Fix PrivateKey.toBuffer

[fanatid]

Issue: #47

[braydonf]

Thanks for taking a look at this!

This has come up before (bitpay/bitcore#1270), and the solution was to use:

privateKey.bn.toString(16, 32); // base 16, zero padded to 32 bytes

The solution in this PR is also useful:

privateKey.bn.toBuffer({ size: 32 });

There may be implementations that don't make a distinction and would break as a result? So it could be good to keep this as optional for the time being, and put into a next major release watch list.

[fanatid]

Agree, it should be major release, but major should be released ASAP.
Many of this bugs can be avoided if bitcore-lib will use cryptocoinjs/secp256k1-node (related #41)

[afk11]

@fanatid secp256k1-node doesn't work in a browser, so unfortunately an alternative is still required.

@braydonf Padding shouldn't be optional behavior, you should get used to doing it anywhere you serialize to binary/hex, lest something like the following happens:

1. Public key is compressed, and top 8 bits of Px are zero.
2. Key is serialized to a length of 32 bytes instead of 33.
3. Hash this, produce an address.
4. Funds are unrecoverable

[braydonf]

@afk11 FYI, it's not an issue with publicKey.toBuffer()

However there is an issue because of this: https://github.com/bitpay/bitcore-wallet-client/blob/1dba5651ed26e2f20e3fb33d9f66faec0152ce35/lib/common/utils.js#L74

re: @matiu

[jprichardson]

@fanatid secp256k1-node doesn't work in a browser, so unfortunately an alternative is still required.

@afk11 this is incorrect. From the readme:

Works on node version 0.10 or greater and in the Browser via browserify.

[afk11]

@jprichardson woops, I assumed it was just the C bindings.

[braydonf]

@fanatid I've created a 1.0.0 branch for collaboration on changes that need an API change. This issue affects hd key derivation, and will likely need this to be the default, but optionally disabled since there are already keys derived by hashing the non-zero padded value.

[braydonf]

I've brought this into the 1.0.0 branch at: https://github.com/bitpay/bitcore-lib/tree/1.0.0

[braydonf]

How this relates to hd derivation is handled in: #97