CalcSI Blog

Background knowledge and practical explanations for the topics our tools cover — from cryptography to travel and visa rules, web standards, and security.

2026-08-14 · 9 min read

German Bridge Days 2027: turn 6 vacation days into 12 days off

Anyone who places their 2027 vacation cleverly can turn a handful of days off into long stretches of free time. The year has one clear weakness and one clear strength: three nationwide public holidays fall on a weekend…

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2026-08-13 · 9 min read

Create Your Own QR Codes in 2026: Wi-Fi, vCard, SEPA Payment & More, Done Right

A QR code is more than a link hidden in pixelated black and white – it can carry Wi-Fi credentials, contact details, an entire SEPA bank transfer or a calendar entry, all fully offline inside the image. The catch…

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2026-08-12 · 9 min read

URL Encoding in 2026: The Traps of Percent-Encoding, %20, + and Double Encoding

URL encoding looks harmless: a space becomes %20, an umlaut becomes %C3%BC, done. In practice it is one of the most common silent sources of bugs in web applications. Sometimes a space shows up as %20, sometimes as +…

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2026-08-11 · 9 min read

Perseids 2026: How to Plan the Meteor-Shower Night of August 12–13

On the night of August 12–13, 2026, the Perseids reach their peak – up to roughly 100 shooting stars per hour under ideal conditions. And the conditions in 2026 are unusually good: August 12 is a new moon…

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2026-08-10 · 8 min read

Germany Housing Grants 2026: What Replaced the Baukindergeld and Which Programs Matter Now

Germany’s Baukindergeld family building grant is gone, the fund is empty and no new cash subsidy is in sight. Yet there is more state support for a first home in 2026 than most people assume – it just hides…

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2026-08-09 · 9 min read

Hydration for Summer Workouts 2026: How Much Fluid You Really Need

Going for a run, a ride or standing on the pitch at 30 degrees Celsius – and feeling parched afterward. How much you should drink during summer workouts depends on far more than a fixed number of liters. As of…

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2026-08-08 · 9 min read

Base64 vs. Base64url 2026: The Small Difference With a Big Impact

Everyone who has ever inlined an image as a data URL or peeked inside a JWT has met Base64. What fewer people know is that it comes in two flavors: the standard alphabet with + and /, and the URL-safe variant Base64url…

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2026-08-07 · 9 min read

Cut Your Household Water Bill in 2026: Where the Money Really Drains Away

Each person in Germany uses around 125 liters of tap water a day – and the prices for fresh water and wastewater keep climbing. Yet the biggest saving lever is not the water itself, but the energy locked inside…

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2026-08-06 · 10 min read

German No-Claims Class (SF-Klasse) 2026: How the Discount Works, Avoiding Downgrades and Saving Before Nov 30

The no-claims class decides a large part of your German car insurance premium – and few drivers know how it really works. Every accident-free year moves you up and lowers your rate; a single settled claim throws…

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2026-08-05 · 9 min read

Understanding HTTP Status Codes in 2026: From 200 to 500 Explained

Every request on the web gets a three-digit answer back, and most people only ever recognize one of them: 404. Yet each number carries a precise statement. Is the thing gone for good, or just moved? Are you locked out…

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2026-08-04 · 8 min read

Follow-Up Mortgage 2026: Prolongation, Refinancing or Forward Loan?

When your fixed-rate period ends, your loan does not end – only the guaranteed interest rate does. What comes next can decide tens of thousands of euros: the convenient prolongation with your existing bank…

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2026-08-03 · 9 min read

German Travel Expenses & Per Diems 2026: EUR 28, the Three-Month Rule and the Meal Deduction

If you travel for work in Germany, you probably leave money on the table every year – simply because the travel-expense report looks complicated. Yet behind per diems, mileage and lodging there are only a handful…

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2026-08-02 · 10 min read

Beating Jet Lag in 2026: Why Eastward Flights Hurt More and How to Reset Your Clock

Jet lag is not a sign of weakness but plain physics of your body clock: your body still ticks in the old time zone while the clock at your destination says something else entirely. Why a flight east almost always hits…

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2026-08-01 · 8 min read

What Changes in Germany in August 2026? Apprenticeship Start, AI Labeling, Packaging Rules and Money at a Glance

