encrypt SQL connectionstring c#

You should store your connection string in a config file and encrypt that section. See http://www.4guysfromrolla.com/articles/021506-1.aspx or http://msdn.microsoft.com/en-us/library/89211k9b%28VS.80%29.aspx.

There are two ways of doing it:

1. You can use Configuration Secure Section to encrypt and decrypt connection strimng from your source code:

try{    // Open the configuration file and retrieve     // the connectionStrings section.    Configuration config = ConfigurationManager.OpenExeConfiguration(exeConfigName);        ConnectionStringsSection section = config.GetSection("connectionStrings") as ConnectionStringsSection;        if (section.SectionInformation.IsProtected)    {        // Remove encryption.        section.SectionInformation.UnprotectSection();    }    else    {        // Encrypt the section.        section.SectionInformation.ProtectSection("DataProtectionConfigurationProvider");    }    // Save the current configuration.    config.Save();        Console.WriteLine("Protected={0}",    section.SectionInformation.IsProtected);}catch (Exception ex){    Console.WriteLine(ex.Message);}

2. You can Enterprise Library Data Access Application Block to perform the encryption using RSAProtectedConfigurationProvider or DPAPIProtectedConfigurationProvider.

For the full article go to --> http://msdn.microsoft.com/en-us/library/89211k9b(VS.80).aspx

No, you can only make it difficult

It is better to let the application use a special database login which only got access to the tables/procedures necessary.