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How to encrypt client private key using server public key rsa in C#

This is a piece of my code that encrypts a private key:
string pemContent = File.ReadAllText(pemPath);
csp.ImportFromPem(pemContent);
string test = rsa.GetPrivateKey();
var data = Encoding.UTF8.GetBytes(test);
var cypher = csp.Encrypt(data, false);
Console.WriteLine(Convert.ToBase64String(cypher));
This is the GetPrivateKey() function:
public string GetPrivateKey()
{
return rsa.ToXmlString(true);
}
I get this error:
Internal.Cryptography.CryptoThrowHelper.WindowsCryptographicException: 'Bad Length.'
I know by now that private key is to big to encrypt it with client public key and the question is how to make is possible?
I can't find anything similar to what I am doing, the only clue I have is that wannary used the same technique while its file encryption process.
I use RSACryptoServiceProvider to handle rsa encryption.
EDIT:
Maybe i should describe my case more in detail. I am building a simple ransomware (i (i am a cybersecurity enthusiast and i do it just to learn how this viruses works in depth, so it's for educational purposes only). More advanced ransomware uses hybrid encryption. The scheme i am trying to implement is well described in this video. I am stuck in the last step which is encrypting client private key with server public key.

I'm presuming RSA here, it's not directly in the question, but it can be concluded from the code. Also, because of the second parameter of Encrypt being false, I'll assume PKCS#1 v1.5 padding.
There are two ways to do this. One you have already mentioned, and it is the best option: use hybrid encryption. You first create a random encryption key, encrypt the RSA key, and then encrypt that key.
The second way is to simply use a larger RSA key pair for the server. PKCS#1 v1.5 padding has a minimum overhead of 11 bytes, 8 of which are non-zero random. It's better to use 16 bytes of random data though, so then you'd have 19 bytes / 152 bits of overhead. For more information on the overhead of RSA encryption see my answer here.
The encoded private key needs to be in the remaining bits. Now it is best to use the minimum amount of bits to encode the private key. The best way to do this is to encode the modulus and only the private exponent (i.e. without the CRT parameters). Each of these will take as many bits as the key size if you use a constant sized, unsigned big endian encoding. So the key pair needs to be klen * 2 + 152 bits.
Note that this is not necessarily the best option as there could be schemes that allow you to never generate the private key on the client in the first place, until the private key needs to be released for decryption that is.

Fast checking for encrypted values by X509Certificate2

I have many different string values in a collection.
Some values were encrypted by X509Certificate2. All other values are numbers, non encrypted strings, dates, etc.
My goal to filter possible candidates for decryption. So I want to use a function that can implement first fast filtration of values that were encrypted.
I use this simple check:
private bool IsEncryptedValue(string value)
{
var result = !string.IsNullOrEmpty(value) && IsBase64String(value);
return result;
}
Please advise more correct (more strong) rules for checking encrypted value.
Thanks for any suggestions.

Ciphertext of any modern cipher is binary. If the ciphertext is in a string I would expect that it has been encoded using base 64 (or a dialect) or hexadecimals. Once you've decoded that it should be exactly be the key size if direct RSA encryption has been used. If a hybrid cryptosystem has been used then it should be at least the key size.
Of course, you will have to find out which RSA encryption scheme was used, and in the case of hybrid encryption you'd have to find out which symmetric encryption scheme was used as well. It might be that the encryption uses a known container format such as CMS or PGP, so you could scan for that as well.
Finally, although it is unlikely: in principle the outcome of RSA encryption is a number. If that number is stored in decimals then it should have keysize / 3.32192809 or fewer digits.
In principle the outcome of a cipher is randomized, so we don't have much to separate it from any other encoding. You can try and validate that it is randomized, but if you've only a small ciphertext then estimating the amount of randomness is relatively tricky - so I've not included that in my answer.
This is a strange endeavor though, generally you'd know if something is a ciphertext or not.

