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QARMA

QARMA (from Qualcomm ARM Authenticator) is a lightweight tweakable block cipher primarily known for its use in the ARMv8 architecture for protection of software as a cryptographic hash for the Pointer Authentication Code. The cipher was proposed by Roberto Avanzi in 2016. Two versions of QARMA are defined: QARMA-64 (64-bit block size with a 128-bit encryption key) and QARMA-128 (128-bit block size with a 256-bit key). The design of the QARMA was influenced by PRINCE and MANTIS. The cipher is intended for fully-unrolled hardware implementations with low latency (like memory encryption). Unlike the XTS mode, the address can be directly used as a tweak and does not need to be whitened with the block encryption first.

Architecture

QARMA is an Even–Mansour cipher using three stages, with whitening keys w<sup>0</sup> and w<sup>1</sup> XORed in between:

  1. permutation F is using core key k<sup>0</sup> and parameterized by a tweak T. It has r rounds inside (r = 7 for QARMA-64, r = 11 for QARMA-128);
  2. "central" permutation C is using key k<sup>1</sup> and is designed to be reversible via a simple key transformation (contains two central rounds);
  3. the third permutation is an inverse of the first (r more rounds).

All keys are derived from the master encryption key K using specialisation:

  • K is partitioned into halves as w<sup>0</sup> Concatenation k<sup>0</sup>, each will have halfsize bits;
  • for encryption w<sup>1</sup> = (w<sup>0</sup> >>> 1) + (w<sup>0</sup> >> (halfsize-1));
  • for encryption k<sup>1</sup> = k<sup>0</sup>;
  • for decryption, the same design can be used as long as k<sup>0</sup>+α is used as a core key, k<sup>1</sup> = Q•k<sup>0</sup>, w<sup>1</sup> and w<sup>0</sup> are swapped. α here is a special constant and Q a special involutary matrix. This construct is similar to the alpha reflection in PRINCE.

The data is split into 16 cells (4-bit nibbles for QARMA-64, 8-bit bytes for QARMA-128). Internal state also contains 16 cells, arranged in a 4x4 matrix, and is initialized by plaintext (XORed with w<sup>0</sup>). In each round of , the state is transformed via operations :

  • is ShuffleCells, a MIDORI permutation of cells ([ 0, 11, 6, 13, 10, 1, 12, 7, 5, 14, 3, 8, 15, 4, 9, 2]);
  • is MixColumns: each column is multiplied by a fixed matrix M;
  • is SubCells: each cell is transformed using an S-box.

The tweak for each round is updated using :

  • is a cell permutation from MANTIS ([ 6, 5, 14, 15, 0, 1, 2, 3, 7, 12, 13, 4, 8, 9, 10, 11]);
  • is an LFSR applied to each of the cells with numbers [0, 1, 3, 4, 8, 11, 13]. For QARMA-64, the LFSR is (b3, b2, b1, b0) ⇒ (b0 + b1, b3, b2, b1), for QARMA-128, (b7, b6, ..., b0) ⇒ (b0 + b2, b7, b6, ..., b1),

The rounds of consist of inverse operations . Central rounds, in addition to two rounds ( and ), include multiplication of the state by an involutary matrix Q.

References

Sources

External links