Signing a byte array of 128 bytes with RSA in C sharp

Question

I am completely new to cryptography and I need to sign a byte array of 128 bytes with an RSA key i have generated with C sharp. The key must be 1024 bits.

I have found a few examples of how to use RSA with C sharp and the code I'm currently trying to use is:

public static void AssignParameter()
{
    const int PROVIDER_RSA_FULL = 1;
    const string CONTAINER_NAME = "SpiderContainer";
    CspParameters cspParams;
    cspParams = new CspParameters(PROVIDER_RSA_FULL);
    cspParams.KeyContainerName = CONTAINER_NAME;
    cspParams.Flags = CspProviderFlags.UseMachineKeyStore;
    cspParams.ProviderName = "Microsoft Strong Cryptographic Provider";
    rsa = new RSACryptoServiceProvider(cspParams);
    rsa.KeySize = 1024;
}

public static string EncryptData(string data2Encrypt)
{
    AssignParameter();
    StreamReader reader = new StreamReader(path + "publickey.xml");
    string publicOnlyKeyXML = reader.ReadToEnd();
    rsa.FromXmlString(publicOnlyKeyXML);
    reader.Close();

    //read plaintext, encrypt it to ciphertext

    byte[] plainbytes = System.Text.Encoding.UTF8.GetBytes(data2Encrypt);
    byte[] cipherbytes = rsa.Encrypt(plainbytes, false);
    return Convert.ToBase64String(cipherbytes);
}

This code works fine with small strings (and thus short byte arrays) but when I try this with a string of 128 characters I get an error saying: CryptographicException was unhandled: Wrong length (OK, it might not precisely say 'Wrong length', I get the error in danish, and that is 'Forkert længde' which directly translates to 'Wrong length').

Can anyone tell me how I can encrypt a byte array of 128 bytes with a RSA key of 1024 bits in C sharp?

Thanks in advance, LordJesus

EDIT:

Ok, just to clarify things a bit: I have a message, from which i make a hash using SHA-256. This gives a 32 byte array. This array is padded using a custom padding, so it ends up being a 128 byte array. This padded hash should then be signed with my private key, so the receiver can use my public key to verify that the message received is the same as the message sent. Can this be done with a key of 1024 bits?

Signing and encrypting are NOT the same thing. Which do you want? — President James K. Polk
– President James K. Polk, Commented Jun 10, 2011 at 0:10
What I want is to sign my padded hash (the 128 bytes) with my private key, so that others can verify the message using my public key. — Daniel
– Daniel, Commented Jun 10, 2011 at 7:05

Thomas Pornin · Accepted Answer · 2011-06-10 15:25:32Z

If you want to sign you do not want to encrypt. Signatures and encryption are distinct algorithms. It does not help that there is a well-known signature algorithm called RSA, and a well-known asymmetric encryption algorithm also called RSA, and that the signature algorithm was first presented (and still is in many places) as "you encrypt with the private key". This is just plain confusing.

In RSA encryption, the data to encrypt (with the public key) must be padded with what PKCS#1 (the RSA standard) describes as "Type 2 padding", and the result (which has the same length than the modulus) is then processed through the modular exponentiation which is at the core of RSA (at the core, but RSA is not only a modular exponentiation; the padding is very important for security).

When signing, the data to sign must be hashed, then the hash value is embedded in a structure which describes the hash function which was just used, and the encoded structure is itself padded with a "Type 1 padding" -- not the same padding than the padding for encryption, and that's important, too.

Either way, a normal RSA engine will perform the type 1 or type 2 padding itself, and most RSA signature engines will also handle themselves the structure which identifies the used hash function. A RSA signature engine such as RSACryptoServiceProvider can work either with SignHash(), which expects the hash value (the 32 bytes obtained from SHA-256, without any kind of encapsulating structure or type 1 padding -- RSACryptoServiceProvider handles that itself), or SignData(), which expects the data to be signed (the engine then does the hash computation too).

To sum up, if you do any kind of padding yourself, then you are doing it wrong. If you used Encrypt() to compute a signature, then you are doing it wrong, too.

ok removed my comment - I wasn't sure about editing it directly. I will drop this comment shortly so we can have a clean comment trail :)

Andrew Savinykh · Accepted Answer · 2023-11-23 20:19:41Z

The minimum key size for encrypting 128 bytes would be 1112 bits, when you are calling Encrypt with OAEP off. Note that setting the key size like this rsa.KeySize = 1024 won't help, you need to actually generate they key of the right size and use them.

This is what worked for me:

using System;
using System.IO;
using System.Security.Cryptography;

namespace SO6299460
{
    class Program
    {
        static void Main()
        {
            GenerateKey();
            string data2Encrypt = string.Empty.PadLeft(128,'$');
            string encrypted = EncryptData(data2Encrypt);
            string decrypted = DecryptData(encrypted);

            Console.WriteLine(data2Encrypt);
            Console.WriteLine(encrypted);
            Console.WriteLine(decrypted);
        }

        private const string path = @"c:\";
    
        public static void GenerateKey()
        {
            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(1112);
            string publickKey = rsa.ToXmlString(false);
            string privateKey = rsa.ToXmlString(true);

