data size and RSA encryption

19 Sep 2005 4:34 PM

Tim Wallace

I am getting an error stating "...data to be encrypted exceeds the maximum
for this modulus of 256 bytes". I am at a loss as to how this could be, as
I'm only attempting to encrypt less than 30 characters. Does anyone have
any ideas as to what could cause this error? I'm creating my keys in a
separate application like so:

RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(2048);
string rsaXml = rsa.ToXmlString(true);
StreamWriter sw = new StreamWriter("c:\\some_file_ name.key");
sw.Write( rsaXml );
sw.Close();

Then, in the application where I encrypt, I do it like so:

RSACryptoServiceProvider rsacspServer = new RSACryptoServiceProvider(2048);
string rsaXml = string.Empty;
StreamReader sr = new StreamReader("c:\\some_file_name.key");
rsaXml = sr.ReadToEnd();
sr.Close();
rsacspServer.FromXmlString(rsaXml);

Next, I encrypt like this: rsacspServer.Encrypt( a, false ); // a is a
byte[]

Over in the app that is decrypting (the bytes are being sent using
TCPClient), I do the same as the encrytion, save for calling Decrypt.

Any ideas?

Tim

19 Sep 2005 6:16 PM

Valery Pryamikov

Tim,
you should not use RSA for encrypting data, but rather use it for encrypting
symmetric encryption key that you use for encrypting your data (as it for
eg. done here:
http://msdn.microsoft.com/library/en-us/cpguide/html/cpcongeneratingkeysforencryptiondecryption.asp
or here:
http://msdn.microsoft.com/library/en-us/dncapi/html/encryptdecrypt2a.asp). I
would not comment on the rest of your post - since I've only briefly looked
over your code.

-Valery.
http://www.harper.no/valery

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"Tim Wallace" <twallace-ThisDoesNotBelong-AT-emailDOTcom> wrote in message
news:uP2SOgTvFHA.2312@TK2MSFTNGP14.phx.gbl...
>I am getting an error stating "...data to be encrypted exceeds the maximum
>for this modulus of 256 bytes". I am at a loss as to how this could be, as
>I'm only attempting to encrypt less than 30 characters. Does anyone have
>any ideas as to what could cause this error? I'm creating my keys in a
>separate application like so:
>
> RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(2048);
> string rsaXml = rsa.ToXmlString(true);
> StreamWriter sw = new StreamWriter("c:\\some_file_ name.key");
> sw.Write( rsaXml );
> sw.Close();
>
> Then, in the application where I encrypt, I do it like so:
>
> RSACryptoServiceProvider rsacspServer = new
> RSACryptoServiceProvider(2048);
> string rsaXml = string.Empty;
> StreamReader sr = new StreamReader("c:\\some_file_name.key");
> rsaXml = sr.ReadToEnd();
> sr.Close();
> rsacspServer.FromXmlString(rsaXml);
>
> Next, I encrypt like this: rsacspServer.Encrypt( a, false ); // a is a
> byte[]
>
> Over in the app that is decrypting (the bytes are being sent using
> TCPClient), I do the same as the encrytion, save for calling Decrypt.
>
> Any ideas?
>
> Tim
>

19 Sep 2005 6:29 PM

Tim Wallace

Valery:

Thank you for your reply. Usage notwithstanding, what would cause this
error? I'm encrypting much less than if I were to be encrypting a symmetric
key. Any thoughts?

Tim

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"Valery Pryamikov" <val***@harper.no> wrote in message
news:urStbZUvFHA.3080@tk2msftngp13.phx.gbl...
> Tim,
> you should not use RSA for encrypting data, but rather use it for
> encrypting symmetric encryption key that you use for encrypting your data
> (as it for eg. done here:
> http://msdn.microsoft.com/library/en-us/cpguide/html/cpcongeneratingkeysforencryptiondecryption.asp
> or here:
> http://msdn.microsoft.com/library/en-us/dncapi/html/encryptdecrypt2a.asp).
> I would not comment on the rest of your post - since I've only briefly
> looked over your code.
>
> -Valery.
> http://www.harper.no/valery
>
> "Tim Wallace" <twallace-ThisDoesNotBelong-AT-emailDOTcom> wrote in message
> news:uP2SOgTvFHA.2312@TK2MSFTNGP14.phx.gbl...
>>I am getting an error stating "...data to be encrypted exceeds the maximum
>>for this modulus of 256 bytes". I am at a loss as to how this could be,
>>as I'm only attempting to encrypt less than 30 characters. Does anyone
>>have any ideas as to what could cause this error? I'm creating my keys in
>>a separate application like so:
>>
>> RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(2048);
>> string rsaXml = rsa.ToXmlString(true);
>> StreamWriter sw = new StreamWriter("c:\\some_file_ name.key");
>> sw.Write( rsaXml );
>> sw.Close();
>>
>> Then, in the application where I encrypt, I do it like so:
>>
>> RSACryptoServiceProvider rsacspServer = new
>> RSACryptoServiceProvider(2048);
>> string rsaXml = string.Empty;
>> StreamReader sr = new StreamReader("c:\\some_file_name.key");
>> rsaXml = sr.ReadToEnd();
>> sr.Close();
>> rsacspServer.FromXmlString(rsaXml);
>>
>> Next, I encrypt like this: rsacspServer.Encrypt( a, false ); // a is a
>> byte[]
>>
>> Over in the app that is decrypting (the bytes are being sent using
>> TCPClient), I do the same as the encrytion, save for calling Decrypt.
>>
>> Any ideas?
>>
>> Tim
>>
>