隱私與監管之間：央行數位貨幣的貨幣搜尋分析

Abstract

本文透過建立貨幣搜尋模型，分析央行數位貨幣（CBDC）在隱私保障與監管之間的政策權衡，特別聚焦於不同隱私程度之 CBDC 並存的情境。我們以交易過程是否可能受到政府審查，刻畫不同隱私程度的資產：離線 CBDC 提供高隱私保障，不受審查但設有支付限額；線上 CBDC 則具低隱私性，雖無交易限額，惟交易資訊可能遭政府即時審查，且其審查機率愈高代表隱私程度愈低。此外，離線 CBDC 不具利率報酬功能，線上 CBDC 則設有利率給付機制。在模型中，人們可能進行合法交易，或進行伴隨負外部性的非法交易。一旦非法交易遭政府查獲，則交易立即終止並被沒收相關產出。人們需根據各情境的預期利得，內生決定是否在非法交易中使用線上 CBDC。本文發現，在離線 CBDC 設有支付限額的情境下，人們傾向避免持有超出該限額的餘額。另一方面，降低線上 CBDC 的隱私程度，雖可直接抑制非法交易量並減少外部性，卻也因交易成交機率下降而降低買家的預期利得與持幣誘因，進而減損實質購買力並對合法交易造成負面影響。當隱私程度過低時，上述負面效果可能超過減少外部性的正面效益，致使社會福利呈現下降趨勢。就線上 CBDC 利率而言，若人們進行非法交易時完全不使用線上 CBDC，提高利率可引導買家調整資產組合配置，於促進合法交易量的同時，減少離線 CBDC 的持有並抑制非法交易量，且因合法交易不受政府審查影響，利率對社會福利的貢獻具直接性。然而，若人們進行非法交易時有使用線上 CBDC 的可能，提高利率亦同時促進非法交易量，導致因審查而遭政府沒收的非法產出隨之增加，使其對社會福利的貢獻不如前述情境明確。

This paper develops a monetary search model to analyze the policy trade-off between privacy protection and regulatory compliance in Central Bank Digital Currency (CBDC) when high-privacy offline CBDC and lower-privacy online CBDC coexist. Privacy is modeled as exposure to government scrutiny: offline CBDC is not monitored, carries a payment limit, and pays no interest; online CBDC is subject to real-time monitoring, where a higher probability of scrutiny implies lower privacy, has no payment limit, and pays interest. In the model, agents may undertake legal transactions or illegal ones that generate negative externalities; detected illegal transactions are terminated and their output is confiscated. Agents choose whether to use online CBDC endogenously based on expected payoffs from illegal transactions. We find that when the offline CBDC has a payment limit, agents avoid holding balances above the cap. Reducing online CBDC privacy curbs illegal transactions and reduces externalities, but lowers the probability of successful transactions, depresses buyers’ expected payoffs and money demand, weakens real purchasing power, and harms legal transactions. If privacy is reduced excessively, losses outweigh benefits, lowering welfare. Regarding the online CBDC interest rate, if agents do not use the online CBDC in illegal transactions, a higher interest rate triggers portfolio rebalancing that expands legal transactions, reduces offline CBDC holdings, and suppresses illegal transactions—thus its welfare effect is direct, as legal transactions are not subject to scrutiny. If agents may use the online CBDC in illegal transactions, a higher rate also stimulates illegal transactions and increases confiscated output, making the net welfare effect weaker or ambiguous.