digital signature ->  Information that is encrypted with an entity private key and is appended to a message to assure the recipient of the authenticity and integrity of the message. The digital signature proves that the message was signed by the entity that owns, or has access to, the private key or shared secret symmetric key.

smart cards -> Smart cards help businesses evolve and expand their products and services in a rapidly changing global market. In addition to the well known commercial applications (banking, payments, access control, identification, ticketing and parking or toll collection), in recent years, the information age has introduced an array of security and privacy issues that have called for advanced smart card security applications (secure logon and authentication of users to PC and networks, storage of digital certificates, passwords and credentials, encryption of sensitive data, wireless communication subscriber authentication, etc.)

Abstract class can have method with implementation, interface can only have method definitions
You can inherit multiple interface but cannot inherit multiple abstract class

An abstract class is a special kind of class that cannot be instantiated. So the question is why we need a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In other words, it only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.

An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class. Since C# doesn't support multiple inheritance, interfaces are used to implement multiple inheritance.

When we create an interface, we are basically creating a set of methods without any implementation that must be overridden by the implemented classes. The advantage is that it provides a way for a class to be a part of two classes: one inheritance hierarchy and one from the interface.

When we create an abstract class, we are creating a base class that might have one or more completed methods but at least one or more methods are left uncompleted and declared abstract. If all the methods of an abstract class are uncompleted then it is the same as an interface but with the restriction that it cannot make a class inherit from it. The purpose of an abstract class is to provide a base class definition for how a set of derived classes will work and then allow the programmers to fill the implementation in the derived classes.

There are some similarities and differences between an interface and an abstract class that I have arranged in a table for easier comparison:

Feature

Interface

Abstract class

Multiple inheritance

A class may inherit several interfaces.

A class may inherit only one abstract class.

Default implementation

An interface cannot provide any code, just the signature.

An abstract class can provide complete, default code and/or just the details that have to be overridden.

Constants

Only Static final constants.

Both instance and static constants are possible.

Core VS Peripheral

Interfaces are used to define the peripheral abilities of a class. In other words both Human and Vehicle can inherit from a IMovable interface.

An abstract class defines the core identity of a class and there it is used for objects of the same type.

Homogeneity

If the various implementations only share method signatures then it is better to use Interface.

If the various implementations are of the same kind and use common behaviour or status then abstract class is better to use.

Speed

Requires more time to find the actual method in the corresponding classes.

Fast

Adding functionality

If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.

If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.