zcash-sign: ensure signing 2 inputs works

[conradoplg]

During the demo we got this error while trying to sign a transaction with 2 inputs:

zcash-sign sign --tx-plan ~/Downloads/tx-plan.json --ufvk uview1y2wm5u5yu0f0p475x4ux0us368gplnla57v8cal9td623x5npm2537xa6cfh0j9qvnsged62r5q23ehj5kufgdt67qm7zth2f0fhaswh02y4a2ddlha4p7k2q0j78g3suggk79e5actcg0zqxq7gq6klkqdw6wt49rdt4hyxw2qugen6aqncxvmsptgqlm8h0lj3qdlvrvna2cqel2a4j9nxm78ltem5t0h06y7eer59jz43cxmfd0p3xqrw2rpsfmkhhne8472xvwrsnyc2ucuft7c42zk5p5erh3dqmyg4ea2rf5xkmrwdh44qhqsd6runnxccvqjcje9r7tpv3rdmput0wlfchntkdq0ns8ce50ds29rj34fr0gfkmc27wfnpgcqqsfrqu -o ~/Downloads/tx-signed.raw
SIGHASH: ca1a0b0faabddb928daa24b85b54f72924683710227f09d821912dc32f76ec62
Randomizer #0: 80deb7e680c00dc8165419df9c88e7e8d1f90bbe4bc559611b4f7e890137ee04
Input hex-encoded signature #0: 
5d4a58827b4e7e54e1e447339b290f6a0fe9d0a87711b2b84a0385913ff3f20e629d59aa2a4f8ae29e2e9030be0019bb55d5f3772d0959fc10c6a87992077321
Randomizer #1: a7ef6173a96a4128e94afee2507b1fcfcfa74ff074e8e12880722f218478d20d
Input hex-encoded signature #1: 
ae7484849b1f9bfe72ec1025f828402e7d44e3bb414b7f1c9c30e220aaf9461aba472486d35172ebbb1f6f9943a7c691342258f5fcfd9edc069d553c30c1f334
thread 'main' panicked at /home/conrado/.cargo/git/checkouts/frost-zcash-demo-cebbfb4290b095fb/3613c1b/zcash-sign/src/sign.rs:257:14:
called `Result::unwrap()` on an `Err` value: InvalidExternalSignature
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

However it was caused by me copying the wrong value to be signed (I copied the signature just above the randomizer, instead of the sighahs):

frost-client coordinator -s frostd.zfnd.org --group af66f02a7fefeb7ae041ede0fba85656c7d2ee3e738c0b316dc69a05909d3f33 -S 97e89b49517f97972c827c76ca4872eb6341ee5d3b99b0cae9d4b748c12bc67d,e76d2f673eb91aca53d6054c0f5271c25790cf6c15805ff3324ee2b83ab96c6d -m - -r -
The message to be signed (hex encoded)
5d4a58827b4e7e54e1e447339b290f6a0fe9d0a87711b2b84a0385913ff3f20e629d59aa2a4f8ae29e2e9030be0019bb55d5f3772d0959fc10c6a87992077321
Enter the randomizer (hex string):
a7ef6173a96a4128e94afee2507b1fcfcfa74ff074e8e12880722f218478d20d
Logging in...
Creating signing session...
Waiting for participants to send their commitments........
Sending SigningPackage to participants...
Waiting for participants to send their SignatureShares...
...
Signature:
ae7484849b1f9bfe72ec1025f828402e7d44e3bb414b7f1c9c30e220aaf9461aba472486d35172ebbb1f6f9943a7c691342258f5fcfd9edc069d553c30c1f334

Nevertheless we haven't really tested this case thoroughly. Check if it really works.