August 1 is a quiet deadline: a new apprenticeship year begins, several EU rules take effect, and in the background 3.2 percent inflation plus the first ECB rate hike in years press on purchasing power. This article…

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2026-07-31 · 9 min read

Missed the German Tax Deadline of 31 July 2026: Extensions, Late Fees and What You Can Still Do Today

Today, on 31 July 2026, the filing deadline for the 2025 German income tax return runs out – for everyone with a mandatory return who files without an adviser. If you realize you will not make it, that is no…

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2026-07-30 · 9 min read

Checking Your German Utility Bill in 2026: Deadlines, Chargeable Costs and the Most Common Errors

German tenant associations estimate that roughly one in two annual utility statements (the Nebenkostenabrechnung) contains errors – sometimes in the landlord favor, sometimes just sloppy math. If you skim the…

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2026-07-29 · 9 min read

German Retirement Planning 2026 Compared: Ruerup, Riester, Company Pension and ETF Plan After the Reform

Private retirement planning in Germany is being rebuilt in 2026: parliament has passed the Riester reform, the retirement securities account arrives in 2027, and the early-start pension is about to launch. At the same…

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2026-07-28 · 9 min read

Password Managers in 2026: Is the Switch Really Worth It? An Honest Fact-Check

Over 19 billion leaked passwords are in circulation, 60 to 85 percent of people reuse the same password across sites – and according to the Verizon DBIR 2025, 22 percent of all breaches trace back to stolen…

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2026-07-27 · 9 min read

German Long-Term Care Insurance 2026: 3.6 Percent Contribution, the Child Discount, and What the Fund Actually Pays

Germany’s statutory long-term care insurance holds its contribution rate at 3.6 percent in 2026 – but only because the federal government is propping it up with a billion-euro loan. People without children…

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2026-07-26 · 9 min read

Planning a 2026 Cookout: Portions, Quantities and Calories Done Right

Too much bought, too little chilled, half of it in the trash at the end – hardly any barbecue works out cleanly on quantities. Yet with a few rules of thumb, the shopping list can be planned surprisingly…

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2026-07-25 · 9 min read

Credit Cards on Holiday 2026: Foreign Currency Fees, the DCC Trap and How to Dodge Them

A credit card is the most convenient way to pay when you travel – and a quiet fee machine at the same time. Between the foreign transaction surcharge, the notorious DCC trap at the terminal and pricey ATM charges…

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2026-07-24 · 9 min read

CSV Injection in 2026: When an Export Turns Excel Into a Security Hole

A CSV file is widely seen as the most harmless format there is: plain text, no code, no macros. That is exactly what makes it dangerous. The moment a user opens the export in Excel, LibreOffice, or Google Sheets, a cell…

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2026-07-23 · 9 min read

Cabin Bags 2026: Sizes, Weight by Airline and the EU Fight Over Free Carry-On

A cabin bag that was free yesterday can cost you 70 euros at the gate today – and that is exactly what the fight between the EU Parliament and the budget airlines is about. On 7 July 2026 the Parliament decided…

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2026-07-22 · 9 min read

Trip Cancellation Insurance 2026: When It Actually Pays – and When It Does Not

Trip cancellation insurance sounds like a worry-free package: if something comes up, the insurer covers the cancellation fees. In practice, everything hinges on one phrase in the fine print – the covered reason…

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2026-07-21 · 9 min read

Summer Jobs in Germany 2026: Minijob 603-Euro Cap, Short-Term Work, Taxes and Take-Home Pay

Summer 2026, and plenty of pupils and students in Germany want to turn their free weeks into cash. But a summer job is not just a summer job: whether you work as a minijobber under the new 603-euro cap, as a short-term…

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2026-07-20 · 9 min read

Instant-Access and Fixed-Term Savings After the ECB Hike: Where Rates Stand in July 2026

The ECB surprised markets in June 2026 by hiking instead of cutting, and instant-access savings accounts are suddenly paying over four percent again. But the headline rates often last only a few months and only for new…

