The uplm package provides simple utilities for doing arbitrary-precision unsigned integer arithmetic for cryptographic purposes, such as "manually" performing Diffie-Hellman key exchanges rather than using specialized tools like OpenSSL.
They are intended to follow the UNIX-principle of being small and simple and act primarily as a filter, i. e. they read something from the standard input stream and write something else to standard output stream.
Multiple utilities can be combined using pipes or temporary files to accomplish more complex tasks than any individual utility can achieve on its own.
The utilities are also intended to be portable, i. e. they do not depend on more than a C89 (or better) compliant C compiler and the standard C library.
All utilities assume that they process highly confidential key material, which dictates the actual numbers being processed will never be provided as command line arguments, but will always be read from a file, pipe or file descriptor (such as /dev/fd/*) instead.
To avoid side-channel attacks which measure the processing time or power consumption of some calculations, the implementation avoids short-circuit evaluations or data-dependent branches whereever possible. The run-time of the utilities should therefore be dominated only by the magnitude of the numeric operands, but not on their particular bit patterns.
The utilities do not try to mlock() their data while processing or "burn" variables after use, because such functionality is not provided by the C standard and the reliability of such features is debatable even if it is available, such as in POSIX.
Therefore, make sure that your confidental data is read either from encrypted storage or from volatile storage such as from a RAM disk.
Make also sure that either no swapping is enabled, or that your swap space is encrypted as well. Because otherwise the memory image of the running process could be swapped out to disk unprotected, where it could be found by an attacker later.
Of course, all this only matters if you are actually using uplm for processing confidential data. You do not need to consider such issues at all if you use uplm just for doing long math.