Modes of Operation
The mode of operation used by VeraCrypt for encrypted partitions, drives, and virtual volumes is XTS.
XTS mode is in fact XEX mode [12], which was designed by Phillip Rogaway in 2003, with a minor modification (XEX mode uses a single key for two different purposes, whereas XTS mode uses two independent keys).
In 2010, XTS mode was approved by NIST for protecting the confidentiality of data on storage devices [24]. In 2007, it was also approved by the IEEE for cryptographic protection of data on block-oriented storage devices (IEEE 1619).
Description of XTS mode:
Ci =
EK1(Pi ^ (EK2(n)
ai)) ^ (EK2(n)
ai)
ai)) ^ (EK2(n)
ai)
Where:
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denotes multiplication of two polynomials over the binary field GF(2) modulo x128+x7+x2+x+1 |
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K1 |
is the encryption key (256-bit for each supported cipher; i.e, AES, Serpent, and Twofish) |
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K2 |
is the secondary key (256-bit for each supported cipher; i.e, AES, Serpent, and Twofish) |
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i |
is the cipher block index within a data unit; for the first cipher block within a data unit, i = 0 |
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n |
is the data unit index within the scope of K1; for the first data unit, n = 0 |
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a |
is a primitive element of Galois Field (2128) that corresponds to polynomial x (i.e., 2) |
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Note: The remaining symbols are defined in the section Notation. |
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The size of each data unit is always 512 bytes (regardless of the sector size).