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2026-07-19 · 8 min read

World Cup 2026 Final: Kickoff Is 9 PM in Germany – Every Time Zone Explained

On Sunday, July 19, 2026, the World Cup final kicks off at MetLife Stadium near New York. Kickoff is 3:00 PM Eastern Time on the US East Coast – which means 9:00 PM in Germany. For a World Cup final in North…

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2026-07-18 · 9 min read

Prime Day and Summer Sales 2026: How to Spot a Real Discount

Amazon ran Prime Day 2026 from June 23 to 26 – but the sale season is far from over, it stretches across the whole summer. These are exactly the weeks when retailers lean on struck-through prices, countdown timers…

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2026-07-17 · 9 min read

Renting a Car Abroad in 2026: Excess, Deposit and the Costly Traps

A rental car with »full coverage« sounds like a worry-free package – until a scratch in the parking garage suddenly costs you 1,500 euros off your credit card. The reason is the excess, and it hides in almost…

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2026-07-16 · 8 min read

Is a Balcony Solar System Worth It in 2026? The Honest Payback Math

A plug-in balcony solar system costs between 300 and 600 euros in 2026, the 800-watt limit applies, registration takes under 20 minutes, and in Germany tenants now have a genuine legal right to install one. Sounds like…

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2026-07-15 · 8 min read

German Tax Return 2025 at the Last Minute: July 31, 2026 Deadline and the 7 Costliest Mistakes

On July 31, 2026 the deadline for the 2025 German income tax return runs out – at least for everyone required to file who does it without a tax advisor. That leaves barely two weeks. Rush through the forms in a…

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2026-07-14 · 8 min read

E-Invoicing in Germany 2026: XRechnung, ZUGFeRD and the Deadlines to 2028

Since 1 January 2025, every company in Germany must be able to receive B2B e-invoices – with no transition period at all. For sending them, staggered deadlines run through to 2028. Yet one misconception persists…

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2026-07-13 · 8 min read

Travel Health Insurance 2026: Why 10 Euros a Year Covers Your Costliest Vacation Risk

A broken leg in Mallorca is annoying but affordable. A heart attack in Florida followed by an air ambulance back to Germany can run into six figures – and your statutory health fund pays none of it. This is…

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2026-07-12 · 8 min read

Mortgage Rates Mid-2026: Wait or Lock In? The Honest Rate Check

German mortgage rates for a ten-year fixed period are back above three percent in July 2026, and the ECB surprised markets in June by hiking instead of cutting. If you are financing a home right now, the question is…

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2026-07-11 · 8 min read

Checking and calculating an IBAN: what the 22 characters really mean

An IBAN is more than a long string of digits: the 22 characters of a German account number hold a country code, a check digit, a bank code and an account number, all built according to fixed rules. Once you understand…

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2026-07-10 · 8 min read

GiroCode explained: how the QR code for SEPA transfers works

More and more invoices, donation flyers and club letters now carry a small QR code labeled "GiroCode". Scan it with your banking app and you no longer have to type in the IBAN, amount and payment reference by hand: the…

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2026-06-17 · 8 min read

Germany inflation May 2026: 2.6 % CPI, 2.7 % HICP — what the headlines hide

Today, 17 June 2026, the Federal Statistical Office confirmed the final inflation figures for May 2026: consumer price index (CPI) at +2.6 % year-on-year, harmonised consumer price index (HICP) at +2.7 %. Core inflation…

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2026-06-16 · 8 min read

OKLCH and OKLab in 2026: why modern CSS colour replaces your RGB

Since early 2026 Chrome 111+, Edge 111+, Firefox 113+ and Safari 15.4+ all support oklch(), oklab() and color-mix() — covering over 95 % of browsers worldwide. What was an experimental feature behind browser flags two…

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2026-06-15 · 8 min read

2026 heat pump subsidy: up to 70 % BAFA grant — what actually lands in your pocket

A heat pump in Germany in 2026 costs on average EUR 27,000–32,000 including installation. Sounds like a lot — and it is. But: the BEG (Federal Subsidy for Efficient Buildings) pays back up to 70 % of that. Anyone…

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2026-06-14 · 9 min read

Passkeys vs. passwords 2026: NIST 800-63-4 and the real-world rollout

2026 is the year passkeys jumped from 'cool but who actually uses them?' to 'why do you still have passwords?' The FIDO Alliance reports over 1 billion activated passkeys in June 2026, 48 % of top-100 sites support the…

