The Speck cipher
is geared toward good performance in software, which makes it attractive
for smaller, often embedded, systems with underpowered CPUs that lack
hardware crypto acceleration. But it also
comes from the US National Security Agency (NSA), which worries lots of
people outside the US—and, in truth, a fair number of US citizens as well.
The NSA has earned a reputation for promulgating various types of
cryptographic algorithms with dubious properties. While the technical
arguments against Speck, which is a fairly simple and straightforward
algorithm with little room for backdoors, have not been all that
compelling, the political arguments are potent—to the point where it is
being dropped by the main
proponent for including it in the kernel.