Unable to sign text block with Private Key - public-key-encryption

Question

I am trying to create a digital signature of a load of clipboard text. I am:
Creating a SHA-256 hash of the plain text.
Encrypting this hash with my PRIVATE key.
Attempting to decrypt this with my PUBLIC key.
I'm doing this as I am of the understanding that anything signed with my PRIVATE key can be decrypted with my PUBLIC - such as this for verification? Is this wrong?
I'm encrypting the hash with these methods:
public static string EncryptText(string text, int keySize, string publicKey)
{
var encrypted = Encrypt(Encoding.UTF8.GetBytes(text), keySize, publicKey);
return Convert.ToBase64String(encrypted);
}
public static byte[] Encrypt(byte[] data, int keySize, string publicKeyXml)
{
if (data == null || data.Length == 0) throw new ArgumentException("Data are empty", "data");
int maxLength = GetMaxDataLength(keySize);
if (data.Length > maxLength) throw new ArgumentException(String.Format("Maximum data length is {0}", maxLength), "data");
if (!IsKeySizeValid(keySize)) throw new ArgumentException("Key size is not valid", "keySize");
if (String.IsNullOrEmpty(publicKeyXml)) throw new ArgumentException("Key is null or empty", "publicKeyXml");
using (var provider = new RSACryptoServiceProvider(keySize))
{
provider.FromXmlString(publicKeyXml);
return provider.Encrypt(data, OptimalAsymmetricEncryptionPadding);
}
}
but passing my PRIVATE KEY down instead of my PUBLIC KEY.
Then to verify the signature, I am using:
public static string DecryptText(string privateKey, int keySize, string text)
{
var decrypted = Decrypt(Convert.FromBase64String(text), keySize, privateKey);
return Encoding.UTF8.GetString(decrypted);
}
public static byte[] Decrypt(byte[] data, int keySize, string publicAndPrivateKeyXml)
{
if (data == null || data.Length == 0) throw new ArgumentException("Data are empty", "data");
if (!IsKeySizeValid(keySize)) throw new ArgumentException("Key size is not valid", "keySize");
if (String.IsNullOrEmpty(publicAndPrivateKeyXml)) throw new ArgumentException("Key is null or empty", "publicAndPrivateKeyXml");
using (var provider = new RSACryptoServiceProvider(keySize))
{
provider.FromXmlString(publicAndPrivateKeyXml);
return provider.Decrypt(data, OptimalAsymmetricEncryptionPadding);
}
}
but passing down my PUBLIC KEY rather than the PRIVATE KEY. At this point, I am getting an error "Key does not exist".
I'm presuming this is the case because the PRIVATE KEY contains the relevant key information for the PUBLIC KEY so it can decode this one-way.
How can I sign a block of text in this way that allows me to distribute a signature with it, that can be decrypted by anybody who knows my PUBLIC KEY?

Answer 1

After much trawling, I found this page:
http://juzzbott.com.au/blog/signing-and-verifying-data-within-csharp