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2026-06-13 · 6 min read

2026 caffeine study: why your deep sleep suffers even when you fall asleep

In May 2026 a team at the University of Szczecin published a meta-analysis of 32 sleep studies covering 1980 to 2026. The headline finding is subtle but important: even when you fall asleep after the afternoon espresso…

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2026-06-12 · 9 min read

EES & ETIAS 2026: what the new EU border systems change for travellers

Since 10 April 2026 the new Entry/Exit System (EES) has been live at every Schengen external border. In Q4 2026, ETIAS arrives — an online pre-authorisation for visa-exempt travellers. What is genuinely relevant if you…

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2026-06-11 · 9 min read

ECB rate hike 11 June 2026: what +25 basis points mean for you

On Thursday, 11 June 2026, the European Central Bank raised its key rates for the first time in more than a year — all three by 25 basis points. The deposit rate is now at 2.25 %, the main refinancing rate at 2.40 %…

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2026-06-10 · 11 min read

Y2038 — the second millennium bug (June 2026 update)

On 19 January 2038, at 03:14:07 UTC, the signed 32-bit Unix timestamp overflows. That sounds like tomorrow's problem, but in 2026 it is already today's: contracts with 30-year terms, database schemas being frozen now…

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2026-06-09 · 3 min read

QR Codes: From the Toyota Factory to Your Restaurant Menu

Everyone recognizes the black-and-white square: on the restaurant menu, on a parcel label, on the EPC payment in online banking, on a train ticket. What hardly anyone knows: the QR code was not invented for consumers…

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2026-06-08 · 6 min read

Lorem Ipsum: Where the Placeholder Text Comes From and When It Hurts Your Design

Lorem ipsum dolor sit amet, consectetur adipiscing elit is probably the most-read and least-understood sentence in the design world. It looks like Latin but doesn't quite make sense. It has been preinstalled in every…

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2026-06-07 · 8 min read

Markdown in 10 Minutes: The Essential Syntax You Actually Need

Markdown is probably the most productive language youll ever learn -- a few dozen rules, useful for the rest of your career. README files on GitHub, issues on GitLab, messages on Discord, docs in MkDocs, blog posts in…

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2026-06-06 · 8 min read

JSON vs YAML vs TOML: Which Config Format Fits Which Project?

Three letters, three worlds: JSON, YAML, and TOML all solve the same problem -- structured data in plain text -- but they do it in fundamentally different ways. Pick the wrong one for your project and you pay with bugs…

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2026-06-05 · 7 min read

Slugs and Permalinks in 2026: why URL structure still matters

A URL is more than a technical address — it's a tiny piece of brand communication, an SEO signal, a trust lever in a shared tweet, and a page's first impression in Google's search results. Even so, the 'slug' — the…

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2026-06-04 · 9 min read

VAT in Europe 2026: why every country has different rates

19 percent in Germany, 20 percent in France, 27 percent in Hungary, 8.1 percent in Switzerland — and in Luxembourg there is a rate of exactly 3 percent for certain books. Anyone who invoices across Europe, runs an…

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2026-06-03 · 8 min read

Tipping Around the World 2026: Where, When, and How Much

Few topics are as awkward in everyday travel as tipping. Tip 10 percent in New York and you'll get cold looks; tip 10 percent in Tokyo and you insult the server. In Paris tipping is optional, in Berlin expected, in…

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2026-06-02 · 8 min read

Building-Savings vs. ETF Plan in 2026: Is Bausparen Still Worth It?

The Bausparvertrag is one of the most German financial products of all: 28 million contracts are active in Germany, the market share of Schwaebisch Hall, BHW and LBS has been stable for decades, and an ad brochure hangs…

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2026-06-01 · 5 min read

Personal Inflation — Why Your Inflation Rate Is Never 2%

When the evening news reports that inflation stands at 2.4 percent, and at the same time you feel your weekly shop is now a third more expensive than two years ago — that isn't perception bias. It's because the official…

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2026-05-31 · 4 min read

Regex Lookaheads and Lookbehinds — Understand Them in 5 Minutes

Lookarounds are the feature that turn regular expressions from a toy into a professional tool. They let you write conditions like "check whether a word follows without consuming it" or "find a comma that isn't wrapped…

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2026-05-30 · 9 min read

TDEE, BMR & PAL — how many calories you actually need

Three calorie calculators, three results, all different — and none of them tells you why. Behind it sits the math of BMR (basal metabolic rate), PAL (activity factor), and TDEE (total daily energy expenditure), plus…

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2026-05-29 · 10 min read

UUID v4 vs. v7 vs. ULID vs. Snowflake — which ID when?

