1. Introduction 1.1 The Relay Attack Problem in Digital Payment Systems In cryptographic payment systems, a relay attack occurs when an adversary intercepts and forwards valid authentication messages between a legitimate user device and a verifier, creating a transaction in a different physical, temporal, or jurisdictional context than the user intended. Critically, relay attacks do not require cryptographic breaks, key compromise, or secure element vulnerabilities. The attack succeeds by exploiting a fundamental architectural assumption: that possession of valid credentials implies legitimate execution context. 1.2 Digital Euro: Design Requirements and Security Challenges The European Central Bank's Digital Euro program, initiated in 2020 and entering its preparation phase in 2021, aims to deliver an operational CBDC by 2029. The Digital Euro faces a uniquely complex security challenge due to the intersection of three mandated design requirements: Requirement 1: Offline Transaction CapabilityAs established in the December 2024 EU Council agreement, the Digital Euro must function "anytime, anywhere, whether users are connected to the internet or offline." This cash-equivalence requirement eliminates continuous real-time validation and introduces deferred settlement windows during which relay attacks can be executed without immediate detection. Requirement 2: GDPR-Compliant PrivacyUnlike CBDCs in other jurisdictions, the Digital Euro operates under the General Data Protection Regulation, which constrains the metadata collection permissible for fraud detection. Article 5 (data minimization) and Article 25 (privacy by design) restrict the use of device fingerprinting, location tracking, and transaction graph analysis—techniques commonly employed in conventional relay attack defenses. Requirement 3: Cross-Border InteroperabilityThe Digital Euro must function seamlessly across 27 EU member states, each with distinct legal frameworks for payment regulation, consumer protection, and law enforcement. Relay attacks exploiting jurisdictional boundaries present elevated risks when attackers can relay transactions from high-regulatory environments to lower-oversight jurisdictions or vice versa. 1.3 Current State of Digital Euro Relay Attack Mitigation As of January 2025, publicly available Digital Euro technical documentation—including the ECB's progress reports (2022, 2023), the rulebook framework consultations (2023-2024), and various ECB working papers—has not published a comprehensive solution to relay attacks that simultaneously satisfies all three requirements above. The existing literature discusses: Proximity verification mechanisms for contactless payments, which function in online point-of-sale scenarios but remain unspecified for offline modes Device authentication protocols that verify credential validity but do not bind authorization to execution context AML/CFT compliance frameworks addressing illicit finance but not addressing relay attack vectors specifically This gap does not represent an oversight but rather reflects the genuine technical difficulty of the problem. Relay attack prevention in offline, privacy-preserving, multi-jurisdictional payment systems constitutes an open research challenge that existing cryptographic protocols have not fully resolved. The ECB's deliberate, methodical approach to this challenge is appropriate given the systemic risks associated with premature CBDC deployment. 1.4 Research Contribution and Scope This paper makes three contributions to Digital Euro security architecture: First, we formalize the relay attack threat model specific to Digital Euro requirements, distinguishing it from relay attacks in traditional card payments or single-jurisdiction CBDCs. Second, we analyze why conventional relay attack countermeasures—distance-bounding, RF constraints, timing analysis, and heuristic detection—remain incompatible with the combined requirements of offline functionality, GDPR compliance, and cross-border operation. Third, we present the Virtual Identity + Compliance Jurisdiction Token (VI+CJT) framework as a candidate architectural approach that addresses relay attacks through context-binding cryptographic primitives rather than detection heuristics. We demonstrate formal security properties and evaluate compatibility with Digital Euro specifications. This analysis is offered in the spirit of constructive technical dialogue with the ECB, European Commission DG FISMA, and the broader CBDC research community. The objective is to contribute to the collective effort of solving one of the Digital Euro's most challenging security problems, not to critique existing work but to build upon it. 2. Digital Euro Architectural Context 2.1 Technical Requirements from EU Council and ECB Specifications The Digital Euro architecture must satisfy requirements established through multiple governance documents: EU Council Conclusions (December 2024): "Usable anytime, anywhere, whether users are connected to the internet or offline" Pan-European acceptance at no cost to consumers Privacy protections equivalent to physical cash for peer-to-peer transactions Full accessibility for persons with disabilities ECB Rulebook Framework (2023-2024 consultations): Interoperability with existing payment infrastructure Settlement finality guarantees Compatibility with AML/CFT regulations (6th Anti-Money Laundering Directive) Support for programmable payment features GDPR Compliance (Regulation 2016/679): Data minimization in transaction processing (Article 5.1.c) Privacy by design and by default (Article 25) Purpose limitation for collected data (Article 5.1.b) Right to erasure for non-compliance-related transaction data (Article 17) 2.2 Current Offline Transaction Model Based on ECB technical presentations and pilot program disclosures, the Digital Euro offline mode is envisioned to operate approximately as follows: Initialization Phase: User device provisions offline payment capabilities from supervised intermediary (bank). Device receives cryptographic credentials and offline balance allocation. Transaction Phase: When network connectivity is unavailable, user device and merchant terminal execute payment protocol using local cryptographic verification. Transaction is recorded locally but not settled. Settlement Phase: When connectivity resumes, both user device and merchant terminal synchronize with settlement infrastructure. Transactions are validated and balances updated. This model introduces a temporal gap between authorization (Phase 2) and settlement finality (Phase 3) that relay attackers can exploit. 2.3 Open Questions in Current Specifications Several aspects of relay attack prevention in this model remain publicly unspecified: Question 1: How does the system verify that an offline authorization occurred in the jurisdictional context claimed by the merchant terminal during settlement? Question 2: What cryptographic mechanisms prevent a valid offline authorization from being relayed to a different merchant or jurisdiction during the transaction phase? Question 3: How can the system distinguish between a legitimate cross-border offline transaction and a relay attack exploiting jurisdictional boundaries, without violating GDPR's location tracking restrictions? Question 4: What prevents a compromised merchant terminal in one member state from replaying or relaying offline authorizations collected from users in different member states? These are not simple questions, and their absence from current public documentation does not imply inadequate security analysis by the ECB. Rather, they represent frontier research problems in privacy-preserving payment cryptography that the broader academic and technical community has not yet fully solved. The ECB's preparation phase timeline extending to 2025-2026 provides opportunity for addressing these challenges before operational deployment. 