Stack
- Node.js
- PostgreSQL
- AWS
- Cybrid
- Yellow Card
01/
“Where is the user’s money?”
- The hard problem
- Cross-border payments flowed through three separate regulated providers — bank (Plaid) → custodial crypto (Cybrid) → African fiat ramps (Yellow Card). When any step failed mid-transfer — a KYC re-check, an FX window closing, a provider rate-limit, a delayed webhook — money would be parked somewhere across three systems with no way to see where. Fine when it happens once; company-existential when a real customer calls support asking where their $500 went and no one can answer.
- What the team built
- A transaction-journey model — every step of every transfer logged with provider, state, input, response, timestamp. Webhook handlers for all three providers feeding one journey log, with BullMQ retries and exponential backoff for transient failures. Idempotent processing so a webhook arriving twice or out-of-order never double-applies or corrupts state. Counterparty validation so sender and receiver are legally matched before money moves. Result: any transfer could be traced to its exact location and failure reason, instead of three black boxes.
- Why it matters
- It’s distributed-systems thinking — idempotency, out-of-order tolerance, reconciliation across untrusted third parties — applied to real money. And the insight came from product empathy — imagining the support call — not just code.
02/
“Someone is attacking our API”
- The hard problem
- The team woke to unusual traffic — someone had rooted a device and was hitting open backend endpoints directly, especially the phone-verification endpoint that gates account creation. The API surface was exposed beyond design.
- What the team built
- A layered mobile security defense, in native code rather than a library wrapper: SSL pinning on iOS (Swift) and Android (Kotlin) so traffic can’t be intercepted via proxy; HMAC-SHA256 request signing with timestamp + body to defeat tampering and replay; root/jailbreak detection to refuse compromised devices. The signing secret lives in Keychain/Keystore, unextractable from a decompiled APK.
- Why it matters
- Native OS-layer security work that most senior engineers have never shipped. It defends against MITM, tampering, replay, and stolen-key attacks simultaneously.
“Payments is unforgiving — a single reconciliation bug costs real money. They built rails we could trust from day one, and the mobile apps held up under real transaction volume.”
Want something like this built?
Book a call and tell us what you're shipping — we'll talk scope, timeline, and the fastest path to production.
Book a Call