The CompTIA Security+ (SY0-201 exam) objectives state that 15 percent of the exam will be on cryptography. Cryptography has several elements and an important one is the use of encryption.
Encryption is used to enforce confidentiality (one of the three elements in the security triad of confidentiality, integrity and availability). If you want to keep your data secure and prevent unauthorized disclosure, you can encrypt the data to make it unreadable.
Encryption uses an algorithm and a key. If either the algorithm is weak or the key is weak, the encryption can easily be broken. Most algorthms are well known but keys are either frequently changed, or kept secret.
Two major types of encryption are used: symmetric and assymmetric.
Symmetric encryption uses a single key to both encrypt the data and decrypt the data.
The most popular type of symmetric encryption is AES. AES is considered a fast, highly secure encryption algorithm. It is significantly faster (using less processor and memory resources) than both DES and 3DES. AES256 uses a key size of 256 bits.
Asymmetric encryption uses two keys that are created as a matched pair (a public key and a private key). Data encrypted with a public key can only be decrypted with the matching private key. Data encrypted with the private key can only be decrypted with the public key
A Public Key Infrastrcuture (PKI) is required to support asymmetric encryption. Any entity can have a private key (which is always kept private). The matching public key is embedded in a certificate issued from a certificate authority (CA).