Note: Everything in this answer relates to US paper currency. Many countries have similar features and laws and security measures.
How do I get undetectable counterfeit money online?
Because everything from the paper to the ink to the printers required to reproduce a high quality counterfeit is nearly impossible to obtain either because of price or exclusive contracts on production of those materials.
First, there’s the paper. Paper for US dollars is a cotton and linen blend that is exclusively produced for the US government. You simply cannot buy it, and to get it right you would have to produce it yourself. Even then, there is a security thread in larger bills woven directly into the bill at the time of production, and a watermark placed in everything larger than the one dollar note that cannot be done later in the process. This paper is produced by the Crane Company, and has been since the mid 1800s (although the features have changed over time, including the new holographic strip in the $100 bill that uses a technique which puts microprinting to shame, more on microprinting a little later). You can get shredded currency from the Treasury, as a souvenir or for art projects, but buying it in any quantity is both expensive and it gets your name put on a list. If you use these shred to make printable paper you are committing a felony.
Here you can see the watermark image of Ben Franklin and the security strips in the $100, $5, and $20 notes. Did I mention that they all glow different colors under UV light?
Second is the ink. US currency uses color gradients in the paper production process and then uses offset printing for the background image which often includes microprinting (this can’t be reproduced with digital printing unless you have a far better printer than pretty much everyone has access to, it requires offset printing or intaglio printing with hyperfine detail), followed by intaglio printing for the portraits, borders, and denominations (which are done with a proprietary color shifting ink on the $10 bill upwards). More microprinting is added in this stage.
Here we see microprinting in the background printing in one of the stars on the $50, and in the intaglio printing on Grant’s collar. This type of microprinting is all over bills, from the border of the five to the sides and tops of several numbers on various bills. If you haven’t noticed already, there is an insane amount of detail in the portraits and backgrounds, including complex cross hatching patterns that you would need to hand etch plates to print.
The next feature may be the absolute coolest, and the least talked about…
Commercial printers, scanners, and photocopiers will not replicate US bills. Photoshop and other image manipulation programs will cease to function if you try to replicate money in them.
This comes down to a pattern on the notes called the EURion constellation. It’s a pattern of rings that major software companies use to detect currency on their devices (I believe Photoshop goes a few steps further, so even if you black out rings, it still recognizes the currency). This deprives would be counterfeiters of their most powerful tools and leaves them at square one: trying to figure out how to replicate the paper closely enough to pass as the real thing which often involves bleaching smaller bills and reprinting them with larger sums, which of course leads us to the fact that…
Counterfeiters still produce fake bills, and relatively convincing ones at that. Without being trained in the security features, and being vigilant about the bills you’re being handed, you could easily mistake a counterfeit bill for a real bill. A lot of the hardest to copy features aren’t routine knowledge or easily checked without a magnifying glass or UV light. It is estimated that roughly $70 million (and possibly up to $200 million) in counterfeit bills are in circulation in the US. That’s about 1 out of every 10,000 bills, and far more common in large denominations. Would you notice a touch of cyan or magenta ink on a fake $100 bill? Here is an excellent article on some more recent counterfeiting operations and the bills they produced.
How It Is Made, How It Moves
Bonus Info: Do you know how those counterfeit detecting pens work? They have an iodine solution that reacts chemically to wood pulp. All the pen really does is check to see if the paper is made of cloth fibers or wood fibers, and can be prevented from working by simply using non-wood paper, or a variety of solvents to prevent the reaction. Add this to the fact that small impurities or changes in the paper makeup for the production of real bills can cause a real bill to present as fake to a pen, and they really aren’t a very strong indicator of counterfeit money. You’re better off buying a small UV light than those pens as a business owner
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