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Learn how to effectively create an RSA key from raw data using OpenSSL 3.0 in C, with detailed guidance on setting up parameters for successful cryptographic operations.
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How to Properly Create an RSA Key from Raw Data in OpenSSL 3.0 using C Language

In the world of cryptography, handling key creation correctly is critical, particularly when transitioning to a new library version, like moving from OpenSSL 1.1.1 to 3.0. If you're finding yourself stuck while trying to create an RSA key from raw binary data, you're not alone. This guide will guide you through the solution effectively.

The Problem

When creating an RSA key from binary modulus and exponent data, developers often face issues, especially when the process does not yield usable keys for signature decryption or verification.

The Scenario

Suppose you have the following:

Modulus: A binary representation of the RSA modulus.

Exponent: A binary array representing the public exponent.

You might think you have followed the manual correctly, as you’ve initialized your context and set up your modulus and exponent as BIGNUM objects. Yet attempts to perform cryptographic operations on this key fail.

The following pseudo-code illustrates the challenge you're encountering:

[[See Video to Reveal this Text or Code Snippet]]

Your code runs without apparent errors but fails at the crucial stage of using the key.

The Solution

The error lies in how the OSSL_PARAM array is constructed. It must be built using the OSSL_PARAM_BLD API. The manual's omission of this detail can lead to confusion and error.

Step-by-Step Implementation

Follow these steps to correctly build your parameters and create the RSA key:

Create a Parameter Builder:
Utilize the OSSL_PARAM_BLD to create the parameter set.

[[See Video to Reveal this Text or Code Snippet]]

Push Parameters:
Push your modulus (n) and exponent (e) into the builder correctly.

[[See Video to Reveal this Text or Code Snippet]]

Convert Builder to Parameters:
Convert the builder to the parameter array that can be used by the key creation function.

[[See Video to Reveal this Text or Code Snippet]]

Create the Key:
Finally, use the built parameter set to create your key.

[[See Video to Reveal this Text or Code Snippet]]

Important Reminders

Check Parameters: Make sure you are aware of what parameters are settable for the type of key you're creating. This knowledge can save you significant debugging time.

If you face issues while using other types of keys, the same OSSL_PARAM_BLD method is likely applicable.

Conclusion

By building the OSSL_PARAM array using the proper builder function, you can successfully create an RSA key from raw data in OpenSSL 3.0. Understanding how to navigate the changes in the library can greatly enhance your development experience and ensure your cryptographic operations function as intended. Happy coding!