Bad data when RSA Decrypt a text file which saved as in Byte by RSA Encryption

Here is the method for encryption and saving to a file (both saving as String and Byte file)
public void Encrypt()
{
RSACryptoServiceProvider myrsa = new RSACryptoServiceProvider();
System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();//Encode String to Convert to Bytes
string data = tbSerial.Text;//whatever you want to encrypt
Byte[] newdata = encoding.GetBytes(data);//convert to Bytes
Byte[] encrypted = myrsa.Encrypt(newdata, false);
msg = "";
for (int i = 0; i < encrypted.Length; i++)
{
//textBox1.Text = "" + encrypted[i];
string[] msg_temp = { encrypted[i].ToString() };
foreach (string val in msg_temp)
msg += val + " ";
}
File.WriteAllBytes("C:\\Marketing\\Serial.txt", encrypted);
}
This is the decryption method. Yet this caused a problem as Bad data.
public void Decrypt()
{
Byte[] bytelist;
String mgs1 = "";
RSACryptoServiceProvider myrsa = new RSACryptoServiceProvider();
//this code segment Read Byte to file.
bytelist= File.ReadAllBytes("C:\\Marketing\\Serial.txt");
Byte[] decrypted = myrsa.Decrypt(bytelist, false);//decrypt
string dData1 = System.Text.Encoding.UTF8.GetString(decrypted); //Encode String to Convert to Bytes
for (int i = 0; i < decrypted.Length; i++)
{
string[] mgs1_temp = { dData1[i].ToString() };
foreach (string val in mgs1_temp)
mgs1 += val;
}
MessageBox.Show("Decrypted Data: " + mgs1);
}
=====================================
Byte[] decrypted = myrsa.Decrypt(bytelist, false);//decrypt ==> **Bad data appears here!**
After encrypting and saving the file, the decrypting cannot read the file. Therefore, as I thought, RSACryptoServiceProvider caused this problem. Can anyone give me some advice?

You should either base 64 encode your ciphertext or you should rely on a binary encoded file. Currently you are storing string representation of bytes as ciphertext. In the decryption method you handle them as bytes which is not correct.
Note that using just RSA is not correct for text. RSA can only encrypt a limited amount of plaintext. Instead, hybrid encryption is normally used. A random AES key is used to encrypt the data, then this key is encrypted with the RSA public key.

A few facts about cryptography and your particular piece of code that may help you:
RSA is NOT for data encryption, it is for signatures. Being an asymmetric cipher, it was designed to serve for message signing purposes, and not encryption. While it generally works for encryption, too, it's applicability is limited. You should you a symmetric cipher like AES. Further reading: Schneier, Bruce, Applied Cryptography. Also see update below.
You don't provide encryption keys anywhere in your code. Think how it could work without keys? Encryption algorithms require, maybe quite surprisingly, two inputs Encrypt(data, key) / Decrypt(data, key). You've provided just the data portion.
Why, why, lord WHY (←this emphasis is here to underline the importance of distinguishing between text and binary data) do you store BINARY data as TEXT? Learn to store BINARY as BINARY, unless you have to store them in a generally textulal file format like XML, in which case let the XML serializer decide how best to store a chunk of BINARY data.
Go again through your code and think whether what you pass as input to the decryption routine actually is the output of the encryption. Generally, when there's a sequence of transformation, inverse transformations in (generally) reverse order must be applied to turn a output into a input. Like this:
input -> A -> B -> C -> output
output -> C^-1 -> B^-1 -> A^-1 -> input
So, your encryption algorithm does:
covert string ("ABC") to byte[] -> encrypt -> covert byte[] to string ("120 130 140")
but your decryption algorithm is far from the inverse, which should be:
convert string ("120 130 140") to byte[] -> decrypt -> covert byte[] to string ("ABC")
UPDATE: Further commentary on (1) for nitpickers, downvoters, and curious readers. When the authors of RSA first published their paper, they stated two important problems that had to be solved:
establishing privacy (=encryption of messages),
establishing signatures.
While the nitpickers and downvoters may claim I do not understand RSA is an ecryption algorithm per se, a smart reader quickly recognizes—typically from the usage of the word cipher—I actually do.
However, as Mr. Rivest, Mr. Shamir, and Mr. Adleman state in their paper in section III Privacy, i.e. in respect to encryption of messages, the cipher has certain properties that make it less practical for common data encryption in comparison to symmetric ciphers:
A public-key cryptosystem can be used to “bootstrap” into a standard encryption scheme such as the NBS method. Once secure communications have been established, the rst message transmitted can be a key to use in the NBS scheme to encode all following messages. This may be desirable if encryption with our method is slower than with the standard scheme.
And yes, the vast majority of real-world RSA applications truly is for things such as key exchange and signatures. Hence for a less-experienced user of cryptography a rule of thumb should be:
Do I need to encrypt data (protect privacy)? ⇒ Symmetric ciphers are the head-start.
Do I need to create an electronic signature? ⇒ Public-key ciphers like RSA.
Plus asking yourself the question “Is there a proven library to solve this problem?” at the very first place. Why? There's a lot more needed to reach acceptable level of security than just a single “cryptoAlg.Encrypt(data, key)” call.
Further downvotes are welcome :-)

