Definition
Encryption
is a process of translating a message, called the Plaintext,
into an encoded message, called the Ciphertext. This is
usually accomplished using a secret Encryption Key and a cryptographic Cipher.
Two
basic types of Encryption are commonly used:
Some
interesting politics surround strong Encryption:
- Strong
(i.e., hard to break) Encryption algorithms are considered to be a munitions by
the United States government. Exporting such algorithms therefore amounts to
arms smuggling -- a very serious offence!
- Some
countries (i.e., France) forbid their citizens from using strong Encryption.
- Strong
encryption algorithms are freely available everywhere in the world, on the
Internet.
In
the United States, it is possible to patent an algorithm, including an
Encryption Cipher.
This can limit who can make such algorithms.
Public Key Encryption and Digital Signatures
Public
key encryption, also known as asymmetric encryption, is based on a
public/private key pair. The keys are mathematically linked, so that data
encrypted with the public key can only be decrypted with the corresponding
private key. X509 certificates use public key encryption as an alternative to
shared symmetric keys, which are discussed in the Data Confidentiality pattern in Chapter 2,
"Message Protection Patterns."
With
public key encryption, the sender converts the plaintext message into
ciphertext by encrypting it with the public key in the message recipient's
X.509 certificate. The message recipient converts the ciphertext back into the
plaintext message by decrypting it with the corresponding private key.
Figure
1 illustrates how public key encryption and decryption take place.
Figure 1. Public key data encryption and decryption
By
using public key encryption, a message sender has assurance that only the
recipient will be able to read the message.
In
addition to providing data confidentiality through encryption, you can use the
public key in the X.509 certificate to verify digital signatures created by a
message sender. A digital signature is a value produced by the message sender
to bind message data to the sender's identity and to provide a means of
verifying the integrity of the message to detect tampering. In this case, the
private key of the message sender is used to create the digital signature. The
corresponding public key, which is found in the sender's X.509 certificate, is
used to verify the signature. Digital signatures are used to assure the message
recipient that the message originated from the identified sender, and that the
message contents have not been altered since they were signed by the sender.
Note With digital signatures that use
public key cryptography, the origin of the signed message can be traced to the
sender's identity, thereby satisfying nonrepudiation requirements. This differs
from symmetric key integrity, where a message may have been signed by either
party with knowledge of the shared secret key.
The
public key can be distributed openly to encrypt messages and to verify digital
signatures, but the private key in a key pair should be carefully guarded by
its owner. This is necessary because it is used to prove the identity of the
certificate subject and to decrypt messages that are intended for that subject.
Figure
2 illustrates the process of using public keys to sign a message.
Figure 2. Creation and
verification of a digital signature
