programming-for-dummies

Symmetric/Asymmetric Encryption Algorithms 453
One type of algorithm commonly associated
with encryption is a hash function. A hash function
takes data as input and based on the size
and content of that data, the hash function calculates
a unique mathematical value. This
value isn’t used as part of the encryption but as
a way to authenticate that certain data hasn’t
been altered.
Hash functions are often used when downloading
files. A Web site might offer a file for
download and display its hash value. Now if
Hash functions
you download that file and run that file through
the hash function, you should get the same
hash value. If you get a different value, the file
you downloaded is corrupt and missing some
information. In encryption, hash functions can
verify that an encrypted message hasn’t been
altered. If a file has been altered, it could mean
the file simply got corrupted, or that someone
tried to insert or remove data from the encrypted
message, which means you shouldn’t trust the
validity of that message.
The biggest problem with symmetric encryption is that both parties need the
same password to encrypt and decrypt data, so if you can’t securely transfer
the password to someone else, that person can never read the message.
A second problem with symmetric encryption is that the weakest link is the
password itself. The encryption algorithm could be the strongest in the
world, but if someone steals the password, that’s like giving someone the
key to unlock the ten-foot-thick steel doors guarding all the gold in the vault
of Fort Knox.
Some popular symmetric encryption algorithms include the Data Encryption
Standard (DES) and the Advanced Encryption Standard (AES). DES was the
original government encryption standard approved in 1976. After computers
became fast enough, they could crack DES encryption; so after a five-year
contest between cryptographers, the government selected a new encryption
standard — AES.
Symmetric encryption is often called private-key encryption because both the
sender and the receiver need an identical copy of the key to encrypt and
decrypt a message. Another type of encryption algorithm is the asymmetric,
or public-key encryption. Unlike symmetric encryption, asymmetric encryption
requires two keys for both the sender and the receiver.
These two keys are the public key and the private key. You can make a million
copies of your public key and give them out, but you want only one copy of
your private key. If someone wants to send you a message, he needs to encrypt
a message with your public key. After someone encrypts a message with
your public key, the only way to decrypt that message is to use your private
key, as shown in Figure 5-5.
Book IV
Chapter 5
Encryption
Algorithms