d4 · independent software studio · 
England
d4 is the studio of Davinder Singh, building native apps, network tools and sovereign cyber defence, and the infrastructure they run on. It also publishes Decrypted, the daily UK cyber security briefing, and nineteen free security tools, both free and free of advertising. Fast, private by design and honest about what they do. No trackers, no cloud middlemen, no fuss.
Decrypted, read aloud. One story that matters, in about four minutes, twice a day. Free, and no advertising.
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The daily UK cyber security briefing. Every weekday morning and evening, one story that matters to UK organisations, explained in plain English, with the NCSC guidance and the Secure by Design lesson that goes with it. Free to read, free to listen to, and no advertising. Read them all
Latest post
31 october 2026 · vulnerabilities and patching · supply chain
Two counterfeit Claude Code releases on Cursor's marketplace planted an SSH key and persistence on developer machines. What a defender should check, and the design lesson. Plus Cisco's exploited firewall bug, GCHQ's missing media and Binance's laptop breach.
Read the full five minutes →More from Decrypted
Nineteen browser-based tools for UK defenders and the merely curious. No sign-up, no adverts, and nothing you look up is stored. Open the toolbox
New and actively exploited vulnerabilities, with the CISA KEV catalogue alongside.
The full enterprise matrix, readable on a phone.
Claims posted by ransomware crews on their own leak sites.
SPF, DKIM and DMARC for any domain, graded and explained.
Whether a password appears in a breach corpus, checked without sending it.
Certificates, chain and protocol support for a hostname.
Security headers on any site, scored with the fix for each.
Build a Content-Security-Policy without breaking the page.
What a page loads behind your back, third parties named.
Records and a resolver race, so you can see who answers fastest.
Whether a domain's DNS is signed and who may issue its certificates.
Trace a message's path and see where authentication failed.
Read a certificate or CSR without pasting it into someone else's site.
Decode a token and inspect its claims, in the browser.
Subnets, ranges and masks, worked out as you type.
What your browser and network reveal about you.
Memorable passphrases generated on your own device.
Hashes, base64 and URL encoding, worked out in the browser.
Check a disclosure policy parses the way researchers expect.
One design language across everything d4 makes: three native apps for iPhone and iPad, and one piece of hardware. Each earns its place, and each says plainly where it has got to.
Your pfSense firewall, on iPhone and iPad.
Monitor, control and analyse a pfSense firewall over SSH, with live dashboards, full rule editing and a Wireshark-style packet analyser. Nothing is installed on the firewall and nothing passes through a cloud service. Your credentials never leave the device.
The diary made for driving instructors. In testing.
A private iPhone app for UK driving instructors. Book lessons around your real availability, track every pupil's progress on the DVSA scale, send invoices, log mileage and expenses, get your Self Assessment figures totalled, and score lessons hands-free in CarPlay. With Tap to Pay on iPhone and Driving School Diary, you can take the lesson fee on your own iPhone before the pupil is out of the car, and the money goes straight to your own account. Terms apply. No accounts, nothing shared, all on your device. It is with testers now and going to the App Store next.
The player the gig actually needs.
The app working musicians run on stage: a no-nonsense backing-track player with zero distractions and high-contrast visuals that stay readable under stage lighting. Built for the British Bhangra group B21, free to download, and it works with no signal in the room. A larger version is in testing, adding a sampled keyboard workstation, an ideas scratchpad and the bookings behind the band.
Hardware
Know when your car is being attacked.
The studio's first hardware product: a small, passive radio sensor for the driveway. It listens to the bands car keys use, recognises the signature of a jamming or relay attack, and sends you a message while it is happening. It detects and alerts, it does not prevent, and it has no ability to interfere with anything. What it gives you is knowing at the time, rather than in the morning.
More is on the bench. New work appears here when it is ready, and not before.
A studio that builds software and, when it makes sense, the infrastructure it runs on.
iOS and iPadOS apps that feel at home on the platform, with credentials on the device and nothing in the cloud. pfAdmin, Driving School Diary and B21 Stage Time so far.
Decrypted, the daily UK cyber briefing, and nineteen free tools that anyone can use. Published openly because the studio would rather be judged on the work than on a claim.
Firewalls, routing, VPN and monitoring. The unglamorous plumbing everything else depends on, done properly.
Private AI platforms and enterprise deployments on hardware you control, with your data kept inside your walls.
Least privilege, encryption and host-key pinning, decided at the start and not patched in afterwards.
Beyond the apps, d4 designs and builds platforms and sites for others. A few from the workbench.
Benchmarking security operations maturity and compliance readiness, mapped to NIST CSF, ISO 27001, MITRE ATT&CK, NCSC CAF and more.
A free DEFCON 658 and Defence Cyber Certification self-assessment, scoring you against Def Stan 05-138 the way an assessor would.
The site for the British Bhangra group B21: thirty years of Handsworth sound, designed and built by d4.
06 · About
d4 is a studio of one, run by Davinder Singh. The design, the code, the servers and the support inbox all pass through the same hands.
Nothing here is outsourced, tracked or rented out. The studio would rather ship one considered thing than ten careless ones. If it carries the d4 mark, it was built here, and d4 stands behind it.
· Davinder Singh, founder
07 · Contact
A project, an app, or support for one you already use. Every message is read and answered by a person.
Or use the Feedback button in the corner. It lands on the studio desk.