RSACryptoServiceProvider doesn't produce consistent output

I need to encrypt some text with RSA, and then recover it later using the private key. My problem is that RSACryptoServiceProvider.Encrypt() outputs a different value every time, even when using the same key. Here is my code which I put into LINQpad to test:
CspParameters cp = new CspParameters();
cp.KeyContainerName = "MyKey";
cp.Flags = CspProviderFlags.UseMachineKeyStore | CspProviderFlags.UseExistingKey;
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(cp);
// using LINQpad to verify the key is loaded properly -- same every time
rsa.ToXmlString(true).Dump();
byte[] rgb = new ASCIIEncoding().GetBytes("Hello world");
byte[] xx = rsa.Encrypt(rgb, false);
string b64 = Convert.ToBase64String(xx);
// this changes every time:
b64.Dump();
I'm guessing that the class must be using something else as well as the key to affect the output, but I'm struggling to find out what.

The fact that the cipher text is different each time the same cleartext is encrypted doesn't mean that it cannot be decrypted consistently.
This is indeed the sign of a good cryptographic algorithm to have be able to have this behavior, making it more resilient to various attacks.
This is because the the encryption logic introduces randomness in the process, for example by systematically adding some random bytes before the cleartext itself. So long as the decryption logic knows to ignore these bytes after the whole ciphertext is decrypted then it can reproduce the original cleartext.
I suggest you take any instance of this b64 text, submit it to the reverse process and see that the "rgb" produced is "Hello world" in all cases.

The different output is perfectly normal. This is due to the fact that your data is being padded by PKCS#1 or OAEP - and both are using/adding some random data.
Now this is not how you should be using RSA. Many reasons but the most direct, for you, is because the padding / block size is limiting the number of bytes you can encrypt (and RSA is too slow to consider looping encrypting blocks).
I wrote a blog entry on the subject that describe how you can mix symmetric (better speed, no size limit) with asymmetric encryption - getting the best of both worlds :-)

RSA Encrypt / Decrypt Problem in .NET

I'm having a problem with C# encrypting and decrypting using RSA. I have developed a web service that will be sent sensitive financial information and transactions. What I would like to be able to do is on the client side, Encrypt the certain fields using the clients RSA Private key, once it has reached my service it will decrypt with the clients public key.
At the moment I keep getting a "The data to be decrypted exceeds the maximum for this modulus of 128 bytes." exception. I have not dealt much with C# RSA cryptography so any help would be greatly appreciated.
This is the method i am using to generate the keys
private void buttonGenerate_Click(object sender, EventArgs e)
{
string secretKey = RandomString(12, true);
CspParameters param = new CspParameters();
param.Flags = CspProviderFlags.UseMachineKeyStore;
SecureString secureString = new SecureString();
byte[] stringBytes = Encoding.ASCII.GetBytes(secretKey);
for (int i = 0; i < stringBytes.Length; i++)
{
secureString.AppendChar((char)stringBytes[i]);
}
secureString.MakeReadOnly();
param.KeyPassword = secureString;
RSACryptoServiceProvider rsaProvider = new RSACryptoServiceProvider(param);
rsaProvider = (RSACryptoServiceProvider)RSACryptoServiceProvider.Create();
rsaProvider.KeySize = 1024;
string publicKey = rsaProvider.ToXmlString(false);
string privateKey = rsaProvider.ToXmlString(true);
Repository.RSA_XML_PRIVATE_KEY = privateKey;
Repository.RSA_XML_PUBLIC_KEY = publicKey;
textBoxRsaPrivate.Text = Repository.RSA_XML_PRIVATE_KEY;
textBoxRsaPublic.Text = Repository.RSA_XML_PUBLIC_KEY;
MessageBox.Show("Please note, when generating keys you must sign on to the gateway\n" +
" to exhange keys otherwise transactions will fail", "Key Exchange", MessageBoxButtons.OK, MessageBoxIcon.Information);
}
Once i have generated the keys, i send the public key to the web service which stores it as an XML file.
Now i decided to test this so here is my method to encrypt a string
public static string RsaEncrypt(string dataToEncrypt)
{
string rsaPrivate = RSA_XML_PRIVATE_KEY;
CspParameters csp = new CspParameters();
csp.Flags = CspProviderFlags.UseMachineKeyStore;
RSACryptoServiceProvider provider = new RSACryptoServiceProvider(csp);
provider.FromXmlString(rsaPrivate);
ASCIIEncoding enc = new ASCIIEncoding();
int numOfChars = enc.GetByteCount(dataToEncrypt);
byte[] tempArray = enc.GetBytes(dataToEncrypt);
byte[] result = provider.Encrypt(tempArray, true);
string resultString = Convert.ToBase64String(result);
Console.WriteLine("Encrypted : " + resultString);
return resultString;
}
I do get what seems to be an encrypted value. In the test crypto web method that i created, i then take this encrypted data, try and decrypt the data using the clients public key and send this back in the clear. But this is where the exception is thrown. Here is my method responsible for this.
public string DecryptRSA(string data, string merchantId)
{
string clearData = null;
try
{
CspParameters param = new CspParameters();
param.Flags = CspProviderFlags.UseMachineKeyStore;
RSACryptoServiceProvider rsaProvider = new RSACryptoServiceProvider(param);
string merchantRsaPublic = GetXmlRsaKey(merchantId);
rsaProvider.FromXmlString(merchantRsaPublic);
byte[] asciiString = Encoding.ASCII.GetBytes(data);
byte[] decryptedData = rsaProvider.Decrypt(asciiString, false);
clearData = Convert.ToString(decryptedData);
}
catch (CryptographicException ex)
{
Log.Error("A cryptographic error occured trying to decrypt a value for " + merchantId, ex);
}
return clearData;
}
If anyone could help me that would be awesome, as i have said i have not done much with C# RSA encryption/decryption.

