AES

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Jun 9, 2009 4:55 AM
Jun 9th, 2009

Introduction

The Advanced Encryption Standard (AES) computer security standard is a symmetric block cipher that encrypts and decrypts 128-bit blocks of data. Standard key lengths of 128, 192, and 256 bits may be  used. The algorithm consists of four stages that make up a round which is iterated 10 times for a 128-bit length key, 12 times for a 192-bit key, and 14 times for a 256-bit key.

The first stage “SubBytes” transformation is a non-linear byte substitution for each byte of the block. The second stage “ShiftRows” transformation cyclically shifts (permutes) the bytes within the block. The third stage “MixColumns” transformation groups 4-bytes together forming 4-term polynomials and multiplies the polynomials with a fixed polynomial mod (x^4+1). The fourth stage “AddRoundKey” transformation adds the round key with the block of data.

In most ciphers, the iterated transform (or round) usually has a Feistel Structure. Typically in this structure, some of the bits of the intermediate state are transposed unchanged to another position (permutation). AES does not have a Feistel structure but is composed of three distinct invertible transforms based on the Wide Trial Strategy design method.

The Wide Trial Strategy design method provides resistance against linear and differential cryptanalysis. In the Wide Trail Strategy, every layer has its own function:

  •     The linear mixing layer: guarantees high diffusion over multiply rounds
  •     The non-linear layer: parallel application of S-boxes that have the optimum worst-case non-linearity properties.
  •     The key addition layer: a simple XOR of the round key to the intermediate state

Terminology


  •       Plaintext refers to the data to be encrypted. Ciphertext refers to the data after going through the cipher as well as the  data that will be                   going into the decipher.
  •       The state is an intermediate form of the cipher or decipher result usually displayed as a rectangular table of bytes with 4 rows and 4 columns.

Features

  • Key lengths of 128, 192, and 256 bits are supported. Each step in key size requires only two additional rounds.

  • The decipher is simply the inverse of the cipher.

Implementations

  • Optimized Software Implementation. The pure software implementation  is bounded by the load/store behavior and byte arithmetic of the  algorithm. The encryption requires 774 cycles per block on a MIPS32  processor and the decryption requires 837 cycles.

  • AES Primitives. This is the simplest form of VOCAL’s hardware  acceleration. The AES Primitives extend the capabilities of the MIPS32  processor by taking advantage of MIPS Technologies CorExtend capability  to decrease the number of cycles to 393 cycles to encrypt and 460 cycles  to decrypt per block on the MIPS32 processor.

  • AES Round Accelerator. The Round Accelerator requires 1024 bytes of  local memory, but increases the performance to 117 cycles per block to  encrypt and 127 cycles per block to decrypt.

  • AES 32-bit Block Accelerator. The Block Accelerator is designed to  be a good mid-scale solution. It uses 2048 bytes of local memory. The  number of cycles to process a block on a MIPS32 cpu falls to 64 cycles  for both encryption and decryption using this implementation.

  • AES 32-bit Co-Processor. The Co-Processor implementation uses 2048  bytes of memory to deliver performance of 45 cycles per block on the  MIPS32.

  • AES 64-bit Co-Processor. The same amount of the memory is required  for the 64-bit implementation, but the performance increases to just 25  cycles per block on the MIPS32.

  • Hardware/firmware.

RFCs:

  • Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS) - RFC 3268
  • Advanced Encryption Standard (AES) Key Wrap Algorithm - RFC 3394
  • The AES-XCBC-MAC-96 Algorithm and Its Use With IPsec - RFC 33566
  • The AES-CBC Cipher Algorithm and Its Use with IPsec - RFC 3602
  • Advanced Encryption Standard (AES) Encryption for Kerberos 5 - RFC 3962
  • The AES-CMAC Algorithm - RFC 4493

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Posted June 9, 2009 at 4:55 AM
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