2.4 Relay Attack Implications for Digital Euro Sovereignty Beyond technical security, relay attacks carry specific implications for the Digital Euro's strategic objective of payment sovereignty. As articulated by Economic Commissioner Valdis Dombrovskis (January 28, 2025): "Ceding such a degree of technological control over the EU's economy to others could impede our ability to act autonomously. It poses real threats to our resilience and economic security." Relay attacks directly threaten this sovereignty goal through three mechanisms: Jurisdictional Arbitrage: Attackers relay transactions from high-regulation member states to lower-oversight jurisdictions, circumventing EU regulatory frameworks. Foreign Infrastructure Dependency: If relay attack prevention requires reliance on external payment rails or verification services (e.g., Visa/Mastercard risk scoring), the Digital Euro fails to achieve independence. Compliance Circumvention: Relay attacks can be weaponized to evade sanctions, AML controls, or capital flow restrictions by misrepresenting transaction geography. A relay-resistant architecture is therefore not merely a security feature but a prerequisite for digital monetary sovereignty. 3. Relay Attack Mechanics in Digital Euro Context 3.1 Threat Model Specific to Digital Euro Consider the following relay attack scenario tailored to Digital Euro offline functionality: Setup: Legitimate user U in Munich, Germany possesses Digital Euro credentials Legitimate merchant M₁ in Munich requests offline payment Attacker A controls merchant terminal M₂ in Zurich, Switzerland (non-EU) Germany enforces strict AML reporting thresholds; Switzerland has different regime Attack Execution: Phase 1 - Authorization Interception: User U enters store in Munich. M₁ initiates payment request for €500. Attacker A, positioned near M₁ with relay equipment, intercepts request. Phase 2 - Cross-Border Relay: A forwards payment request to controlled terminal M₂ in Zurich. M₂ presents request to U's device via standard contactless interface (NFC, QR code, etc.). Phase 3 - User Authorization: U's device, operating in offline mode, validates M₂'s request cryptographically. Finding valid merchant credentials and sufficient balance, device generates authorization signature. Phase 4 - Settlement Relay: A relays authorization back to M₂ in Zurich. M₂ commits transaction to offline queue. Phase 5 - Jurisdictional Exploitation: When M₂ reconnects to settlement infrastructure, transaction is processed under Swiss regulatory framework, not German. AML reporting thresholds differ, enabling potential compliance circumvention. Attack Success Conditions: U authorized payment in Germany, expecting German consumer protections and AML regime Actual settlement occurred under Swiss jurisdiction without U's knowledge or consent No cryptographic protocols were violated; all signatures and credentials were valid 3.2 Why This Attack Succeeds Against Conventional Defenses Distance-Bounding Failure: Distance-bounding protocols measure round-trip time between challenge and response to verify proximity. In this scenario: U's device is physically proximate to relay equipment (A's receiver near M₁) Relay to Zurich introduces latency, but offline mode has no real-time validation By the time settlement occurs (hours or days later), timing evidence is irrelevant NFC Range Constraints Failure: While NFC typically operates at <10cm range: Attackers use high-gain antennas to extend effective range to several meters Multiple relay hops (M₁ → A₁ → A₂ → M₂) can span arbitrary distances QR code payment modes bypass NFC constraints entirely Heuristic Detection Failure: Machine learning anomaly detection fails because: Offline mode provides no real-time telemetry for analysis GDPR restricts collection of device fingerprints or location data needed for behavioral models Cross-border transactions are legitimate use cases, indistinguishable from attacks without additional context 3.3 Multi-Jurisdictional Relay Attack Variants The Digital Euro's 27-member-state scope introduces additional relay attack vectors: Variant 1: Regulatory Arbitrage RelayAttacker relays transactions from strict-regulation member states (e.g., Germany, Netherlands) to lenient-regulation states (e.g., jurisdictions with higher AML thresholds or weaker consumer protection enforcement) to avoid reporting requirements. Variant 2: Sanctioned Entity RelayAttacker in sanctioned jurisdiction uses relay to execute payments through merchant terminals in compliant member states, circumventing sanctions enforcement. Variant 3: VAT Fraud RelayCross-border relay used to misrepresent transaction geography for VAT purposes. Merchant claims transaction occurred in low-VAT member state when actual authorization occurred in high-VAT state. Variant 4: Double-Spending via RelayIn offline mode, attacker relays same authorization to multiple merchant terminals in different jurisdictions before settlement synchronization occurs, exploiting delayed double-spend detection. Each variant exploits the gap between cryptographic validity (credentials, signatures) and contextual validity (jurisdiction, purpose, timing). 3.4 Privacy-Security Tension in Relay Prevention The most straightforward relay attack countermeasures conflict with GDPR requirements: Location Tracking: Continuous GPS monitoring could verify transaction geography but violates: Article 5.1.c (data minimization): Location history unnecessary for payment processing Article 9 (special category data): Location data can reveal sensitive information (healthcare facilities, religious sites, political activities) Device Fingerprinting: Collecting hardware identifiers, IP addresses, and behavioral telemetry enables relay detection but conflicts with: ePrivacy Directive (2002/58/EC): Prohibits terminal equipment tracking without consent GDPR Article 25: Privacy by design prohibits excessive data collection Transaction Graph Analysis: Monitoring payment patterns to detect unusual cross-border activity violates: Article 5.1.b (purpose limitation): Analysis exceeds purpose of payment processing Article 22: Creates automated decision-making affecting individuals This tension explains why existing relay attack solutions from card payments (which freely employ these techniques) cannot be directly adopted for Digital Euro. 4. Analysis of Conventional Relay Attack Countermeasures in Digital Euro Context 4.1 Distance-Bounding Protocols Technical Approach: Distance-bounding measures round-trip time for cryptographic challenge-response to verify that the responding party is within a specified physical distance. Application to Digital Euro: Advantages: Well-studied cryptographic protocols with formal security proofs Can be implemented in contactless payment terminals Compatible with existing NFC infrastructure Fundamental Limitations for Digital Euro: Offline Incompatibility: Distance-bounding requires interactive challenge-response. In offline mode, no infrastructure exists to issue challenges or verify responses in real-time. Cross-Border Ambiguity: Even if distance-bounding confirms proximity, it cannot determine which jurisdiction the transaction occurred in. A user near German-French border could legitimately be in either country. Network Heterogeneity: Digital Euro must function across 4G, 5G, WiFi, and potentially satellite networks. Distance-bounding assumes bounded communication channels; internet routing invalidates timing assumptions. Early-Detect-Late-Commit Attacks: Attacker can satisfy distance-bounding check during authorization but relay the resulting authorization token for later use. Assessment: Distance-bounding provides insufficient protection for Digital Euro offline cross-border scenarios. 