Keys¶
A key is a block of data. The size of the block is called the key size. For strong encryption, the required key size depends on the algorithm that is used to generate the key. For example, a symmetric algorithm used for AES keys requires 128 or 256 bits for strong encryption. However, an asymmetric algorithm, like RSA, requires 1024 or 2048 bits for strong encryption. RSA keys are commonly used for publicly purchased certificates.
A public key is mathematically linked to a private key. A public key can be shared, but the associated private key should be kept secret. Without the private key, anything encrypted with the public key cannot be decrypted.
A good analogy to help understand public and private keys is to think of a lock and a key.
Think of the public key as being the lock. It is not actually a key; it is a padlock that you can make lots of copies of and distribute wherever you want. A computer that has a copy of the padlock can use it to lock or encrypt data. Think of the private key as being the actual key that can unlock the padlock. Since anyone who has access to the private key can unlock the padlock, you want to keep your private key as safe and secure as possible.
You can share copies of the padlock (usually as a certificate) with anyone and everyone that you want. Anyone who has a copy of your padlock can use it to lock or encrypt the data that they want to send to you securely. The only way to access the data that has been locked with the padlock is to use the private key to unlock or decrypt the data. Since you are the only one that has a copy of the private key, you do not have to worry about any data that has been locked with the public key getting into the wrong hands. In fact, you can transmit that data openly over the public internet and anyone who tries to decipher the data cannot be able to.
Note
Be sure to keep your private key (and password) in a safe place. Do not ever send the private key to a 3rd party. If the private key is lost the certificate might not be available for use.