Symmetric key encryption is a category of encryption methods in which the same or two related keys are used to encrypt and decrypt the message. The alphabetic substitution cipher in the example under Basic Encryption Theory is one example of symmetric key encryption. The author of the message used the "offset of five" key to encrypt the message and then communicated the key to the recipient. The recipient then used the same key to decrypt the message. A major problem with symmetric key encryption is that the key must be passed along to the recipient before the message can be decrypted. When this happens, there is the risk that someone in between will also obtain the key. When this happens, the encryption can be broken very easily. In fact, the adversary will be able to decrypt the message just as easily as the one for whom the message was intended. One benefit to using symmetric key encryption compared to asymmetric key encryption, which we will discus next, is symmetric key encryption's speed. Symmetric key encryption is much less computationally intense and can be up to a thousand times faster than asymmetric key encryption. DES, AES, Blowfish and IDEA are examples of widely used symmetric key ciphers.
There are three main methods by which symmetric key ciphers can be broken. The first simply occurs when the key used to encrypt and decrypt the message is compromised to an outside source. When an adversary has the key, he can decrypt the data just as easily as the author and recipient. Secondly, known-plaintext attacks occur when an adversary already possesses or can find a piece of data encrypted with the same key and the unencrypted version. A quick comparison of the unencrypted and encrypted data will reveal the key with which the adversary can decrypt the new encrypted data as well. Lastly, a chosen-plaintext attach is one in which the adversary chooses a piece of data, encrypts it with the same cipher and a different key, and compares the three types of data (the desired encrypted data, the chosen plaintext, and the chosen ciphertext) for clues that may lead to the discovery of the key.