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. 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.
One design language across everything d4 makes. Three apps, one hardware product, and each earns its place.
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.
A private iPhone app for UK driving instructors. Take the lesson fee with Tap to Pay on iPhone before the pupil is out of the car, book lessons around your real availability, track every pupil's progress on the DVSA scale, log mileage and expenses, get your Self Assessment figures totalled, and score lessons hands-free in CarPlay. No accounts, nothing shared, all on your device.
Everything the gig needs. On stage, and after it.
The app working musicians run at the gig: a no-nonsense backing-track player with zero distractions and high-contrast visuals that stay readable under stage lighting, a sampled keyboard workstation played from any MIDI controller, an on-device songwriting scratchpad, and the bookings, contracts and invoices behind the band. Built for the British Bhangra group B21, and free to download and run your set.
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.
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.
Short, plain-English reads on cyber and sovereignty news, with UK security and Secure by Design front of mind. Read them all
Latest post
15 september 2026 · vulnerabilities and patching · uk policy and law
Cisco confirms active exploitation of a critical, unauthenticated SQL injection flaw in Secure Email Gateway that hands attackers root access via a single crafted email, with no workaround available. Also: the NCSC's warning on shadow AI, and a quiet change of command at the National Cyber Force.
Read the full five minutes →More from Decrypted
05 · 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
06 · 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.
