librustzcash_sapling_generate_r generates alpha from 64 rather than 80 bytes of randomness

[zcash]

librustzcash_sapling_generate_r generates alpha from 64 rather than 80 bytes of randomness

[daira]

Note that there's no compatibility issue with making this change.

[daira]

Here's where librustzcash_sapling_generate_r is used to generate alpha. (Arguably its name should be changed.)

https://github.com/zcash/zcash/blob/625797a0371cac7044a9eb2872ee3820728c1d7d/src/transaction_builder.cpp#L14-L21

[str4d]

We did get this right when generating the randomness for T here. If the justification is the same, then we should refactor this so both sources use the same function (with associated comment).

[teh-f00l]

@str4d thanks for making these labels. This seems like a great place for me to start contributing if that is alright!

[str4d]

@teh-f00l absolutely! I look forward to seeing your PR 😀 Feel free to comment here or chat to us in Rocket.Chat if you have any questions.