Allow me a bit of terminology. There is asymmetric encryption and there is digital signature.
Asymmetric encryption is about keeping confidentiality. Some sensitive data is transformed into something unreadable, save for the entity who knows the decryption key. The decryption key is necessarily the private key: if the decryption key is the public key, then everybody can decrypt the data (the public key is, well, public) and there is no confidentiality anymore. In asymmetric encryption, one encrypts with the public key and decrypts with the corresponding private key.
Digital signatures are meant to prove integrity. Someone computes a kind of keyed checksum over the data, in such a way that the link between the checksum and the data can be verified later. This is a "signature" only because the power to compute that checksum requires knowledge of something which is not public -- in plain words, signing uses the private key. Verification, however, should be doable by anybody, and thus use the public key.
A fair bit of confusion is implied by the fact that "the" RSA algorithm is actually a mathematical operation which can be declined into both an asymmetric encryption system, and a digital signature system. The confusion is further enhanced by the RSA standard, aka PKCS#1, which implicitly relies on how RSA digital signatures were first described, i.e. as a "reversed encryption" ("the signer encrypts the data with his private key"). Which leads to things like RSA signatures called "sha1WithRSAEncryption". This is quite unfortunate.
Therefore, you must first decide whether you want confidentiality or signatures. For confidentiality, for data sent from clients to the server, the server shall own a private key, and the clients use the server public key to encrypt the data. For signatures, each client shall have his own private key and use it to sign the data, and the server verifies the signatures. From your description I cannot tell what you are really after, thanks to the confusion I allude to above.
Also, there is something called authentication which may look like digital signatures, but is weaker. The point of signatures is than anybody can verify the signature. In particular, the signature can be shown to a judge and thus serve as legal weapon against the signer (the signature is legally binding -- at least if you do it right, and in the current state of regulations over electronic signatures, this is not easy). In most situations you only need something weaker and simpler, in which the server is convinced that it talks to the right client, but cannot afterwards convince anybody else that this client was really there. Any web site with user passwords is using such authentication.
With that being said...
RSA asymmetric encryption covers only short messages. For a 1024-bit RSA key (i.e. a key where the most important part, the "RSA modulus", is a big number with a value between 2^1023 and 2^1024, and encrypted messages will be of length 128 bytes), the maximum size of an encrypted message is 117 bytes (that's the actual source of your error message). When we want to send longer messages, we use an hybrid system, in which we only encrypt a small bunch of random bits (say 128 bits) and use that bunch as a key for a symmetric encryption system (e.g. AES) which can process much longer messages (and much faster, too).
RSA signatures, similarly, can be computed only on short messages, hence the PKCS#1 standard mandates that a signature is actually computed over a hash value. The hash value is the output of a specific hash function, which is computed over the message to sign. The hash function has a fixed-sized output (e.g. 256 bits for SHA-256) but accepts input messages of (almost) arbitrary length. Hash functions are public (there is no key in them) and, for proper security, must have some special properties. SHA-256 is, right now, not a bad choice. SHA-1 (a predecessor of SHA-256) has been proven to have some weaknesses and should be avoided. MD5 has (a kind-of uncle of SHA-1) has bigger weaknesses and shall not be used.
Proper use of asymmetric encryption, especially in an hybrid scheme, and digital signatures, is trickier than what the text above may suggest. It is awfully easy to get it wrong at some point, invisibly, i.e. the code will appear to work but will leak data useful for an attacker. The right way to use asymmetric encryption or digital signatures is to rely on existing, well-thought protocols. A protocol is an assembly of cryptographic elements into a coherent system, where leaks are taken care of. The prime example is TLS, also known as SSL. It is a protocol which ensures confidential data transmission, with integrity and authentication (possibly mutual authentication). The HTTPS protocol is a mixture of HTTP and SSL. The bright side is that HTTPS has existing implementations, notably in C#. The code which is easiest to implement and debug is the code which has already been implemented and debugged. So use HTTPS and you will live longer and happier.