4.2 Radio Frequency and Contactless Constraints Technical Approach: Rely on short-range properties of NFC (typically <10cm) to physically enforce proximity. Application to Digital Euro: Advantages: No additional cryptographic overhead Aligns with existing contactless payment user experience Simple to implement Fundamental Limitations for Digital Euro: Relay Equipment: Commercial NFC relay devices extend effective range to 50+ meters through antenna amplification. Multi-Modal Payments: Digital Euro specifications require support for QR codes, Bluetooth, and other payment initiation methods that have no inherent range constraints. Offline Peer-to-Peer: Person-to-person offline payments may legitimately occur at greater distances (e.g., via visual QR code scanning). Physical Security Insufficiency: RF constraints provide physical security, not cryptographic security. They can be defeated through engineering rather than cryptanalysis. Assessment: RF constraints alone cannot provide the security assurances required for a sovereign digital currency operating across diverse payment modalities. 4.3 Timing Analysis and Latency Detection Technical Approach: Monitor transaction timing characteristics to detect anomalous latencies introduced by relay equipment. Application to Digital Euro: Advantages: Passive detection requiring no protocol changes Can potentially identify sophisticated relay attacks Fundamental Limitations for Digital Euro: Network Variability: Legitimate transactions over mobile networks exhibit high latency variance (50-500ms). Relay-induced delays may fall within normal range. Offline Mode: Timing analysis requires online connectivity for validation infrastructure. Adaptive Attacks: Sophisticated attackers can measure and compensate for relay latency to remain within expected bounds. False Positive Risk: Network congestion, device performance variation, and geographic distance create timing anomalies indistinguishable from attacks. Assessment: Timing analysis offers probabilistic detection in online scenarios but cannot provide deterministic relay prevention in offline contexts. 4.4 Device Attestation and Secure Elements Technical Approach: Use hardware security modules (HSMs) or trusted execution environments (TEEs) to attest device integrity and prevent credential extraction. Application to Digital Euro: Advantages: Strong protection against key compromise Can verify device has not been tampered with Aligns with existing mobile payment infrastructure (Apple Secure Enclave, Android StrongBox) Fundamental Limitations for Digital Euro: Attestation ≠ Context Verification: Device attestation confirms the device is legitimate but not that the transaction is occurring in the claimed context. Relay Transparency: Relay attacks do not compromise the device. The secure element correctly signs the transaction; the problem is the transaction is being executed in the wrong context. Jurisdictional Blindness: Secure elements have no inherent mechanism to determine or attest to jurisdictional context. Assessment: Device attestation is necessary for credential protection but insufficient for relay attack prevention. It secures the authorization primitive but not the execution context. 4.5 Behavioral Analytics and Fraud Detection Technical Approach: Apply machine learning to transaction patterns to identify anomalous behaviors indicative of fraud. Application to Digital Euro: Advantages: Successfully employed in credit card fraud detection Can identify sophisticated attack patterns Adapts to evolving threats Fundamental Limitations for Digital Euro: GDPR Incompatibility: Behavioral profiling requires extensive data collection that violates data minimization principles. Article 22 restricts automated decision-making affecting individuals. Offline Inapplicability: Machine learning requires real-time data feeds. Offline transactions provide no telemetry until settlement. False Positive Harm: In a CBDC context, false positives block legitimate payments without recourse (no alternative payment method). Card payments can fall back to alternative cards or cash; Digital Euro may be sole legal tender in some scenarios. Reactive Not Preventive: Behavioral analytics detect attacks after they occur. CBDC settlement finality requirements may prevent reversal of fraudulent transactions. Legitimate Cross-Border Patterns: EU citizens routinely travel and transact across borders. Cross-border activity cannot be presumed fraudulent without excessive false positives. Assessment: Behavioral analytics may complement other defenses in online scenarios but cannot serve as primary relay attack prevention mechanism for privacy-preserving offline CBDC. 4.6 Summary: The Unsolved Problem The analysis above demonstrates that conventional relay attack countermeasures, individually or in combination, do not satisfy Digital Euro's combined requirements: Countermeasure Offline Support GDPR Compliance Cross-Border Capable Deterministic Security Distance-Bounding ❌ No ✓ Yes Partial ✓ Yes RF Constraints ✓ Yes ✓ Yes ✓ Yes ❌ No Timing Analysis ❌ No ✓ Yes ❌ No ❌ No Device Attestation ✓ Yes ✓ Yes ✓ Yes Partial Behavioral Analytics ❌ No ❌ No Partial ❌ No This is not a critique of these techniques, which function effectively in their designed contexts (card payments, online banking, single-jurisdiction systems). Rather, it reflects the genuine technical novelty of Digital Euro requirements. The ECB faces a security challenge that existing payment cryptography has not fully solved. The question then becomes: Is there an architectural approach that can provide relay attack prevention while satisfying all four requirements simultaneously? 5. The VI+CJT Architecture: A Proposed Solution Framework 5.1 Design Philosophy The Virtual Identity + Compliance Jurisdiction Token (VI+CJT) framework approaches relay attack prevention through a fundamental reframing of the authorization model. Rather than attempting to detect relay attacks through probabilistic means, the architecture eliminates the possibility of successful relay through cryptographic construction. Core Principle: Make authorization validity inseparable from execution context. Traditional payment authorization asks: "Is this credential valid?" VI+CJT authorization asks: "Is this exact execution, in this jurisdiction, for this purpose, at this moment, cryptographically permitted?" This reframing aligns with Digital Euro sovereignty requirements: payment authorization becomes a cryptographic policy enforcement mechanism rather than merely an authentication check. 5.2 Virtual Identity (VI): Eliminating Reusability 5.2.1 Conceptual Foundation A Virtual Identity in the VI+CJT framework is not a persistent identifier but an ephemeral cryptographic capability bound to a specific transaction context. Key Properties: Ephemeral: VI validity is constrained to a single transaction or session Context-Derived: VI generation cryptographically incorporates execution parameters Non-Transferable: VI cannot be meaningfully reused outside its derivation context Pseudonymous: VI reveals no biographical information about the user 5.2.2 Relay Attack Mitigation Mechanism Relay attacks fundamentally depend on credential reusability: an attacker relays valid authentication material from context A (legitimate) to context B (malicious). The VI design breaks this by ensuring: If execution context A ≠ execution context B, then VI_A ≠ VI_B More formally: VI = DeriveVirtualIdentity( MasterKey, TransactionParameters, // amount, merchant, transaction type TemporalScope, // timestamp, nonce, validity window JurisdictionalContext, // encoded via CJT (see Section 5.3) DeviceAttestation // secure element state ) When a relay attacker intercepts and forwards messages: User device generates VI_local for local transaction context (Munich, Germany, Merchant M₁) Attacker relays VI_local to remote context (Zurich, Switzerland, Merchant M₂) Remote verifier evaluates VI_local against remote execution context Validation fails because VI was not derived for remote context parameters The VI is not a bearer token but a context-specific proof. Transporting the proof to a different context invalidates it cryptographically, not merely heuristically. 