The Basics of Encryption in Java

Encoder QR Code ISO/IEC18004 in Java The Basics of Encryption

Electronic codebook (ECB)

After a block cipher divides plaintext into blocks, it has several different ways to encrypt that data The simplest way to encrypt data is to encrypt each block of data separately with the same key, which is the electronic codebook method, as shown in Figure 5-2

Encrypting with the electronic codebook method is simple and fast, but because it uses the same key to encrypt data, it tends to encrypt redundant data in identical chunks So the message I am Sam Sam I am might create two blocks of encrypted data that would look nearly identical, such as X*4d*34d^ and 34d*X*4d^ A cursory examination of these two encrypted blocks can reveal that X represents the letter I, * represents a space, 4d represents am, 3 represents S, and ^ represents a period

Cipher-block chaining (CBC)

The ideal encryption algorithm takes identical data and scrambles it in two different ways to avoid revealing any redundant data So the idea behind the cipher-block chaining (CBC) method is to use the encrypted output from one block as input to encrypt a second block Because the output from one encrypted block directly affects the encryption of another block, identical plaintext data gets converted into completely different ciphertext, as shown in Figure 5-3

Plaintext Figure 5-3: Cipherblock chaining uses the output from one block as the input for encrypting a second block

Altered cipher encryption #1

Altered cipher encryption #2

The most common type of encryption algorithm is a symmetric algorithm, which uses the same password to encrypt and decrypt data Basically, this means that the password that scrambles the data can also reverse the process and unscramble the data, as shown in Figure 5-4

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 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