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System.Security.SecureString class

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Important

We recommend that you don't use the SecureString class for new development on .NET (Core) or when you migrate existing code to .NET (Core). For more information, see SecureString shouldn't be used.

This article provides supplementary remarks to the reference documentation for this API.

SecureString is a string type that provides a measure of security. It tries to avoid storing potentially sensitive strings in process memory as plain text. (For limitations, however, see the How secure is SecureString? section.) The value of an instance of SecureString is automatically protected using a mechanism supported by the underlying platform when the instance is initialized or when the value is modified. Your application can render the instance immutable and prevent further modification by invoking the MakeReadOnly method.

The maximum length of a SecureString instance is 65,536 characters.

Important

This type implements the IDisposable interface. When you have finished using an instance of the type, you should dispose of it either directly or indirectly. To dispose of the type directly, call its Dispose method in a try/catch block. To dispose of it indirectly, use a language construct such as using (in C#) or Using (in Visual Basic). For more information, see the "Using an Object that Implements IDisposable" section in the IDisposable interface topic.

The SecureString class and its members are not visible to COM. For more information, see ComVisibleAttribute.

String versus SecureString

An instance of the System.String class is both immutable and, when no longer needed, cannot be programmatically scheduled for garbage collection; that is, the instance is read-only after it is created, and it is not possible to predict when the instance will be deleted from computer memory. Because System.String instances are immutable, operations that appear to modify an existing instance actually create a copy of it to manipulate. Consequently, if a String object contains sensitive information such as a password, credit card number, or personal data, there is a risk the information could be revealed after it is used because your application cannot delete the data from computer memory.

A SecureString object is similar to a String object in that it has a text value. However, the value of a SecureString object is pinned in memory, may use a protection mechanism, such as encryption, provided by the underlying operating system, can be modified until your application marks it as read-only, and can be deleted from computer memory either by your application calling the Dispose method or by the .NET garbage collector.

For a discussion of the limitations of the SecureString class, see the How secure is SecureString? section.

SecureString operations

The SecureString class includes members that allow you to do the following:

Instantiate a SecureString object You instantiate a SecureString object by calling its parameterless constructor.

Add characters to a SecureString object You can add a single character at a time to a SecureString object by calling its AppendChar or InsertAt method.

Important

A SecureString object should never be constructed from a String, because the sensitive data is already subject to the memory persistence consequences of the immutable String class. The best way to construct a SecureString object is from a character-at-a-time unmanaged source, such as the Console.ReadKey method.

Remove characters from a SecureString object You can replace an individual character by calling the SetAt method, remove an individual character by calling the RemoveAt method, or remove all characters from the SecureString instance by calling the Clear method.

Make the SecureString object read-only Once you have defined the string that the SecureString object represents, you call its MakeReadOnly method to make the string read-only.

Get information about the SecureString object The SecureString class has only two members that provide information about the string: its Length property, which indicates the number of UTF16-encoded code units in the string; and the IsReadOnly, method, which indicates whether the instance is read-only.

Release the memory allocated to the SecureString instance Because SecureString implements the IDisposable interface, you release its memory by calling the Dispose method.

The SecureString class has no members that inspect, compare, or convert the value of a SecureString. The absence of such members helps protect the value of the instance from accidental or malicious exposure. Use appropriate members of the System.Runtime.InteropServices.Marshal class, such as the SecureStringToBSTR method, to manipulate the value of a SecureString object.

The .NET Class Library commonly uses SecureString instances in the following ways:

SecureString and interop

Because the operating system does not directly support SecureString, you must convert the value of the SecureString object to the required string type before passing the string to a native method. The Marshal class has five methods that do this:

Each of these methods creates a clear-text string in unmanaged memory. It is the responsibility of the developer to zero out and free that memory as soon as it is no longer needed. Each of the string conversion and memory allocation methods has a corresponding method to zero out and free the allocated memory:

Allocation and conversion method Zero and free method
Marshal.SecureStringToBSTR Marshal.ZeroFreeBSTR
Marshal.SecureStringToCoTaskMemAnsi Marshal.ZeroFreeCoTaskMemAnsi
Marshal.SecureStringToCoTaskMemUnicode Marshal.ZeroFreeCoTaskMemUnicode
Marshal.SecureStringToGlobalAllocAnsi Marshal.ZeroFreeGlobalAllocAnsi
Marshal.SecureStringToGlobalAllocUnicode Marshal.ZeroFreeGlobalAllocUnicode

How secure is SecureString?

When created properly, a SecureString instance provides more data protection than a String. When creating a string from a character-at-a-time source, String creates multiple intermediate in memory, whereas SecureString creates just a single instance. Garbage collection of String objects is non-deterministic. In addition, because its memory is not pinned, the garbage collector will make additional copies of String values when moving and compacting memory. In contrast, the memory allocated to a SecureString object is pinned, and that memory can be freed by calling the Dispose method.

Although data stored in a SecureString instance is more secure than data stored in a String instance, there are significant limitations on how secure a SecureString instance is. These include:

Platform

On the Windows operating system, the contents of a SecureString instance's internal character array are encrypted. However, whether because of missing APIs or key management issues, encryption is not available on all platforms. Because of this platform dependency, SecureString does not encrypt the internal storage on non-Windows platform. Other techniques are used on those platforms to provide additional protection.

Duration

Even if the SecureString implementation is able to take advantage of encryption, the plain text assigned to the SecureString instance may be exposed at various times:

  • Because Windows doesn't offer a secure string implementation at the operating system level, .NET still has to convert the secure string value to its plain text representation in order to use it.

  • Whenever the value of the secure string is modified by methods such as AppendChar or RemoveAt, it must be decrypted (that is, converted back to plain text), modified, and then encrypted again.

  • If the secure string is used in an interop call, it must be converted to an ANSI string, a Unicode string, or a binary string (BSTR). For more information, see the SecureString and interop section.

The time interval for which the SecureString instance's value is exposed is merely shortened in comparison to the String class.

Storage versus usage More generally, the SecureString class defines a storage mechanism for string values that should be protected or kept confidential. However, outside of .NET itself, no usage mechanism supports SecureString. This means that the secure string must be converted to a usable form (typically a clear text form) that can be recognized by its target, and that decryption and conversion must occur in user space.

Overall, SecureString is more secure than String because it limits the exposure of sensitive string data. However, those strings may still be exposed to any process or operation that has access to raw memory, such as a malicious process running on the host computer, a process dump, or a user-viewable swap file. Instead of using SecureString to protect passwords, the recommended alternative is to use an opaque handle to credentials that are stored outside of the process.