            WriteStringToFile(publickKey, path + "publickey.xml");
            WriteStringToFile(privateKey, path + "privatekey.xml");
        }

        public static void WriteStringToFile(string value, string filename)
        {
            using (FileStream stream = File.Open(filename, FileMode.Create, FileAccess.Write, FileShare.Read))
            using (StreamWriter writer = new StreamWriter(stream))
            {
                writer.Write(value);
                writer.Flush();
                stream.Flush();
            }
        }
    
        public static string EncryptData(string data2Encrypt)
        {
            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
            StreamReader reader = new StreamReader(path + "publickey.xml");
            string publicOnlyKeyXML = reader.ReadToEnd();
            rsa.FromXmlString(publicOnlyKeyXML);
            reader.Close();

            //read plaintext, encrypt it to ciphertext

            byte[] plainbytes = System.Text.Encoding.UTF8.GetBytes(data2Encrypt);
            byte[] cipherbytes = rsa.Encrypt(plainbytes,false);

            return Convert.ToBase64String(cipherbytes);
        }

        public static string DecryptData(string data2Decrypt)
        {
            RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
            StreamReader reader = new StreamReader(path + "privatekey.xml");
            string key = reader.ReadToEnd();
            rsa.FromXmlString(key);
            reader.Close();

            byte[] plainbytes = rsa.Decrypt(Convert.FromBase64String(data2Decrypt), false);
            return System.Text.Encoding.UTF8.GetString(plainbytes);

        }


    }
}

Note however, that I'm not using a crypto container, and thus, I don't need your AssignParameter, but if you need to use it, modifying the code should be easy enough.

If you ever need to encrypt large quantities of data (much larger than 128 bytes) this article has sample code on how to do this.

Community · Accepted Answer · 2017-05-23 12:20:00Z

Apparently, according to this question — how to use RSA to encrypt files (huge data) in C# — RSA can only encrypt data shorter than its key length.

Bizarre. The MSDN docs for`RSACryptoServiceProvider.Encrypt() say that a CryptographicException may be thrown if the length of the rgb parameter is greater than the maximum allowed length.

Well. That seems odd, especially since there doesn't seem to be much in the way of documentation regarding said maximum.

A little further digging, under Remarks has this:

The following table describes the padding supported by different versions of Microsoft Windows and the maximum length of rgb allowed by the different combinations of operating systems and padding.

If you are running XP or later and you're using OAEP padding, then the limit is stated to be

Modulus size -2 -2*hLen, where hLen is the size of the hash

No idea what the "size of the hash" might be, since the docs, AFAICS, don't mention "hash" anywhere except in regards to digital signatures.
If you are running Windows 2000 or later with the "high encryption pack" installed (again, no idea how you find that out), then the limit is stated to be

Modulus size - 11. (11 bytes is the minimum padding possible.)
Otherwise (Windows 98, Millenium or Windows 2000 or later without the aforementioned "high encryption pack" then you get "Direct Encryption and OAEP padding not supported", where the limitation is

The maximum size allowed for a symmetric key.

Say...wait a second... RSA is an asymmetric algorithm, right?

Worthless documentation. Sheesh.

I think I finally got it. I tested the different paddings on Windows 10. These are the valid values of the RSAEncryptionPadding enum which you can use as a parameter for the RSA.Encrypt method: Pkcs1, OaepSHA1 (digest size is 160 bits = 20 bytes), OaepSHA256 (32 bytes), OaepSHA384 (48), OaepSHA512 (64). If you choose the PKCS#1 then the maximum data size is (modulus size - 11) bytes. If you choose any of the OAEP's, the max data size is (modulus size - 2 - 2*OAEP hash digest size). For example with a 512 bit public key and the OaepSHA1 setting, it's 512 bits - 2 - 2*160 bits=64-2-40=22 bytes.

Tomas Voracek · Accepted Answer · 2011-06-09 21:33:07Z

0

See http://msdn.microsoft.com/en-us/library/system.security.cryptography.rsacryptoserviceprovider.encrypt.aspx. The exception thrown is probably "The length of the rgb parameter is greater than the maximum allowed length."

answered Jun 9, 2011 at 21:33

Tomas Voracek

5,9441 gold badge28 silver badges41 bronze badges

Comments

Nickolay Olshevsky · Accepted Answer · 2011-06-09 21:49:17Z

0

Usually RSA encryption has padding, and since your encrypted data size goes to the key size, there is no space for padding. Try to use longer key or less data size to encrypt.

answered Jun 9, 2011 at 21:49

Nickolay Olshevsky

14.2k2 gold badges38 silver badges49 bronze badges

2 Comments

Daniel Over a year ago

Problem is, I have to use a custom padding which is applied, thus making my data exactly 128 bytes. And the RSA key has to be 1024 bits. Is there a way to turn off the built-in padding of the RSA?

President James K. Polk Over a year ago

@LordJesus: Then you're almost certainly doing it wrong. If you don't know the difference between signing and encrypting how can you know how to do proper padding?

Patrick · Accepted Answer · 2011-06-10 13:12:42Z

0

Do you real need the custom padding? If not you could just use RSACryptoServiceProvider.SignData Method

answered Jun 10, 2011 at 13:12

Patrick

8,3387 gold badges58 silver badges73 bronze badges

Collectives™ on Stack Overflow

Signing a byte array of 128 bytes with RSA in C sharp

6 Answers 6

1 Comment

Comments

1 Comment

Comments

2 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

1 Comment

Comments

1 Comment

Comments

2 Comments

Comments

Linked

Related