Cryptographic Basics

 

Cryptography while essential in modern software engineering is a complicated subject. While there is no need to gain an understanding of the complex mathematics that underly modern cryptographic techniques, a well rounded engineer should understand the available tools and the situations in which they should be used. 

What is presented below is by no means an in depth examination of cryptography but is a primer into the topics that are likely to come up as you try to ensure your code base is well protected.

Encryption vs. Hashing

Encryption and hashing are probably the two primary applications of cryptography but the use case for each is different.

Encryption is a two-way i.e. reversible process. In order to protect data either at rest or in transit encryption can be applied such that only those that have the corresponding key can view the underlying data. Encryption is therefore used to protect data in situations where access to the data needs to be maintained but also protected from unauthorised disclosure.  

Hashing is a one-way i.e. irreversible process. Taking data as an input a hashing algorithm produces a unique digest that cannot be used to get back to the original data source. Hashing is therefore used in situations where either the integrity of data needs to be verified or where the data being stored is very sensitive and therefore only a representation of the data should be stored rather than the data itself. A common example of the latter would be the storage of passwords.

Stream vs Block Ciphers

Encryption is implemented by the application of ciphers, algorithms that given an input (referred to as plain text) will output the same data in an encrypted form (referred to as cipher text).

These ciphers are often categorised based on how they view the input data.

Stream ciphers view the data as a constant stream of bits and bytes, they produce a corresponding stream of pseudo random data that is combined with the input data to produce the encrypted output. A block cipher divides the data up into fixed size blocks, using padding to ensure the overall size of the encrypted data is a whole number of these fixed sized blocks. 

Stream ciphers have proven to be complicated to implement correctly mainly because of their reliance on the true randomness of the generated key stream. Because of this the most popular ciphers are mostly block ciphers such as the Advanced Encryption Standard (AES).

While block ciphers are now the most widely used attention also needs to be paid to the mode they are used in. The mode largely controls how the blocks are combined during the encryption process. When using Electronic Code Book (ECB) mode then each block is encrypted separately and are simply concatenated to form the encrypted output. While this may seem logical it leads to weaknesses, when separate blocks contain the same data they will lead to the same output which can present an advantage to a possible attacker. For this reason other modes such as Cipher Block Chaining (CBC) combine each block as the algorithm progresses to ensure even if blocks contain the same data they will produce different encrypted output.

Cryptographic Hashing

As we discussed earlier a hashing function is a one-way function that produces a unique digest of a message. Not all hashing algorithms are explicitly designed for cryptographic purposes. 

A cryptographic hashing function should have the following properties:

• It should be deterministic, meaning the same input message will always lead to the same digest.
• It should be a fast operation to compute the digest of a message.
• It should be computationally infeasible to generate a message that gives a specific digest.
• It should be computationally infeasible to find two messages that produce the same digest.
• A small change in the input message should produce a large change in the corresponding digest.

When an algorithm has these qualities it can be applied to provide digital signatures of Message Authentication Codes (MACs) to protect the integrity and authenticity of data either at rest or in transit.

We said earlier that there is no need to to understand the complex mathematics behind these cryptographic techniques, to take this a step further it's important that you don't attempt to understand or implement these techniques yourselves. The complexity involved means the likelihood of making a mistake in the implementation is high, this can lead to bugs that can be exploited by attackers to undermine the security you are trying to implement.

Instead you should view cryptography as a tool box providing implements you can use to protect you and your users, the important thing to learn is which tool should be used for which job and become and expert in its application.