Re: RSACryptoServiceProvider decrypt with public key

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Date: Mon, 24 May 2004 18:04:02 -0700

Hello Martin,

You are correct that it is possible to both encrypt with the public
key/decrypt with the private key and encrypt with the private key/decrypt
with the public key. However, it's a little more complicate than just that.
Here's why:

Your public key is meant for public consumption by anyone. Your private key
must always stay private and must only be known by you.

This basic scheme offers the following scenarios:

If Alice gives Bob her public key (by sending it across the wire), Bob can
encrypt data and send it back to Alice. Because only Alice has the private
key, only she can decrypt Bob's data. Now, let's assume that a third party
named Eve is able to listen in to the conversation. Eve would be able to
see the public key and the encrypted data, but she could not decrypt Bob's
data and listen in. So, the scenario where Bob encrypts data and sends it
to Alice using her public key is a useful way for Bob to share his data
without Eve being able to listen in.

Now let's look at the scenario where Alice encrypts data with her private
key and sends it to Bob. In this situation Alice sends her public key to
Bob and then sends some data to Bob. Bob then uses the public key to
decrypt the data. However, let's assume that Eve is still lurking out there
and that she is able to see both the public key and the encrypted data. In
this situation, Eve has everything she needs to decrypt Alice's data. So,
as you can see, when Alice encrypts data with her private key, there isn't a
way for her to keep the data secret. In fact this scheme buys you
absolutely nothing if your goal is keeping data secret! If Alice wants to
send Bob a secret using asymmetric cryptography, she must use Bob's public
key (not her private key).

But . there is one thing that this second scheme does buy Alice and Bob.
When Alice encrypts data with her private key and sends it to Bob along with
her public key, Bob can verify that the data came from Alice. As long as
Bob trusts that only Alice has access to the private key, he knows that data
that will decrypt with Alice's public key actually came from Alice, and not
from say .Eve. So, imagine if Alice sent Bob a message and then sent a
hashed version of the message that was encrypted with her private key. Bob
could then decrypt the hash and compare it to a hash of the plaintext
version that he generates. If both hashes match, then Bob knows that Eve
did not hijack the message and send him a false one. He knows that this is
an unaltered message that only came from Alice. This is how digital
signatures work!

Now, let's get back to the RSACryptoServiceProvider class. The Encrypt
method ONLY encrypts using the public key and the Decrypt method only
decrypts using the private key. This makes sense because we have already
established that encryption with a private key for the purpose of keeping
data secret is quite pointless.

The RSACryptoServiceProvider.ToXmlString method will return a public key
only if you pass "false". It will return both a public and private key when
you pass "true". So, when you try to decrypt data using the
RSACryptoServiceProvider.Decrypt method, but you only initialize the
RSACryptoServiceProvider using an XML file with a public key, you will
always get an exception because there is no private key to perform
decryption. This is the expected behavior, but hopefully it makes more
sense now.

You can use the RSACryptoServiceProvide class to create a digital signature
using methods like SignData, SignHash, VerifyData, and VerifyHash. These
are the methods that use the private key to encrypt a value (a hash) and a
public key to verify the value, but they are designed to be used only for
digital signatures. They can't be used to encrypt arbitrary data with a
private key and decrypt the data with a public key.

As always, there are lots of caveats to cryptography, so the real story is
slightly more complicated than what I have presented. However, I hope this
clears things up. Let me know if you have any more questions.

Stephen McCloskey

Programmer/Writer

.NET Framework SDK

"Martin M?ller" <mav.northwind@web.de> wrote in message
news:e81857a.0405190403.2fab6118@posting.google.com...
> Dear community, please help:
> For several days now I've been trying to implement something that
> should be possible according to all the sources you find on asymmetric
> encryption but just can't get it to work.
>
> The main idea behind asymmetric encryption is that there's a public
> and a private key and that the public key can be derived from the
> private key, but not the other way round.
> Data encrypted with the public key can be decrypted using the private
> key.
> Data encrypted with the private key can be decrypted using the public
> key.
>
> The first way (encrypt with public, decrypt with private) is shown in
> several examples you can find, but the other one (encrypt with
> private, decrypt with public) doesn't seem to work and I can't find a
> working example for it either.
>
> I'm using .NET 1.0 on a WinXP machine and tried the following:
> Create a new RSACryptoServiceProvider, save each key to a separate
> file using ToXmlString().
>
> For encryption I read the private key file, use a new
> RSACryptoServiceProvider's FromXmlString() method to set the
> parameters just read and encode the data to a Base64 string.
> The data encrpyted is very short (about 20 bytes) and key length is
> sufficient (1024, but you get the same result with other key lengths).
> Encryption works fine.
>
> Now I create a new CSP like before, read the public key file, convert
> the Base64 string to a byte array again and try to decrypt it and I
> always get a CryptographicException stating "invalid key", if I don't
> use OAEP padding and "Error occurred while decoding OAEP padding."
> when using padding.
>
> The other way (encrypt with public and decrypt with private) does
> work, as does encryption and decryption both with the private key, but
> if I always need the private key what's the point of asymetric
> encryption ?
>
> Can anyone enlighten me? Is it a bug in .NET 1.0? Does 1.1 behave
> differently? What other options do I have?
>
>
> Here are the code fragments I described above:
> ---------------- Create a key pair -------------------------------
> RSACryptoServiceProvider csp = new RSACryptoServiceProvider(1024);
>
> string s = csp.ToXmlString(true);
> StreamWriter sw = new StreamWriter("KeyPriv.xml");
> sw.WriteLine(s);
> sw.Close();
>
> s = csp.ToXmlString(false);
> sw = new StreamWriter("KeyPub.xml");
> sw.WriteLine(s);
> sw.Close();
>
> csp.Clear();
> ------------------------------------------------------------------
> ---------------- Encrypt with private key ------------------------
> RSACryptoServiceProvider csp = new RSACryptoServiceProvider(1024);
>
> StreamReader sr = new StreamReader("KeyPriv.xml");
> string s = sr.ReadToEnd();
> sr.Close();
> csp.FromXmlString(s);
>
> byte[] inp = System.Text.Encoding.Unicode.GetBytes(clearText);
> byte[] outp = csp.Encrypt(inp, false);
> cypherText = Convert.ToBase64String(outp);
> csp.Clear();
> ------------------------------------------------------------------
> ---------------- Decrypt with public key -------------------------
> RSACryptoServiceProvider csp = new RSACryptoServiceProvider(1024);
>
> StreamReader sr = new StreamReader("KeyPub.xml");
> string s = sr.ReadToEnd();
> sr.Close();
> csp.FromXmlString(s);
>
> byte[] inp = Convert.FromBase64String(cypherText);
> // This always throws an exception "invalid key" :(((
> byte[] outp = csp.Decrypt(inp, false);
> clearText = System.Text.Encoding.Unicode.GetString(outp);
> csp.Clear();
> ------------------------------------------------------------------

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