Dialog RSA Demonstration

You can access this dialog either via the menu entry Crypt/Decrypt \ Asymmetric \ RSA Demonstration or via the menu entry Indiv. Procedures \ RSA Cryptosystem \ RSA Demonstration.

The most well-known asymmetric cryptosystem is the RSA algorithm, named after its authors, Ron Rivest, Adi Shamir and Leonard Adleman. In this dialog you can carry out all three steps of the RSA cryptosystem:

  1. RSA key generation ,

  2. RSA encryption of messages and

  3. RSA decryption of messages,

    and also perform a

  4. Factorisation attack on RSA.

For RSA encryption and RSA signature verification it is sufficient to enter the public parameters:

In this case the secret parameters (the Euler phi function of N and the secret key) don't need not to be known.

Within the upper group box RSA using the private and public key – or using only the public key you can indicate how much information is known.

The following default settings are used after each start of CrypTool:

The values of the prime numbers, the RSA parameters and the options are retained if the dialog is closed and invoked again, as long as the CrypTool application is not terminated. So you do not have to enter them again if they shall be used a second time. But you always can modify them.

In order to confidentially transmit a message, the public key of the recipient must be known. The procedure of encoding the message with the recipients public key for privacy reasons is called encryption.

It is a special property of the RSA cryptosystem, that digital signatures can be created with same operation, if the private key of the signer is used instead of the public key of the recipient. Everyone can then verify using the holder's public key, that the signature was generated by the holder of the corresponding private key.

The story, The Dialogue of the Sisters [c't magazine, issue 25/1999], describes a variant of the RSA cryptosystem that is suitable for hand calculation. This variant can be selected under Options for the alphabet and the number system after a small RSA key has been generated.

The example below illustrates the RSA encryption of the following message:

"Sell my shares on 14 July 2000 and use the sales revenues to buy me a South Sea island".

rsa_demo_1.gif

This encryption operation needs the following steps:

First of all the two prime numbers p and q have to be chosen (the simplest way to achieve this is to click on Generate prime numbers), then the public key e has to be entered. After that the secret key d can be calculated by clicking on "Update parameters".

Prior to carrying out the encryption or decryption you can make further adjustments to the settings for encryption in the Options for the alphabet and the number system dialog:

To initiate encryption, enter the plaintext and then click on the Encrypt button.