Here’s a concise guide to secure payment gateways for online transactions: what they are, what security features to require, common integration types, recommended providers by use-case, and an implementation/security checklist you can use.

What a payment gateway does

• Authorizes and processes card and alternative payments between your customer, the acquiring bank, and the card networks.
• Handles encryption, tokenization, fraud checks and settlement (or hands off to processors that do).

Essential security features to require

• TLS 1.2/1.3 for all endpoints (HTTPS).
• PCI DSS compliance (and a plan to minimize PCI scope — e.g., hosted/JS tokenization).
• Tokenization of card data (so raw PAN isn’t stored on your servers).
• Strong fraud tools: velocity checks, AVS, CVV verification, device fingerprinting, ML-based risk scoring.
• 3-D Secure / SCA support (for Europe and many card issuers).
• Signed, verifiable webhooks and HMAC/secret verification.
• Encryption at rest for any stored sensitive fields; key management/HSM support for enterprise users.
• Good audit logging and dispute/chargeback tools.
• Strong API key credentialing, scoped keys, and key rotation.
• Transparent reporting, settlement, and payout controls.

Common integration models (pros & cons)

• Redirect / Hosted Checkout (very low PCI scope, fastest to secure; less control over UX).
• Embedded/Hosted iFrame (good compromise: merchant keeps UX, gateway hosts card fields → reduced PCI).
• Client-side tokenization + server-side charge (best for control + limited PCI scope; requires secure JS SDK).
• Direct API (full control, but highest PCI burden — generally avoid unless necessary).

Recommended gateways by use-case (widely used, developer- and security-focused)

• Stripe — developer-friendly, strong tokenization, fraud tools (Radar), subscription and marketplace support. Good for startups to mid-market and scale-ups.
• PayPal / Braintree — broad consumer reach, PayPal wallet + card processing; Braintree offers tokenization and vaulted payment methods.
• Adyen — enterprise-grade, global acquiring, strong risk platform and multi-currency support.
• Authorize.Net — long-established gateway, good for U.S. merchants and traditional integrations.
• Square — good if you need unified POS + online payments.
• Regional providers (choose local leaders if you need local schemes/regulatory coverage): e.g., Razorpay (India), PayU, etc.

How to choose (priority checklist)

• Supported payment methods/countries and currencies you need.
• PCI scope & compliance approach (hosted vs direct).
• Fraud prevention capabilities and false-positive tuning.
• Pricing: transaction fee, monthly, chargeback fees, settlement timing.
• Integration complexity and SDKs for your platform (web, mobile).
• Payout options and requirements for marketplaces (split payments, KYC).
• Reliability, SLA, and support/enterprise features.

Implementation & security checklist (practical steps)

• Use the gateway’s hosted fields or client-side tokenization SDK to avoid handling raw PANs.
• Ensure all endpoints use TLS 1.2+ and HSTS.
• Never store CVV. Store only what is permitted and encrypted (token, last4, expiration).
• Verify and validate webhooks using signatures; do not blindly trust incoming webhook payloads.
• Enforce least-privilege API keys, separate keys for production vs. test, and rotate keys regularly.
• Implement rate limiting, WAF, and input validation to guard APIs.
• Enable 3-D Secure / SCA where required and support fallback flows.
• Monitor transactions and alerts; tune fraud rules to reduce false positives.
• Maintain PCI documentation and perform required scans/assessments (ASV scans, SAQ).
• Log access and use SIEM/alerting for suspicious admin or payment activity.
• Test thoroughly in sandbox: edge cases, chargebacks, refunds, partial captures, webhook retries.

Fraud-reduction tools to enable

• 3-D Secure 2.0 (friction where needed).
• AVS and CVV checks.
• Device fingerprinting and geolocation/velocity checks.
• Machine-learning scoring and manual review queue for high-risk transactions.
• BIN blocking, IP reputation, card verification history.

Operational notes

• Have a chargeback management process and clear refund policy.
• Keep reconciliation and accounting flows automated.
• For marketplaces, ensure you meet KYC/AML and consider a managed marketplace product (Connect, MarketPay, etc.) to simplify compliance.

Bottom line

• For most online businesses seeking secure, fast integration: prefer a gateway offering client-side tokenization or hosted fields (Stripe, Braintree, Adyen), enable 3-D Secure, and use built-in fraud tooling. Minimize your PCI scope by never touching raw card numbers and verify webhooks/signatures for integrity.

If you want, I can generate a one-page PCI/security checklist tailored to your tech stack (platform, language, and whether you prefer hosted checkout or direct integration).