AES Encryption

Table of Contents

Basic AES

If you're not familiar with encryption techniques, AES is the Advanced Encryption Standard. This specification was established by the National Institute of Standards and Technology, sub-selected from the Rijndael family of ciphers (128, 192, and 256 bits) in 2001. Furthering its popularity and status, the US government chose AES as their default encryption method for top-secret data, removing the previous standard which had been in place since 1977.

AES has proven to be an extremely safe encryption method, with 7-round and 8-round attacks making no material improvements since the release of this encryption standard almost two decades ago.

Though many papers have been published on the cryptanalysis of AES, the fastest single-key attacks on round-reduced AES variants [20, 33] so far are only slightly more powerful than those proposed 10 years ago [23,24].

- Bogdonav, et al.

How Secure is AES?

In theory, AES-256 is non-crackable due to the massive number of combinations that can be produced. However, AES-128 is no longer recommended as a viable implementation to protect important data.

A semi-short comic strip from Moserware quickly explains AES for the general public to understand. Basically AES encrypts the data by obscuring the relationship between the data and the encrypted data. Additionally, this method spreads the message out. Lastly, the key produced by AES is the secret to decrypting it. Someone may know the method of AES, but without the key, they are powerless.

To obscure and spread the data out, AES creates a substitution-permutation network. Wikipedia has a wonderful example of an SP network available. This network sends the data through a set of S boxes (using the unique key) to substitute the bits with another block of bits. Then, a P box will permutate, or rearrange, the bits. This is done over and over, with the key being derived from the last round. For AES, the key size specifies the number of transformation rounds: 10, 12, and 14 rounds for 128-bit, 192-bit, and 256-bit keys, respectively.

The Process

  1. KeyExpansion: Using Rijndael's key schedule, the keys are dynamically generated.
  2. AddRoundKey: Each byte of the data is combined with this key using bitwise xor.
  3. SubBytes: This is followed by the substitution of each byte of data.
  4. ShiftRows: Then, the final three rows are shifted a certain number of steps, dictated by the cipher.
  5. MixColumns: After the rows have been shifted, the columns are mixed and combined.

This process does not necessarily stop after one full round. Steps 2 through 5 will repeat for the number of rounds specified by the key. However, the final round excludes the MixColumns step. As you can see, this is a fairly complex process. One must have a solid understanding of general mathematic principles to fully understand how the sequence works (and to even attempt to find a weakness).

According to research done by Bogdanov et al., it would take billions of years to brute force a 126-bit key with current hardware. Additionally, this brute force attack would require storing 288 bits of data! However, there are a few different attacks that have been used to show vulnerabilities with the use of this technology. Side-channel attacks use inadvertent leaks of data from the hardware or software, which can allow attackers to obtain the key or run programs on a user's hardware.

Please note that this is not something you should run out and try to implement in your Hello, World! app after only a few hours of research. While AES (basically all encryption methods) is extremely efficient in what it does, it takes a lot of time and patience to understand. If you're looking for something which currently implements AES, check out the Legion of the Bouncy Castle for Java implementations of cryptographic algorithms.

Why Does Encryption Matter?

There are limitless reasons to enable encryption at-rest or in-transit for various aspects of your digital life. You can research specific examples, such as: Australia passes new law to thwart strong encryption. However, I will simply list a few basic reasons to always enable encryption, where feasible:

  1. Privacy is a human right and is recognized as a national right in some countries (e.g., US Fourth Amendment).
  2. "Why not?" Encryption rarely affects performance or speed, so there's usually not a reason to avoid it in the first place.
  3. Your digital identity and activity (texts, emails, phone calls, online accounts, etc.) are extremely valuable and can result in terrible consequences, such as identity theft, if leaked to other parties. Encrypting this data prevents such leaks from ruining lives.
  4. Wiping or factory-resetting does not actually wipe all data from the storage device. There are methods to read data from the physical disks/boards inside devices.
  5. Corporations, governments, and other nefarious groups/individuals are actively looking for ways to collect personal information about anyone they can. If someone's data is unencrypted, that person may become a target due to the ease of data collection.

Read More: Federal Information Processing Standards Publication 197 [.pdf]