How to encrypt with private key and decrypt with public key in c# RSA

Question

I found several solutions where I can use the .Net RSA Provider to Encrypt a message with the public key and Decrypt it with the private one.

But what I want to have is to Encrypt with the private key and Decrypt with the public key.

I want t store the public key in my app and encrypt a license for example on my dev machine with the private key, send it to the app and let the information decrypt with a public key.

How can I achieve that?

You can't. You can only encrypt with the public key and decrypt with the private key. What you mean is signature and verification, which is a valid application for a license. You get authentication, but no confidentiality. If you want to make that interactive you can of course achieve more. — Artjom B.
– Artjom B., Commented Dec 20, 2014 at 9:12
Asymmetric encryption/decryption: everyone can encrypt (using a public key), only the host can decrypt (using a private key). Asymmetric signature/verification: only the host can sign (using a private key), everyone can verify (using a public key). — barak manos
– barak manos, Commented Dec 20, 2014 at 9:23
@Zoya If you think that is a good article then you've just proven you completely missed the point about cryptography in general. Strange, your answer seems to indicate otherwise. — Maarten Bodewes
– Maarten Bodewes, Commented Dec 20, 2014 at 10:13
@Maarten Why? I read it and looked and the sample code. It was clear for me. I could understand how it works. But it would be great if you could explain a little bit why this code is bad or wrong about cryptography. ( i have to add that i have no knowledge about cryptography ) — STORM
– STORM, Commented Dec 20, 2014 at 21:20
The security proofs of RSA rely on the padding used. This one uses a few (too few) bytes of padding, and the proprietary scheme has no security proof. — Maarten Bodewes
– Maarten Bodewes, Commented Dec 20, 2014 at 21:23

StayOnTarget · Accepted Answer · 2022-07-02 14:10:40Z

16

You can use signature generation, In which the private key is used to generate a signature that verifies that your message is authentic.

// Create message and signature on your end
string message = "Here is the license message";

var converter = new ASCIIEncoding();
byte[] plainText = converter.GetBytes(message);

var rsaWrite = new RSACryptoServiceProvider();
var privateParams = rsaWrite.ExportParameters(true);

// Generate the public key / these can be sent to the user.
var publicParams = rsaWrite.ExportParameters(false);

byte[] signature =
    rsaWrite.SignData(plainText, new SHA1CryptoServiceProvider());

// Verify from the user's side. Note that only the public parameters
// are needed.
var rsaRead = new RSACryptoServiceProvider();
rsaRead.ImportParameters(publicParams);
if (rsaRead.VerifyData(plainText,
                       new SHA1CryptoServiceProvider(),
                       signature))
{
    Console.WriteLine("Verified!");
}
else
{
    Console.WriteLine("NOT verified!");
}

you can take further help from HERE

edited Jul 2, 2022 at 14:10

StayOnTarget

13.4k11 gold badges65 silver badges116 bronze badges

answered Dec 20, 2014 at 9:32

Syeda Zunaira

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6 Comments

Maarten Bodewes Over a year ago

I've removed the term "signature strategy" as I've never heard of signature generation to be described that way.

Kraang Prime Over a year ago

What if you wish to actually encrypt the data -- not just validate it's authenticity using a hash ?

KevinM Over a year ago

@Syeda "Generate the public key / these can be sent to the user" Can you explain how ? What do I do with the RSAParameters object ? Thanks

Rubens Farias Over a year ago

link is now dead

StayOnTarget Over a year ago

@RubensFarias I added a link to an archive copy

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