When creating a new table, most developers reflexively reach for UUIDv4 as the primary key — it's available everywhere, unique, and simple. But v4 has a stubborn performance problem in large tables that UUIDv7 (RFC…

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2026-05-28 · 9 min read

German real estate transfer tax 2026: state-by-state rates and the biggest legal savings

Buying a house or apartment in Germany costs more than the headline price: real estate transfer tax (Grunderwerbsteuer) sits on top, and it varies a lot by state. On a 500,000 EUR property, depending on the state…

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2026-05-27 · 9 min read

Your real hourly wage — why your gross salary lies

60,000 USD a year sounds decent. 40 hours a week times 4.33 weeks a month — quick math, that's around 30 USD per hour gross. But if you count honestly — commute, unpaid overtime, ironing shirts, decompressing on the…

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2026-05-26 · 9 min read

What counts as good contrast? WCAG 2.2, AA, AAA, and why light grey on white isn't enough

Designers love elegant light grey on white. Users with low vision, older people, or anyone reading their phone in summer sunshine hate it. The Web Content Accessibility Guidelines (WCAG) have provided concrete numbers…

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2026-05-25 · 9 min read

How long does caffeine really last? Half-life, sleep quality, and the last cup of the day

You have an espresso at 3 pm, feel fine in the evening, and then wonder why you're staring at the ceiling at 2 am. Pharmacokinetics are to blame: caffeine has a surprisingly long half-life, and most people underestimate…

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2026-05-24 · 9 min read

Compound interest & the Rule of 72 — why time is your biggest lever when saving

Albert Einstein allegedly called compound interest the eighth wonder of the world — the quote is unverified, but the math behind it is not. Whoever starts saving early gets a head start that even double the monthly…

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2026-05-23 · 9 min read

How secure is my password, really?

Passwords are like shoes: at some point you have too many, they often pinch, and once in a while you lose one. But unlike shoes, passwords protect access to our digital existence — email, banking, taxes, photos. How…

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2026-05-22 · 9 min read

Understanding and writing cron expressions correctly

Cron expressions are a strange mix of brilliant and cryptic. Five fields with digits and special characters are enough to describe arbitrarily complex schedules — from a daily backup at 03:15 to "every other Tuesday of…

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2026-05-21 · 4 min read

How do JSON Web Tokens (JWT) work?

JSON Web Tokens are everywhere in modern web APIs: single sign-on, OAuth flows, OpenID Connect, serverless authentication — almost everything touches this compact token format. Behind the cryptic-looking string made of…

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2026-05-20 · 9 min read

Understanding the Schengen 90/180 rule

The Schengen 90/180 rule sounds simple at first: stay in for 90 days, then out. In practice, the bookkeeping is trickier, because the 180-day window isn't fixed but rolls forward each day. Anyone traveling visa-free…

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2026-05-19 · 10 min read

BMI vs. body fat percentage — which is more meaningful?

The Body Mass Index is one of the best-known fitness metrics. A single number derived from height and weight — quick to compute, easy to compare. But it has blind spots. Anyone who lifts regularly has probably noticed…

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2026-05-18 · 10 min read

Why SHA-256 is more secure than MD5

Hash algorithms are the quiet workhorse behind file integrity, password storage, digital signatures, and blockchains. But not all hash functions are equally secure: MD5, ubiquitous for decades, is now considered broken…

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2026-05-17 · 8 min read

Save 320 € a month for three years — then ride German rail free for life

Since January 2026 Germany's flat-rate transit pass, the Deutschlandticket, costs 63 euros per month — and from 2027 onwards its price will track an index of labour and energy costs, which all but guarantees further…

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