feat(backend): harden Path Payment Service with signature verification, query optimization, error recovery, and security audit

[rohan911438]

Overview

This PR introduces a comprehensive security, reliability, and performance hardening initiative for the Path Payment Service.

Included Issues

• Closes [Backend] Add cryptographic signature verification to Path Payment Service #749 — Add cryptographic signature verification to Path Payment Service
• Closes [Backend] Optimize SQL queries in Path Payment Service #750 — Optimize SQL queries in Path Payment Service
• Closes [Backend] Enhance error recovery for Path Payment Service #751 — Enhance error recovery for Path Payment Service
• Closes [Backend] Conduct security audit on Path Payment Service #752 — Conduct security audit on Path Payment Service

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Cryptographic Signature Verification (#749)

Added

• Request signature verification layer
• Payload integrity validation
• Timestamp validation
• Replay protection safeguards

Security Benefits

• Prevents request tampering
• Validates trusted request origin
• Strengthens payment authorization flow

Test Coverage

• Valid signatures
• Invalid signatures
• Expired requests
• Replay attack scenarios
• Malformed payloads

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SQL Query Optimization (#750)

Improvements

• Reduced unnecessary database scans
• Optimized query execution paths
• Added targeted indexing strategy
• Removed inefficient query patterns

Benefits

• Faster path payment execution
• Reduced database load
• Improved scalability

Validation

• Query performance benchmarks
• Result consistency verification
• Slow-query analysis

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Error Recovery Enhancements (#751)

Added

• Retry handling for transient failures
• Circuit breaker protection
• Recovery logging
• Failure classification

Recovery Scenarios

• Database connectivity issues
• Horizon API timeouts
• Temporary network failures

Benefits

• Improved service resilience
• Reduced payment processing interruptions
• Better operational visibility

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Security Audit (#752)

Audit Scope

• Payment authorization
• Signature validation
• Replay protection
• JWT handling
• SQL injection prevention
• Error handling
• Secret management

Deliverables

• Security audit report
• Remediation documentation
• Validation checklist

Outcome

Improved confidence in Path Payment Service security posture and operational safety.

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Testing

Unit Tests

• Signature verification
• Error recovery
• Query optimization paths

Integration Tests

• End-to-end payment processing
• Failure recovery flows
• Security validation scenarios

Security Validation

• Replay attack testing
• Invalid request handling
• Input validation coverage

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Security Notes

• Signature verification enforced before payment execution
• Parameterized queries used throughout service interactions
• Sensitive errors sanitized before exposure
• Replay protection validated against duplicate request attempts

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Result

This PR significantly improves the security, reliability, performance, and maintainability of the Path Payment Service while maintaining compatibility with existing Stellar payment workflows.

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