029'+
    'BFCDB2DCE28D959F2815B16F81798483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68'+
    '554199C47D08FFB10D4B8');
> uncompressed.compressed
false
> uncompressed.toAddress().toString()
'1EHNa6Q4Jz2uvNExL497mE43ikXhwF6kZm'
```