What is AES?

The Advanced Encryption Standard Instruction Set and the Intel Advanced Encryption Standard New Instructions allows specific Intel/AMD and other CPUs to do extremely fast hardware encryption and decryption. AES (Advanced Encryption Standard), is a symmetric block cipher which means that blocks of text which have a size of 128 bits are encrypted, which is the opposite to a stream cipher where each character is encrypted one at a time. The algorithm takes a block of plain text and applies alternating rounds of substitution and permutation boxes to it which are separate stages. In AES, the size of each box is 128, 192 or 256 bits, depending on the strength of the encryption with 10 rounds applied for a 128-bit key, 12 rounds for the 192-bit key, and 14 rounds for the 256-bit key, providing higher security.

The figure below shows that potential key combinations exponentially increase with the key size. AES-256 is impossible to break by a brute force attack based on current computing power, making it the strongest encryption standard. However longer key and more rounds requires higher performance requirements. AES 256 uses 40% more system resources than AES 192, and is therefore best suited to high sensitivity environments where security is more important than speed.

AES Block Cipher Modes

There are different AES block cipher modes that are part of AES.

Electronic Code Book

The simplest block cipher mode is Electronic Code Book. This cipher mode just repeats the AES encryption process for each 128-bit block of data. Each block is independently encrypted using AES with the same encryption key. For decryption, the process is reversed. With ECB, identical blocks of unencrypted data, referred to as plain text, are encrypted the same way and will produce identical blocks of encrypted data. This cipher mode is not ideal since it does not hide data patterns well.

Cipher Block Chaining

A newer block cipher mode was created called Cipher Block Chaining. CBC’s aim is to achieve an encryption method that encrypts each block using the same encryption key producing different cipher text, even when the plain text for two or more blocks is identical. Cipher Block Chaining addresses security weaknesses with ECB.

AES-XTS Block Cipher mode

AES-XTS Block Cipher Mode is a new block cipher mode and designed to be stronger than other modes. It eliminates potential vulnerabilities from sophisticated side channel attacks used to exploit weaknesses within other modes. XTS uses two AES keys. One key performs the AES block encryption; the other is used to encrypt what is known as a Tweak Value. This encrypted tweak is further modified with a Galois polynomial function (GF) and XOR with both the plain text and the cipher text of each block. The GF function ensures that blocks of identical data will not produce identical cipher text. This achieves the goal of each block producing unique cipher text given identical plain text without the use of initialization vectors and chaining. Decryption of the data is carried out by reversing this process.

What is AES-NI?

Intel AES New Instructions (Intel AES-NI) is a new encryption instruction set which contains improvements to the AES algorithm and accelerates the encryption of data in the Intel Xeon processor family and the Intel Core processor suite. AES is a symmetric block cipher that encrypts/decrypts data through several rounds. It is part of the FIPS standard.

There are seven new instructions. The instructions have been implemented to perform some of the complex and performance intensive steps of the AES algorithm. Hardware is used to accelerate the AES algorithms. Intel say that AES-NI can be used to accelerate the performance of an implementation of AES by 3 to 10x over a total software implementation.

How does it work?

A fixed block size of plain text is encrypted several times to produce a final encrypted output. The number of rounds (10, 12, or 14) used depends on the key length (128, 192, or 256). Each round feeds into the following round. Each round is encrypted using a subkey that is generated using a key schedule

What are the six new instructions?

The new instructions perform several computationally intensive parts of the AES algorithm using fewer clock cycles than a software solution.

• Four of the new instructions accelerate the encryption/decryption of a round
• Two new instructions are for round key generation.

Improved security

The new instructions also improve security by preventing side channel attacks on AES. Encryption and decryption are performed completely in hardware without the need for software lookup tables. By running in data-independent time and not using tables, they help in eliminating the major timing and cache-based attacks that target table-based software implementations of AES. In addition, AES is simple to implement, with reduced code size, which helps reducing the risk of introducing security flaws, such as difficult-to-detect side channel leaks.

Most of the cloud providers such as Amazon, Google, IBM, Microsoft offer instances equipped with this Intel extension and use it as security feature in their products. AES can be used in applications where confidentiality and integrity is of highest priority. If cryptographic strength is a major factor in the application, AES is the best suited algorithm.