genProof throws FailedDecryption trying to decrypt the placeholder message created during poll.init()

[yuetloo]

Prerequisites

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• I am not running a deprecated version
• I checked the documentation and found no answer
• I checked to make sure that this issue has not already been filed

Expected Behavior

Please describe the behavior you are expecting

Decryption failed when trying to decrypt the placeholder message generated during poll.init(). Note that the message is not encrypted (that's why it failed to decrypt).

https://github.com/privacy-scaling-explorations/maci/blob/dev/contracts/contracts/Poll.sol#L142

    // init messageAq here by inserting placeholderLeaf
    uint256[2] memory dat;
    dat[0] = NOTHING_UP_MY_SLEEVE;
    dat[1] = 0;
    (Message memory _message, PubKey memory _padKey, uint256 placeholderLeaf) = padAndHashMessage(dat, 1);
    extContracts.messageAq.enqueue(placeholderLeaf);

    emit PublishMessage(_message, _padKey);

If the error was intentional to get the poll message processor to inject a blank ballot, it should just log a different message and process it properly rather than relying on throwing error to insert blank ballot.

Current Behavior

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Failure Information

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Steps to Reproduce

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1. step 1
2. step 2
3. you get it...

Context

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Failure Logs

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[ctrlc03]

thanks @yuetloo

perhaps we could directly skip processing if message index is 0 and just add the empty ballot? In terms of functionality there would not be any changes. Or, we could keep as is, and with the addition of optional logs for poll.processMessages (#1138) this would be logged with index 0, which we know it's reserved for the blank message. Let me know if I'm missing something?

[yuetloo]

This issue is more to make the log looks cleaner and minimize confusion. I'm good if you just skip processing the message at index 0 and add an empty ballot.

Currently, this is what we see in our log.

[i] Generating proofs of message processing...
Error at message index 0 - failed decryption due to either wrong encryption public key or corrupted ciphertext
[i] Progress: 1 / 1