5.2.3 GDPR Compatibility Virtual Identities preserve GDPR compliance through several properties: Data Minimization (Article 5.1.c): VIs contain no personal information. They are pseudonymous identifiers that cannot be reverse-engineered to reveal user identity. Purpose Limitation (Article 5.1.b): VI generation purpose is exclusively payment authorization. The VI cannot be repurposed for tracking, profiling, or secondary data processing. Unlinkability: VIs from different transactions cannot be correlated without the user's master key. This prevents transaction graph reconstruction. Right to Erasure (Article 17): VI deletion leaves no personal data. Only the cryptographic proof-of-payment remains, which is non-identifying. 5.2.4 Implementation in Digital Euro Offline Context For offline Digital Euro payments: Initialization: User device provisions master key material during supervised intermediary onboarding (bank account linkage). Offline VI Generation: When executing offline payment, device generates VI locally using: Merchant terminal identifier (from terminal's cryptographic certificate) Transaction amount and parameters Current timestamp and transaction nonce CJT provided by merchant (see Section 5.3) Local Validation: Merchant terminal validates VI against its own context parameters. If mismatch detected, transaction rejected immediately. Deferred Settlement: Both user device and merchant queue transaction with VI for later settlement. Settlement infrastructure performs full context verification. This preserves offline functionality while establishing cryptographic binding between authorization and context. 5.3 Compliance Jurisdiction Token (CJT): Binding Legal Context 5.3.1 Conceptual Foundation While Virtual Identities eliminate technical reusability, Compliance Jurisdiction Tokens address the legal and jurisdictional dimension of relay attacks. A CJT is a cryptographic token issued by a supervised intermediary (bank) that encodes: Jurisdictional Scope: Geographic or regulatory jurisdiction where payment is authorized (domestic, specific cross-border corridor, prohibited regions) Transaction Purpose: Category of authorized transaction (retail payment, government disbursement, refund, peer-to-peer, etc.) Temporal Validity: Time-bounded authorization window Usage Constraints: Single-use enforcement, amount limits, merchant category restrictions Revocation State: Capability for real-time or deferred revocation Regulatory Compliance Encoding: AML thresholds, sanctions screening requirements, consumer protection rules 5.3.2 Relay Attack Mitigation Mechanism Relay attacks across jurisdictional boundaries exploit the gap between where authorization appears to occur and where settlement actually occurs. CJTs close this gap by cryptographically encoding jurisdictional permission into the authorization itself. Enforcement Logic: Transaction authorized in jurisdiction J IF AND ONLY IF CJT.authorizedJurisdiction = J Consider the Munich-Zurich relay attack scenario (Section 3.1): User's CJT issued by German bank encodes authorizedJurisdiction = EU_GERMANY Merchant M₂ terminal in Zurich evaluates CJT Terminal determines localJurisdiction = NON_EU_SWITZERLAND Validation check: EU_GERMANY ≠ NON_EU_SWITZERLAND Transaction rejected before settlement commitment The CJT transforms jurisdictional policy from a settlement-time check (reactive) into an authorization-time constraint (preventive). 5.3.3 Cross-Border Transaction Handling For legitimate cross-border Digital Euro transactions: Scenario: German citizen traveling in France wishes to make payment. Implementation: User's German bank issues CJT encoding: authorizedJurisdictions = [EU_GERMANY, EU_FRANCE, EU_SCHENGEN] transactionPurpose = RETAIL_PAYMENT maxAmount = 1000 EUR validity = 7 DAYS French merchant terminal validates CJT and determines EU_FRANCE ∈ authorizedJurisdictions Transaction proceeds normally This enables legitimate cross-border payments while blocking unauthorized jurisdictional relay. For transactions to non-EU jurisdictions (e.g., Switzerland), user must explicitly obtain CJT with extended jurisdictional scope, subject to enhanced AML/CFT checks and reporting requirements. 5.3.4 GDPR and Regulatory Alignment CJTs encode policy not identity: GDPR Compliance: No personal data in CJT itself (pseudonymous token) Jurisdiction field indicates legal framework, not precise location Purpose field categorizes transaction type, not transaction content Regulatory Integration: AML/CFT thresholds encoded in CJT (6AMLD compliance) Sanctions screening rules embedded (EU restrictive measures) Consumer protection rules (Payment Services Directive 2) enforced at protocol level Auditability: CJT issuance creates audit trail at supervised intermediary Settlement validates CJT usage against issuance records Regulatory reporting automated through CJT metadata 5.4 VI-CJT Binding: The Critical Security Property 5.4.1 Inseparable Binding Requirement The security property that definitively closes relay attack vectors is the cryptographic binding between VI and CJT: Valid Transaction ⟺ (VI_valid ∧ CJT_valid ∧ Bound(VI, CJT)) This binding ensures: A valid VI with incorrect CJT → execution fails A valid CJT with substituted VI → execution fails A VI-CJT pair used outside encoded context → execution fails 5.4.2 Cryptographic Binding Mechanisms Several cryptographic constructions can implement VI-CJT binding: Option 1: Commitment Scheme VI = Commit(MasterKey, TransactionContext, CJT) The VI cryptographically commits to the CJT. Any modification to CJT invalidates VI. Option 2: Digital Signature VI = Sign(MasterKey, TransactionContext || Hash(CJT)) The VI signature includes CJT hash as signed material. CJT substitution breaks signature verification. Option 3: Zero-Knowledge Proof VI = ZKProof( MasterKey, TransactionContext, CJT, Proof: "I possess valid credentials for this CJT in this context" ) The VI proves possession of valid credentials for the specific CJT without revealing the credentials themselves. 5.4.3 Relay Attack Impossibility Under Binding With VI-CJT binding enforced, consider the relay attack scenarios: Attack 1: Relay VI and CJT together Attacker relays pair (VI₁, CJT₁) from context C₁ to context C₂ CJT₁ encodes authorizedJurisdiction = C₁ Validation in C₂ checks CJT₁.jurisdiction = C₂ Check fails: C₁ ≠ C₂ Transaction rejected Attack 2: Modify CJT to match new context Attacker attempts to forge CJT₂ with authorizedJurisdiction = C₂ VI₁ remains bound to original CJT₁ Binding verification: Bound(VI₁, CJT₂) = false Transaction rejected Attack 3: Generate new VI for relayed context Attacker attempts to create VI₂ for context C₂ Requires user's master key Contradicts threat model (no key compromise) Attack 4: Relay only VI, omit CJT Transaction validation requires both VI and CJT Missing CJT causes immediate rejection Under binding enforcement, no successful relay attack exists without compromising the user's master key material. 5.5 Execution-Time Enforcement: Eliminating the Authorization Gap 5.5.1 The Authorization-Settlement Gap Problem Traditional payment architectures separate authorization from settlement: Phase 1 - Authorization: User signs transaction requestPhase 2 - Transmission: Signed request transmitted to settlement systemPhase 3 - Settlement: Value transfer occurs (potentially hours/days later) Relay attacks exploit Phase 2 by injecting malicious context between authorization and settlement. 