I understand why you are asking the question. The problem is that RSA is not used like a typical block cypher (like AES or 3DES) that encrypts 8 bytes at a time, all day long. RSA is a math operation that returns the remainder of a division (the modulus). Back in grade school, when you learned long division, remember that the remainder can never be greater than the divisor:if you are dividing 20 by 7, your remainder is 6. No matter what integer you divide by 7, the remainder cannot be greater than six.
RSA math is the same way. For example, if you are using a 1024-bit RSA public key, the remainder can never be greater than 2^1024, which is only 128 bytes. So you can only encrypt 128 bytes at a time with this key. (That's one reason we measure the size of RSA keys by the number of bits.)
Technically you could use this RSA key in a loop to encrypt 128 byte chunks of your data at a time. In reality, we almost never do this because RSA math is BIG and SLOW. Instead, we use what is called "two-phase" encryption. We use RSA to encrypt only a short "session key", and then use that session key in a fast symmetric-keyed block cypher (like AES) to encrypt the actual data.
The whole protocol is:
Obtain the RSA public key of your destination. This is often delivered embedded in a certificate; if it is, be sure to validate the certificate to make sure the key is genuine. Let's say the RSA key is 2048 bits long.
Generate a cryptographically strong pseudo-random number to use as a key for the block cypher (you need 256 bits as the key for AES-256, for example.) Note that 256 < 2048, the max that RSA-2048 can encrypt at once. We call this random number the "session key".
Encrypt the session key using the RSA 2048-bit public key. It will give you 2048 bits of encrypted session key. Note that this operation is very slow.
Encrypt all the secret data using AES-256, using the session key. Note that this is much faster than step 3.
Bundle the public key ID from the certificate, the RSA encrypted session key, and the AES encrypted data together. I'd also tag it with a format identifier and version number, so you know what format it is in and how to decrypt it.
Send the bundle to the destination.
At the destination you use the format identifier and version to take apart the bundle.
Retrieve the private key whose identity is in the public key ID field.
Use this private key in RSA to decrypt the session key.
Use the session key in AES to decrypt the data.
If you are going to do this, you should know that it is exactly what the CMS (PKCS#7) format is for. I would encourage you to learn about the standard and adopt it, rather than trying to invent your own format. Microsoft's CSP supports it, so it should be easy.
If you don't follow a standard you will have to make your own decisions about things like "what format should the AES key bits be in in the RSA encryption process?" More likely, you would almost certainly make security mistakes, weakening your system. Besides, you will find that tools such as the CSP will be very difficult to work with if you don't follow a standard.

In DecryptRSA, is "data" base 64 encoded? If it is, you have to undo that first.
Honestly I think you shouldn't implement that routine yourself to protect "sensitive financial information", unless you have a lot of experience with cryptography. There are too many ways to make errors. Better use some ready solution - maybe SSL and certificates, or just PGP or GnuPG?

RSA is primarily used to validate secure hashes of data - rather than encrypting the data itself. So, given a large blob of data, you might use SHA512 to create a hash of that data, then use RSA to sign that hash.
You'll want to use a symmetric encryption algorithm for large blocks of data - something like AES or 3DES.
Managing secure transactions isn't easy and really ought to be left to those guys that spend all day and night thinking about it. If you're exposing the service as over the web, just use SSL which already encrypts and secures your data.

First decide what you are trying to protect against. If you "encrypt" something using the private key, anyone can "decrypt" it with the public key, since the public key is - well - public.
If you actually want to sign it, you should (as Paul Alexander explains) sign a hash with the private key which can then be verified on the server.
To encrypt data using RSA you should first generate a random symmetric key (f.x. AES), encrypt the key using a public key and encrypt the data using the symmetric key. You can then transmit the encrypted key together with the encrypted data to the holder of the private key, who can then first decrypt the encrypted key with the private key and then decrypt the data with the symmetric key.
You might also consider using SSL, but remember to carefully consider the authentication. You will probably need client authentication and have to decide which certificates to trust (you should not just blindly accept any certificate issued by Verisign).