How do RSA cryptosystems work?

An RSA cryptosystem is an asymmetric encryption that is used in online data transfers due to its ease of use. It consists of a public and a private RSA key. The public key is used for encryption and its private counterpart for decryption. Since there is no algorithm that can determine the private key from the public key, the method is considered secure. In addition to encryption, the RSA cryptosystem is also suitable for generating digital signatures.

The most important feature of asymmetric encryption is that there is not a single key to re-decrypt a file, for example, but two. In RSA encryption, there is a public key, which is freely accessible, and a private key, which should preferably only be known to a single person. The original encryption is done with the public RSA key. However, the private RSA key is required for decryption. If the key is lost, the encryption can almost certainly not be broken.

The basis of the RSA cryptosystem is the trapdoor function known from mathematics. This states that a function can only be reversed if additional information is introduced. If this information isn’t provided, there are too many variables, meaning a solution cannot be calculated in a realistic time frame. In the case of the RSA cryptosystem, the private RSA key contains this additional information.

RSA encryption is used in many areas of everyday digital life. For example, the communication protocol HTTPS (Hypertext Transfer Protocol Secure) or SSL certificates are in most cases secured with an RSA encryption. An RSA cryptosystem is also suitable for encrypting emails, digital messengers, image data, or a hard disk. Since decryption would take too much time even at the highest computing power without the necessary RSA key, the method is considered comparatively secure.

In many cases, an RSA cryptosystem is combined with other methods of encryption to ensure that content or data cannot be opened or read without authorization. Even if you convert your website to SSL, in most cases a combination of different encryptions is used.

In addition to RSA encryption, which is used, for example, in SSL or TLS protocols the technique is also suitable for creating a digital signature. To confirm the authenticity of a message or file, and to also ensure that the file has not been subsequently edited, the signature is created using the private RSA key. The public key can then be used to check whether the signature and the corresponding file still match.