5.5.2 VI+CJT Execution-Time Model The VI+CJT architecture collapses this gap by deferring cryptographic validation to settlement finality: Authorization validity is evaluated at the moment value becomes final, not at signature generation. Implementation: User generates (VI, CJT) pair for intended transaction Pair transmitted through payment system (potentially relayed by attacker) At settlement time, enforcement gate evaluates: Is VI valid in current execution context? Is CJT consistent with settlement jurisdiction? Is VI-CJT binding intact? Are all temporal, usage, and policy constraints satisfied? Settlement capability released only if all checks pass 5.5.3 Offline Transaction Handling For Digital Euro offline payments, execution-time enforcement operates as follows: Offline Transaction Phase: User device generates (VI, CJT) pair locally Merchant terminal performs preliminary validation: CJT jurisdiction matches terminal's registered jurisdiction VI-CJT binding cryptographically valid Temporal validity within bounds If preliminary checks pass, transaction queued locally Both parties store transaction record with (VI, CJT) pair Settlement Phase (when connectivity resumes): Merchant uploads queued transactions to settlement infrastructure User device synchronizes state Settlement system performs full context validation: Verifies VI was derived for correct merchant and jurisdiction Confirms CJT authorizations were valid at transaction time Checks for double-spending across synchronized device state Validates compliance with AML/CFT rules If any check fails, transaction rejected; offline balances restored Critical Property: Even if relay attack succeeds in offline phase (preliminary checks bypassed), the attack fails definitively at settlement when full validation occurs. 5.5.4 Why Timing Attacks Become Irrelevant Under execution-time enforcement: There exists no "pre-authorized" state that can be cached for later misuse No authority exists outside the execution boundary Relay-induced latency provides no advantage because validation occurs at settlement, not during message relay Distance-bounding and timing analysis become unnecessary because the architecture does not rely on proximity inference. Authorization is cryptographically bound to context, not probabilistically inferred from timing. 6. Formal Security Analysis for Digital Euro Context 6.1 Threat Model Formalization We model a relay attacker A operating within the Digital Euro system with the following capabilities: Adversary Capabilities: Message Interception: A can intercept all communications between user U and merchant M Perfect Relay: A can forward messages without detectable modification or excessive delay Context Switching: A can relay messages between different physical locations and jurisdictions Terminal Control: A may control legitimate merchant terminals in various jurisdictions Network Access: A can operate online or offline relay equipment Adversary Constraints: No Cryptographic Breaks: A cannot forge signatures, break encryption, or compromise secure elements No Key Extraction: A cannot extract user's master key or supervised intermediary's signing keys No CJT Forgery: A cannot generate valid CJTs without access to supervised intermediary infrastructure Attack Success Condition: A succeeds if they cause settlement of a transaction in execution context C₂ when user U authorized only for context C₁, where contexts differ in jurisdiction, merchant identity, or other relevant parameters. 6.2 VI+CJT Security Theorem Theorem: Under the VI+CJT architecture with cryptographically secure VI generation and VI-CJT binding, relay attacks as defined above are infeasible assuming: Collision resistance of hash functions Unforgeability of digital signatures Pseudorandom function security of key derivation Binding property of commitment schemes (if used) Proof Sketch: For relay attack to succeed, attacker must present valid (VI, CJT) pair in context C₂ when user authorized only C₁. Case 1: Direct Relay of User's Authorization Attacker relays user's (VI₁, CJT₁) pair to context C₂. VI₁ derived for context C₁: VI₁ = DeriveVI(K, C₁, CJT₁, N₁) Validation in C₂ requires: VI₂ = DeriveVI(K, C₂, CJT₁, N₁) By collision resistance of DeriveVI: C₁ ≠ C₂ ⟹ VI₁ ≠ VI₂ Validation fails in C₂ Case 2: CJT Modification Attacker modifies CJT₁ to CJT₂ matching context C₂ while reusing VI₁. VI-CJT binding check: Bound(VI₁, CJT₂) VI₁ was generated with commitment to CJT₁ By binding property: CJT₁ ≠ CJT₂ ⟹ Bound(VI₁, CJT₂) = false Transaction rejected Case 3: VI Regeneration Attacker generates new VI₂ for context C₂ using user's master key. Requires knowledge of user's master key K Contradicts "no key extraction" constraint Attack infeasible Case 4: CJT Forgery Attacker generates fraudulent CJT₂ for context C₂. CJTs signed by supervised intermediary with key K_bank By signature unforgeability: attacker cannot generate valid signature without K_bank Contradicts "no CJT forgery" constraint Attack infeasible Case 5: Execution Context Manipulation Attacker manipulates enforcement gate to accept incorrect context. Enforcement gate validates VI against received context parameters Context parameters obtained from trusted infrastructure (jurisdiction databases, merchant registries) Attacker cannot manipulate these without broader system compromise beyond relay attack scope Conclusion: No successful relay attack exists under stated adversary model and cryptographic assumptions. ∎ 6.3 Comparison to Existing Digital Euro Security Guarantees Current Digital Euro security specifications (based on publicly available documentation) provide: Authentication: User credentials verified through cryptographic protocols Confidentiality: Transaction details protected in transit Integrity: Transactions cannot be modified without detection Non-repudiation: Users cannot deny authorized transactions What is not explicitly guaranteed in public specifications: Context Binding: Authorization cannot be separated from execution context Jurisdictional Enforcement: Transactions cannot be executed in unintended jurisdictions Relay Resistance: Authorization cannot be successfully relayed to different contexts The VI+CJT framework adds these properties while preserving existing security guarantees. It is compatible with and complementary to ECB's current security architecture. 7. Integration with Digital Euro Technical Specifications 7.1 Compatibility with ECB Rulebook Framework The VI+CJT architecture aligns with Digital Euro rulebook requirements: Settlement Finality (Rulebook §4.2): VI+CJT validation occurs before settlement commitment Provides deterministic accept/reject decision No ambiguity in transaction status Interoperability (Rulebook §5.1): VI+CJT can be implemented as protocol extension to existing standards (ISO 20022, EMVCo) Legacy systems can process transactions without understanding VI+CJT (with reduced security) Gradual migration path available Supervised Intermediary Role (Rulebook §3.3): CJT issuance naturally aligns with supervised intermediary responsibilities Banks already manage KYC, AML, and customer authorization CJT generation integrates with existing compliance workflows Privacy Requirements (Rulebook §6.4): VI pseudonymity preserves transaction privacy CJT policy encoding avoids identity disclosure Unlinkability maintained across transactions 7.2 GDPR Compliance Analysis Article 5 - Principles Relating to Processing: Data Minimization (5.1.c): VIs contain no personal data. CJTs encode policy, not identity. Compliance: ✓ Purpose Limitation (5.1.b): Data processing purpose limited to payment authorization. Compliance: ✓ Storage Limitation (5.1.e): VIs ephemeral, deleted post-settlement. CJTs time-bounded. Compliance: ✓ Article 25 - Privacy by Design: VI+CJT architecture embeds privacy at protocol level No opt-out mechanisms that expose more data Default configuration maximally privacy-preserving Compliance: ✓ Article 22 - Automated Decision-Making: VI+CJT validation is deterministic rule application, not profiling No behavioral analytics or risk scoring based on personal characteristics Users retain ability to challenge rejections (CJT reissuance) Compliance: ✓ Article 17 - Right to Erasure: Post-settlement VI deletion leaves no personal data Only cryptographic receipt remains (non-identifying) Compliance: ✓ 7.3 Cross-Border Implementation for EU Member States Digital Euro must function across 27 jurisdictions with heterogeneous regulatory frameworks. VI+CJT addresses this through: Jurisdictional Encoding Taxonomy: Level 1: EU_MEMBER_STATES (common Digital Euro space) Level 2: SPECIFIC_MEMBER_STATE (e.g., EU_GERMANY, EU_FRANCE) Level 3: CROSS_BORDER_CORRIDORS (e.g., DACH_REGION) Level 4: NON_EU (special authorization required) Example CJT for German Resident: json { "issuedBy": "Deutsche Bank AG", "userId": "pseudonym_xyz789", "authorizedJurisdictions": [ "EU_GERMANY", // Domestic always permitted "EU_MEMBER_STATES", // EU-wide retail payments "DACH_REGION" // Enhanced authorization for AT/CH ], "transactionPurpose": "RETAIL_PAYMENT", "maxSingleTransaction": 1000, "dailyLimit": 5000, "validFrom": "2025-02-01T00:00:00Z", "validUntil": "2025-02-15T23:59:59Z", "amlThreshold": 10000, // German 6AMLD requirement "merchantCategories": ["RETAIL", "SERVICES", "HOSPITALITY"] } When this user travels to France and makes payment, French merchant terminal validates EU_FRANCE ∈ authorizedJurisdictions via parent category EU_MEMBER_STATES. When user attempts payment in Switzerland (non-EU), validation fails unless explicit NON_EU_SWITZERLAND authorization present in CJT. 7.4 Offline Transaction Security for Digital Euro The ECB's offline transaction model faces unique relay attack exposure. VI+CJT provides layered defense: Layer 1 - Offline Preliminary Validation (at transaction time): Merchant terminal verifies CJT jurisdiction matches terminal registration Checks temporal validity Validates VI-CJT binding cryptographically Prevents obviously fraudulent transactions from entering offline queue Layer 2 - Device-Side Enforcement (user protection): User device maintains transaction log with context metadata Detects if same VI being used multiple times (double-spend attempt) Can refuse to sign if context parameters suspicious Layer 3 - Settlement Validation (finality enforcement): Full VI context verification against settlement infrastructure CJT cross-check against issuance records at supervised intermediary Jurisdictional consistency validation Double-spend detection across all synchronized device states Risk Mitigation: Offline transaction limits constrain exposure during vulnerable offline period Time-bounded CJTs prevent indefinite offline authorization Revocation mechanisms allow supervised intermediaries to invalidate compromised CJTs even before settlement 8. Implementation Considerations for 2029 Digital Euro Deployment 8.1 Technical Infrastructure Requirements User Device Layer: Hardware: Secure element or Trusted Execution Environment (TEE) for master key storage Cryptographic accelerator for VI derivation (recommended) NFC/Bluetooth/QR capabilities for payment initiation Offline storage for CJT pool and transaction queue Software: VI derivation function implementation CJT validation logic Context extraction (jurisdiction, merchant identity, time) Transaction queue management for offline mode Merchant Terminal Layer: Hardware: Standard payment terminal (EMV-compliant) Connectivity for settlement synchronization Secure storage for offline transaction queue Software: CJT jurisdiction validation VI-CJT binding verification Offline transaction queueing Settlement protocol integration Supervised Intermediary Layer (Banks): Infrastructure: CJT issuance service integrated with KYC/AML systems User authorization management CJT policy template configuration Revocation service for compromised CJTs Settlement validation integration Central Infrastructure Layer (ECB/Eurosystem): Settlement System: VI+CJT enforcement gates Jurisdiction validation service Double-spend detection across all transactions AML/CFT compliance validation Regulatory reporting automation 8.2 Performance Characteristics Latency Analysis: VI Generation: Cryptographic operation: 1 SHA-256 hash + 1 HMAC Estimated time: <1ms on modern smartphone Negligible impact on user experience CJT Validation: Signature verification: 1 ECDSA verification Jurisdiction lookup: Database query Estimated time: <5ms Acceptable for payment flow VI-CJT Binding Verification: Commitment opening: <1ms OR ZK proof verification: 10-50ms (depending on scheme) Acceptable for online payments; no impact on offline (deferred to settlement) Settlement Validation: Full VI+CJT verification: <100ms per transaction Parallelizable across millions of transactions No bottleneck for batch settlement Scalability Assessment: Target Load: 450M EU citizens, assume 10 transactions/person/day = 4.5B transactions/day VI+CJT Overhead per Transaction: Storage: ~2KB (VI + CJT + binding proof) Computation: <100ms validation Network: Negligible (CJT can be cached, VI transmitted in standard payment message) Infrastructure Capacity: 4.5B transactions/day = 52,000 TPS average Peak load (3x): 156,000 TPS Modern payment infrastructure (Visa: 65,000 TPS) demonstrates feasibility Stateless validation enables horizontal scaling Assessment: VI+CJT overhead is acceptable for Digital Euro scale requirements. 8.3 Deployment Strategy Aligned with ECB Timeline The ECB's 2029 operational target requires phased deployment: Phase 1 (2025-2026): PreparationGoal: Technical standards and pilot integration Finalize VI+CJT cryptographic specifications Publish ISO/ITU standards documentation Integrate into Digital Euro sandbox testing Pilot with 2-3 supervised intermediaries in controlled environment Phase 2 (2026-2027): Supervised Intermediary RolloutGoal: CJT issuance infrastructure deployment Banks deploy CJT issuance systems Integration with existing KYC/AML workflows User enrollment and master key provisioning Cross-border testing across member states Phase 3 (2027-2028): Merchant InfrastructureGoal: Terminal capability deployment Merchant terminal software updates for VI+CJT validation Payment service provider integration Offline transaction queue management deployment Interoperability testing across payment rails Phase 4 (2028-2029): Public RolloutGoal: End-user availability Wallet application updates with VI generation Public education on security features Gradual migration from pilot to production Monitoring and optimization Phase 5 (2029+): Full Operation and EvolutionGoal: Operational CBDC with ongoing enhancement Monitor relay attack attempts (should be zero successful) Refine CJT policies based on usage patterns Extend to additional use cases (programmable payments, IoT, etc.) This timeline aligns with ECB's stated 2029 operational readiness target while providing adequate testing and deployment windows. 8.4 Backward Compatibility and Migration Legacy System Interoperability: VI+CJT can be implemented as optional extension to base Digital Euro protocol Non-VI+CJT transactions processed with conventional security (accept elevated relay risk) Gradual migration: high-value or cross-border transactions require VI+CJT first, expand coverage over time Migration Incentives: Merchants supporting VI+CJT eligible for lower payment processing fees (reduced fraud risk) Users with VI+CJT capability receive higher offline transaction limits Cross-border payments may eventually require VI+CJT for authorization 9. Strategic Implications for Digital Euro Sovereignty 9.1 Addressing Commissioner Dombrovskis' Concerns In his January 28, 2025 statement, Economic Commissioner Valdis Dombrovskis articulated: "Today, our payments landscape is highly dominated by non-European providers. This makes us dependent on foreign-owned companies in an increasingly polarised and fragmented world... Ceding such a degree of technological control over the EU's economy to others could impede our ability to act autonomously." VI+CJT directly addresses these sovereignty concerns: Technological Independence: No reliance on proprietary fraud detection systems (Visa/Mastercard risk engines) Cryptographic enforcement using open standards (ISO, ITU, EMVCo) European institutions retain full control over policy encoding in CJTs Autonomous Operation: Relay attack prevention does not require connection to foreign infrastructure Offline mode fully functional without external validation services Jurisdictional enforcement embedded in protocol, not outsourced to intermediaries Resilience Against External Pressure: Foreign actors cannot weaponize relay attacks to circumvent EU regulations Sanctions and capital controls enforceable at cryptographic level AML/CFT compliance automated, not dependent on third-party screening services 9.2 Competitive Positioning vs. US/China CBDC Models Comparison with Digital Dollar (if developed): US Approach (hypothetical, based on Federal Reserve research): Likely emphasis on integration with existing banking infrastructure Privacy concerns secondary to law enforcement access Less stringent cross-border control (dollar's reserve currency status) Digital Euro with VI+CJT Advantage: GDPR-compliant privacy preservation Explicit jurisdictional sovereignty Protocol-level regulatory enforcement Comparison with e-CNY (China's Digital Yuan): e-CNY Approach: Centralized architecture with full transaction visibility State surveillance capabilities by design Limited cross-border functionality (capital controls) Digital Euro with VI+CJT Advantage: Privacy-preserving while maintaining compliance Decentralized validation (supervised intermediaries) Legitimate cross-border functionality within regulatory framework Market Positioning: VI+CJT enables Digital Euro to occupy unique position: privacy-preserving yet compliant, sovereign yet interoperable. This aligns with EU values and competitive advantages. 