see this

https://www.c-sharpcorner.com/article/best-algorithm-for-encrypting-and-decrypting-a-string-in-c-sharp/

In C# and C++, the IDEA encryption algorithm is not natively supported by the standard libraries. However, you can use third-party libraries or cryptographic providers to add IDEA encryption functionality to your applications. Here are a few options you can consider:

These libraries offer various encryption algorithms, including IDEA, and provide APIs that you can use to integrate the encryption functionality into your C# or C++ applications. Make sure to carefully review the documentation and examples provided by each library to understand how to use IDEA encryption effectively and securely.

also see this

https://www.c-sharpcorner.com/article/introduction-to-aes-and-des-encryption-algorithms-in-net/

IDEA (International Data Encryption Algorithm) is a symmetric-key block cipher that uses a 128-bit key and operates on 64-bit blocks.

You can use the .NET Framework’s cryptographic services to implement IDEA encryption algorithms in C#

You can also use the Aes class in the System.Security.Cryptography namespace to encrypt and decrypt data using symmetric encryption algorithms