Blogs >> Technology >>
Symmetric Encryption
A good cryptosystem requries two components: the algorithm and the variable. The algorithm is a formula—a series of steps—that must be followed to turn the plain text into cipher text, and vice versa. The variable is the secret key to the formula.
In the so-called Caesar shift, the algorithm is to simply put two alphabet strings on top of each other:
a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
The key is to shift the lower alphabet three places to the right:
a b c d e f g h i j k l m n o p q r s t u v w x y z
X Y Z A B C D E F G H I J K L M N O P Q R S T U V W
This method of encryption is called symmetric encryption, since the same key encrypts and (when applied in reverse) decrypts. Symmetric encryption is used today, though, not surprisingly, the algorithm is much more complex and the key is longer.
Symmetric encryption is used because it is quicker. Unfortunately, symmetric encryption suffers from three primary drawbacks:
In the so-called Caesar shift, the algorithm is to simply put two alphabet strings on top of each other:
a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
The key is to shift the lower alphabet three places to the right:
a b c d e f g h i j k l m n o p q r s t u v w x y z
X Y Z A B C D E F G H I J K L M N O P Q R S T U V W
This method of encryption is called symmetric encryption, since the same key encrypts and (when applied in reverse) decrypts. Symmetric encryption is used today, though, not surprisingly, the algorithm is much more complex and the key is longer.
Symmetric encryption is used because it is quicker. Unfortunately, symmetric encryption suffers from three primary drawbacks:
- Key exchange can be problematic. In short, you and I have agreed to use the alphabet-shift method to encrypt our communications. Before I send you a message, I decide that the letters should be shifted eight places to the right. How exactly do I get this information to you?
- Key management becomes difficult as the system grows. Let us say I have a lot of friends I want to exchange information with. Not only do I want to keep the information safe from the outside world, I also don’t want the other members of my circle to eavesdrop on my communications with any one of them. So when I communicate with Bob, we agree to shift the alphabet one place to the left. With Mary, it is two. (We have to assume that no one in the group is smart enough to try various shifts on any intercepted messages not bound for them.) Remembering “Bob / one,” “Mary / two” is easy enough. But managing the various keys grows more complex as my circle of correspondence grows.
- It cannot be used for other important facets of secure communications, such as authentication (I am who I say I am) and non-repudiation (I cannot tell a lie; it was I who sent that email).
Recognizing these limitations, two really smart guys—Whitfield Diffie and Martin Hellman, to give credit where it is due—developed a new system called asymmetric encryption.
Next time: Asymmetric encryption
|
|