9.3 Geopolitical Implications of Relay-Resistant Architecture Scenario 1: Payment System Weaponization Threat: Adversarial state or corporation attempts to exploit relay attacks to circumvent EU sanctions. VI+CJT Defense: CJTs encode prohibited jurisdictions and sanctioned entities. Cryptographic enforcement prevents relays to sanctioned terminals regardless of network routing. Scenario 2: Regulatory Arbitrage Threat: Market participants relay transactions to low-regulation member states to evade AML thresholds. VI+CJT Defense: CJTs encode applicable AML regime based on supervised intermediary jurisdiction. Relay to different jurisdiction triggers settlement rejection. Scenario 3: Cross-Border Data Flow Exploitation Threat: Foreign intelligence services attempt to use relay attacks to extract transaction metadata or compromise privacy. VI+CJT Defense: VI pseudonymity and CJT policy encoding prevent identity disclosure even if relay succeeds technically. No personal data leakage in relayed messages. 10. Open Research Questions and Future Work While the VI+CJT framework addresses core relay attack challenges, several areas merit further investigation: 10.1 Post-Quantum Cryptography Integration Challenge: Current VI+CJT design assumes classical cryptographic hardness (ECDSA signatures, SHA-256 hashing). Quantum computers could break these primitives. Research Direction: Transition to post-quantum signature schemes (CRYSTALS-Dilithium, Falcon) Quantum-resistant commitment schemes for VI-CJT binding Performance impact assessment (PQC signatures larger, slower) Timeline Consideration: NIST post-quantum standards finalized 2024. Integration feasible before 2029 deployment. 10.2 Privacy-Preserving Compliance Verification Challenge: Current CJT design encodes jurisdictional policy, enabling enforcement but potentially revealing transaction metadata. Research Direction: Zero-knowledge proofs of CJT compliance without revealing CJT contents Homomorphic encryption for policy evaluation Selective disclosure mechanisms (reveal only necessary fields) Benefit: Enhanced privacy while maintaining relay attack resistance. 10.3 Decentralized CJT Issuance Challenge: Current model requires supervised intermediaries to issue CJTs. This creates centralization and potential single points of failure. Research Direction: Distributed CJT generation using multi-party computation Blockchain-based CJT registries with revocation User-controlled CJT derivation from master policies Trade-offs: Decentralization vs. regulatory accountability. Further analysis needed. 10.4 Cross-CBDC Interoperability Challenge: If multiple CBDCs adopt VI+CJT-like architectures, how do they interoperate for cross-currency transactions? Research Direction: Standardized CJT schema across jurisdictions VI derivation protocol harmonization Settlement protocols for multi-CBDC atomic swaps Relevance: BIS mBridge project, Project Jura, and other cross-border CBDC initiatives. 10.5 Formal Verification Challenge: Security proof in Section 6 is informal. Formal verification provides higher assurance. Research Direction: Formal modeling in Tamarin or ProVerif protocol verifiers Automated theorem proving of relay attack resistance Verification of implementation correctness against formal model Benefit: Confidence for central bank risk committees evaluating architecture. 11. Conclusion 11.1 Summary of Findings This paper has analyzed relay attack vulnerabilities in Central Bank Digital Currency systems with specific focus on the European Central Bank's Digital Euro program. The analysis demonstrates several key findings: Finding 1: Relay attacks represent a fundamental architectural challenge for Digital Euro, not merely an implementation security concern. The combination of offline functionality, GDPR-compliant privacy, and cross-border interoperability creates a threat surface that conventional payment security approaches do not adequately address. Finding 2: Existing relay attack countermeasures—distance-bounding protocols, radio frequency constraints, timing analysis, device attestation, and behavioral analytics—each fail to satisfy one or more of Digital Euro's core requirements. This is not a deficiency in these techniques but rather reflects the novel security challenge posed by Digital Euro's design goals. Finding 3: The absence of a published relay attack solution in current Digital Euro specifications represents a genuine open research problem, not an oversight. The ECB's methodical approach to this challenge is appropriate given the systemic risks of premature CBDC deployment with unresolved security vulnerabilities. Finding 4: The Virtual Identity + Compliance Jurisdiction Token (VI+CJT) framework offers a potential architectural solution through context-binding cryptographic primitives. The approach provides deterministic relay attack prevention while preserving privacy, supporting offline transactions, and enabling cross-border functionality. Finding 5: VI+CJT aligns with Digital Euro regulatory requirements (GDPR, 6AMLD, PSD2), technical specifications (ECB rulebook framework), and strategic objectives (payment sovereignty as articulated by Commissioner Dombrovskis). 11.2 Contribution to Digital Euro Security Architecture This work contributes to ongoing Digital Euro development in three ways: Theoretical: Formal threat modeling of relay attacks in multi-jurisdictional, privacy-preserving, offline CBDC contexts. Security proofs demonstrating VI+CJT's relay resistance properties. Practical: Concrete implementation guidance for supervised intermediaries, merchant terminals, and settlement infrastructure. Performance analysis demonstrating scalability to Digital Euro requirements. Strategic: Analysis of relay attack implications for payment sovereignty and Digital Euro competitive positioning vs. other CBDC programs. 11.3 Path Forward for Digital Euro Implementation As the ECB progresses toward 2029 operational readiness, several steps could advance relay attack mitigation: Near-Term (2025): Formal evaluation of VI+CJT or similar architectures by ECB cryptography experts Inclusion of relay attack scenarios in Digital Euro sandbox testing Engagement with standardization bodies (ISO TC 68, ITU-T SG 17) on context-binding protocols Medium-Term (2026-2027): Pilot deployment of relay-resistant architecture with selected supervised intermediaries Cross-border testing across multiple member states Privacy impact assessment with European Data Protection Board Long-Term (2028-2029): Gradual rollout to production infrastructure Monitoring of real-world relay attack attempts Refinement based on operational experience 11.4 Broader Implications for CBDC Security The relay attack challenge and VI+CJT-style solutions have implications beyond Digital Euro: For other CBDC programs: Similar architectural approaches may benefit Bank of England (Digital Pound), Bank of Japan (digital yen), and other CBDC initiatives facing offline and privacy requirements. For payment standards: Context-binding cryptographic primitives could be incorporated into EMVCo, ISO 20022, and other payment protocol standards as best practices. For privacy-preserving systems: The balance between privacy and compliance demonstrated by VI+CJT informs broader cryptographic system design for GDPR and similar regulatory frameworks. 