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MultiSwap

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Block cipher and message authentication code
MultiSwap
General
DesignersMicrosoft
First publishedApril 1999
Cipher detail
Key sizes374 bits
Block sizes64 bits
Best publiccryptanalysis
Thedifferential cryptanalysis of Borisov, et al. requires about 213chosen plaintexts or about 222known plaintexts
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Incryptography,MultiSwap is ablock cipher/MAC created byMicrosoft in 1999 as part of itsWindows Media DRM service (WMDRM). Microsoft's internal name for the algorithm is not publicly known; it was dubbed MultiSwap in a 2001 report on WMDRM under the pseudonym "Beale Screamer".

The cipher has ablock size of 64 bits, but the two halves are processed nearly separately. All arithmetic operations are performedmod 232. In the encryption process, each half block has added to it the output of the previous half block. Next it undergoes 5 multiplications by odd 32-bit subkeys, each followed by a swap of its 16-bit halves. Then a final subkey is added to it. As the half blocks use separate subkeys, and the multipliers are forced to be odd, the totalkey size is 374 bits. The name MultiSwap comes from the cipher's multiplications and swaps. WMDRM uses this algorithm only as a MAC, never for encryption.

Borisov, et al. applied a multiplicative form ofdifferential cryptanalysis to break MultiSwap.

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