In cryptography, MultiSwap is a block cipher/MAC created by Microsoft in 1999 as part of its Windows 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 a block size of 64 bits, but the two halves are processed nearly separately.
As the half blocks use separate subkeys, and the multipliers are forced to be odd, the total key size is 374 bits.
Borisov, et al. applied a multiplicative form of differential cryptanalysis to break MultiSwap.