11.5 Final Reflection The Digital Euro represents one of the most ambitious monetary infrastructure projects in modern history. Its success depends not only on technological sophistication but on solving genuinely novel security challenges. Relay attacks exemplify this: they exploit the intersection of offline functionality, privacy requirements, and multi-jurisdictional operation in ways that existing payment security has not addressed. The Virtual Identity + Compliance Jurisdiction Token framework offers one potential path forward. Whether this specific architecture or alternative approaches ultimately secure the Digital Euro, the essential requirement remains: transforming authorization from credential verification into context-bound policy enforcement. As Commissioner Dombrovskis noted, payment system sovereignty is critical for European economic autonomy. Achieving this sovereignty requires not just political will but cryptographic innovation. The relay attack challenge—and efforts to solve it—represent the technical frontier where European digital sovereignty will be won or lost. This paper is offered as a contribution to that effort. Acknowledgments This research was conducted independently and reflects technical analysis of publicly available Digital Euro documentation, cryptographic literature, and regulatory frameworks. The author acknowledges the substantial work undertaken by the European Central Bank, European Commission, and supervised intermediaries in advancing Digital Euro development. Any errors or omissions in this analysis are solely the author's responsibility. References Digital Euro Primary Sources: European Central Bank. (2023). "Progress on the investigation phase of a digital euro." European Central Bank. (2024). "Digital euro rulebook framework: Public consultation." Council of the European Union. (2024). "Council conclusions on the digital euro." European Commission. (2023). "Proposal for a Regulation on the establishment of a digital euro." Relay Attack Security Literature: Cremers, C., Rasmussen, K. B., Schmidt, B., & Capkun, S. (2012). "Distance hijacking attacks on distance bounding protocols." IEEE Symposium on Security and Privacy. Chothia, T., Garcia, F. D., de Ruiter, J., van den Breekel, J., & Thompson, M. (2015). "Relay cost bounding for contactless EMV payments." Financial Cryptography and Data Security. CBDC Architecture Research: Bank for International Settlements. (2023). "Blueprint for the future monetary system: improving the old, enabling the new." Auer, R., & Böhme, R. (2020). "The technology of retail central bank digital currency." BIS Quarterly Review. Privacy-Preserving Payments: Camenisch, J., Hohenberger, S., & Lysyanskaya, A. (2005). "Compact e-cash." EUROCRYPT. Benaloh, J., & De Mare, M. (1994). "One-way accumulators: A decentralized alternative to digital signatures." EUROCRYPT. Cryptographic Protocols: Goldreich, O., Micali, S., & Wigderson, A. (1991). "Proofs that yield nothing but their validity." Journal of the ACM. National Institute of Standards and Technology. (2024). "Post-Quantum Cryptography Standardization." Regulatory Frameworks: European Parliament and Council. (2016). "Regulation (EU) 2016/679 (General Data Protection Regulation)." European Parliament and Council. (2015). "Directive (EU) 2015/2366 on payment services (PSD2)." European Parliament and Council. (2024). "Directive on Anti-Money Laundering (6AMLD)." Geopolitical Context: Dombrovskis, V. (2025). Speech at European Banking Summit. Brussels, January 28. Author ContactFor technical correspondence regarding this research, contact information should be directed through appropriate academic or institutional channels. Document Status: Research paper for technical evaluation and peer review. Not peer-reviewed at time of publication. Comments and feedback welcomed from cryptography experts, central bank technologists, and payment system architects. Disclaimer English is not the author’s first language.Artificial intelligence–based tools were used solely for grammatical correction, language refinement, translation assistance, and tabulation/formatting of content. All technical concepts, system architectures, inventive ideas, and substantive technical descriptions originate from the author.No artificial intelligence system was used to generate, design, or invent the disclosed technologies or technical solutions. Patent Status Disclosure Certain concepts, systems, and methods described in this publication are the subject of patent applications that have already been filed and are currently pending in one or more national and/or international patent offices. This disclosure is provided for defensive publication, academic reference, and transparency purposes only.Nothing herein shall be construed as a waiver of patent rights, a dedication to the public domain, or a license to practice any claimed or claimable invention. All patent rights remain expressly reserved. About the Author (Transparency Disclosure Only) This section is provided solely in the interest of transparency and contextual clarity. It does not request or imply adoption, endorsement, funding, procurement, standard-setting, or regulatory action, nor is it intended to influence policy outcomes. The author is an independent technologist and inventor based in Balasore, India, holding a Bachelor of Engineering in Electronics and Telecommunications. The author’s work focuses on protocol-level privacy, security, and compliance-enforcement architectures designed to make existing legal and regulatory obligations technically enforceable at execution time in automated and AI-driven digital systems. The technical framework and related inventions referenced in this submission have been filed internationally through the World Intellectual Property Organization (WIPO) and through multiple national patent applications in India, comprising more than 2,550 coordinated claims. These filings address execution-time privacy enforcement, jurisdictional control, and automation-safe compliance architectures across terrestrial, satellite, and distributed execution environments. All patent applications were conceived, prepared, and filed independently, without institutional affiliation, external funding, or corporate sponsorship.

Abstract Relay attacks represent a fundamental security challenge in Central Bank Digital Currency (CBDC) implementations, particularly for systems designed to support offline transactions and privacy-preserving features. This paper analyzes the structural vulnerabilities that enable relay attacks in conventional CBDC architectures, with specific attention to the European Central Bank's Digital Euro program. We examine the current state of relay attack mitigation in Digital Euro technical specifications and identify gaps between stated requirements and available countermeasures. The analysis demonstrates that conventional approaches—distance-bounding protocols, timing analysis, and proximity heuristics—remain insufficient for systems requiring simultaneous offline functionality, GDPR-compliant privacy, and cross-border interoperability. We present a technical evaluation of the Virtual Identity + Compliance Jurisdiction Token (VI+CJT) framework as a potential architectural countermeasure that addresses these combined requirements. Unlike probabilistic detection mechanisms, the VI+CJT architecture eliminates relay attack surfaces through context-binding cryptographic primitives. This work contributes to ongoing Digital Euro security architecture discussions by offering a formal analysis of relay attack vulnerabilities specific to the European context and proposing cryptographic mechanisms compatible with both EU regulatory frameworks and the ECB's 2029 operational timeline.