Why You’re Getting No Module Named Crypto and How to Actually Fix It

You’re staring at your terminal. The code looks perfect. You’ve imported the library exactly like the tutorial said, but Python keeps screaming that there is no module named 'crypto'. It’s frustrating. It feels like the computer is gaslighting you because you know you installed something related to cryptography.

Honestly, this is probably the single most common headache for Python developers working with encryption. It’s a mess of naming conventions, legacy packages, and a weird quirk in how Python handles case sensitivity in its import system.

The problem isn't usually that you forgot to install a library. Usually, the problem is that you installed the wrong one, or you’re trying to call it by the wrong name. Python’s packaging ecosystem, specifically regarding the Python Cryptography Toolkit, has a messy history that dates back over a decade.

The Core Conflict: PyCrypto vs. PyCryptodome

If you are seeing the no module named 'crypto' error, you likely tried to run pip install crypto. Here is the thing: that package on PyPI (the Python Package Index) is almost certainly not what you want.

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Years ago, the standard was a library called PyCrypto. It was the go-to. But it died. The last official release was in 2013. It has known security vulnerabilities that will never be patched. Because it’s abandoned, newer versions of Python—especially Python 3.9 and beyond—struggle to even compile it from source.

Then came PyCryptodome. This is a "fork" of PyCrypto. It’s actively maintained, it’s faster, and it’s safer. But here is the kicker that trips everyone up: even though the package is named pycryptodome, you still import it in your code using import Crypto.

Wait, did you notice the capital "C"?

Python is case-sensitive. If you type import crypto (lowercase), but the folder in your site-packages is named Crypto (uppercase), Python might lose its mind depending on your operating system. Linux is strictly case-sensitive. Windows and macOS can be a bit more forgiving, but you shouldn't rely on that.

The "Crypto" Package Trap

There is actually a package on PyPI literally named crypto. If you ran pip install crypto, you didn't get a cryptography library. You got a tiny, mostly useless package that does almost nothing related to AES, RSA, or hashing. It’s basically a name-squatter.

If you have that installed, it will conflict with the real tools you need. You have to get rid of it. Run pip uninstall crypto immediately. Seriously. Do it now. Then, you need to uninstall pycrypto if you have that lingering around. Clean the slate.

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Solving the Error Once and for All

To get things working, you usually need one of two libraries: PyCryptodome or Cryptography.io.

1. The PyCryptodome Path

Most people getting the no module named 'crypto' error are trying to use PyCryptodome. To fix it, use these commands in your terminal:

pip uninstall crypto pycrypto pycryptodome

pip install pycryptodome

Now, look at your Python script. Your import statement must look like this:
from Crypto.Cipher import AES

Notice the capital C. If you use a lowercase 'c', it will fail. This is the "Aha!" moment for about 90% of developers.

2. The Cryptography.io Path

If you aren't tied to a specific tutorial, many experts—including the folks at the Python Packaging Authority—recommend a library simply called cryptography. It’s the modern standard.

Install it with pip install cryptography.

The catch? It doesn't use the Crypto namespace at all. You would import it using from cryptography.fernet import Fernet. If your project specifically demands import Crypto, this library won't satisfy that requirement without a total code rewrite.

Why Case Sensitivity and Namespaces Matter

Python handles modules by looking through a list of directories called sys.path. When it finds a folder that matches your import statement, it stops looking.

If you have a file in your own project folder named crypto.py, and you try to import Crypto, Python might get confused. This is called "shadowing." Never name your own scripts the same thing as a major library. It’s a recipe for a 2:00 AM debugging session that ends in tears.

On Linux systems, crypto and Crypto are two entirely different animals. If your code was written on a Mac where the filesystem is often case-insensitive, it might work fine. Then you deploy it to an AWS Ubuntu server and suddenly—boom—no module named 'crypto'.

Virtual Environments: Your Best Friend

If you’ve done the uninstalls and reinstalls and it still isn't working, your Python paths are likely crossed. You might be installing the library for Python 3.10 while your IDE is running Python 3.12.

This is why we use virtual environments.

Stop installing things globally. It breaks things. Navigate to your project folder and run python -m venv venv. Activate it. On Windows, that’s .\venv\Scripts\activate. On Mac or Linux, use source venv/bin/activate.

Once you’re inside that clean bubble, run pip install pycryptodome. Now, when you run your script, the environment knows exactly where the Crypto folder is. There is no ambiguity. No competition from other versions.

Summary of Actionable Steps

Fixing this isn't about writing better code; it's about cleaning up a messy environment. Follow this specific sequence to resolve the issue:

• Scrub the old stuff: Run pip uninstall crypto pycrypto pycryptodome to ensure no conflicting versions exist.
• Install the right fork: Use pip install pycryptodome for the most compatible experience with older tutorials.
• Check your casing: Ensure your code says from Crypto... and not from crypto....
• Verify the installation: Open a python shell and type import Crypto; print(Crypto.__version__). If it prints a version number without an error, you've won.
• Use a Requirements file: Once it works, run pip freeze > requirements.txt. This ensures that when you move to a new machine, you can just run pip install -r requirements.txt and avoid this entire headache in the future.

The "no module named 'crypto'" error is a rite of passage. It's a sign you're moving into more complex Python development involving security and data integrity. Once you move past the naming confusion, you can actually get back to the real work of encrypting your data.