Bitcoin signing UI mislabels protocol outputs as “network fee” / “OP_RETURN amount” for Taproot + OP_RETURN transactions

[jolestar]

Description

I encountered a confusing and potentially misleading UI issue when signing a Bitcoin transaction that includes Taproot (P2TR) outputs and an OP_RETURN output.

In the signing screen of the OneKey device/app, the UI displays:

• an “estimated network fee” with a large fixed BTC amount, or
• after a firmware update, an “OP_RETURN amount” with a non-zero BTC value

However, based on Bitcoin transaction rules and independent verification, this displayed amount does not correspond to the actual miner fee nor to the OP_RETURN output value.

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Expected behavior

• The network fee shown on the signing screen should reflect the actual miner fee
  (i.e. sum(inputs) − sum(outputs)), typically a small amount in sats.

• OP_RETURN outputs should always display value = 0 BTC, as required by Bitcoin consensus.

• If the UI cannot fully represent multiple outputs or protocol-specific Taproot scripts, it should:

  • either clearly label the value as non-fee / protocol-related, or
  • avoid presenting it as a fee or OP_RETURN amount.

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Actual behavior

• The signing UI shows a large BTC amount labeled as:

  • “estimated network fee”, or
  • “OP_RETURN amount”

• This amount does not match the real miner fee, which is much smaller.

• The OP_RETURN output on-chain has 0 BTC value, yet the UI shows a non-zero amount.

• This can reasonably lead users to believe they are paying an abnormally high fee or burning BTC.

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Why this is problematic

• For advanced Bitcoin transactions (Taproot, protocol registration, OP_RETURN metadata),
  mislabeling outputs as fees introduces unnecessary fear and confusion.
• Hardware wallets are trusted specifically for accurate transaction review before signing.
• Even if the transaction itself is correct, the UI presentation violates the
  “don’t confuse, don’t mislead” principle.

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Suggested improvements

• Clearly distinguish between:

  • miner fee
  • protocol-related outputs
  • OP_RETURN metadata (always 0 BTC)

• If multiple outputs cannot be fully displayed, add an explicit warning such as:

  “This transaction contains protocol-specific outputs. Displayed amounts are not miner fees.”

• Avoid showing any non-fee amount under labels like “network fee” or “OP_RETURN value”.

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Notes

• This issue is reproducible with Taproot-based protocol transactions that include OP_RETURN data.
• The underlying Bitcoin transaction is valid and correct; the issue appears to be UI parsing / labeling, not transaction construction.