A
CipherService uses a cryptographic algorithm called a
Cipher to convert an original input source using a
key to
an uninterpretable format. The resulting encrypted output is only able to be converted back to original form with
a
key as well.
CipherServices can perform both encryption and decryption.
Cipher Basics
For what is known as
Symmetric
Ciphers, the
Key used to encrypt the source is the same
as (or trivially similar to) the
Key used to decrypt it.
For Asymmetric
Ciphers, the encryption
Key is not the same as the decryption
Key.
The most common type of Asymmetric Ciphers are based on what is called public/private key pairs:
A private key is known only to a single party, and as its name implies, is supposed be kept very private
and secure. A public key that is associated with the private key can be disseminated freely to anyone.
Then data encrypted by the public key can only be decrypted by the private key and vice versa, but neither party
need share their private key with anyone else. By not sharing a private key, you can guarantee no 3rd party can
intercept the key and therefore use it to decrypt a message.
This asymmetric key technology was created as a
more secure alternative to symmetric ciphers that sometimes suffer from man-in-the-middle attacks since, for
data shared between two parties, the same Key must also be shared and may be compromised.
Note that a symmetric cipher is perfectly fine to use if you just want to encode data in a format no one else
can understand and you never give away the key. Shiro uses a symmetric cipher when creating certain
HTTP Cookies for example - because it is often undesirable to have user's identity stored in a plain-text cookie,
that identity can be converted via a symmetric cipher. Since the the same exact Shiro application will receive
the cookie, it can decrypt it via the same
Key and there is no potential for discovery since that Key
is never shared with anyone.
CipherServices vs JDK
javax.crypto.Ciphers
Shiro
CipherServices essentially do the same things as JDK
javax.crypto.Ciphers, but in
simpler and easier-to-use ways for most application developers. When thinking about encrypting and decrypting data
in an application, most app developers want what a
CipherService provides, rather than having to manage the
lower-level intricacies of the JDK's
Cipher API. Here are a few reasons why most people prefer
CipherServices:
- Stateless Methods -
CipherService method calls do not retain state between method invocations.
JDK
Cipher instances do retain state across invocations, requiring its end-users to manage the instance
and its state themselves.
- Thread Safety -
CipherService instances are thread-safe inherently because no state is
retained across method invocations. JDK
Cipher instances retain state and cannot be used by multiple
threads concurrently.
- Single Operation -
CipherService method calls are single operation methods: encryption or
decryption in their entirety are done as a single method call. This is ideal for the large majority of developer
needs where you have something unencrypted and just want it decrypted (or vice versa) in a single method call. In
contrast, JDK
Cipher instances can support encrypting/decrypting data in chunks over time (because it
retains state), but this often introduces API clutter and confusion for most application developers.
- Type Safe - There are
CipherService implementations for different Cipher algorithms
(
AesCipherService,
BlowfishCipherService, etc). There is only one JDK
Cipher class to
represent all cipher algorithms/instances.
- Simple Construction - Because
CipherService instances are type-safe, instantiating and using
one is often as simple as calling the default constructor, for example,
new AesCipherService();
. The
JDK
Cipher class however requires using a procedural factory method with String arguments to indicate how
the instance should be created. The String arguments themselves are somewhat cryptic and hard to
understand unless you're a security expert. Shiro hides these details from you, but allows you to configure them
if you want.