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Usage
#include <sodium.h>
int main(void)
{
if (sodium_init() == -1) {
return 1;
}
...
}
sodium.h is the only header that has to be included.The library is called
sodium (use -lsodium to link it), and proper compilation/linker flags can be obtained using pkg-config on systems where it is available:CFLAGS=$(pkg-config --cflags libsodium)
LDFLAGS=$(pkg-config --libs libsodium)
For static linking, Visual Studio users should define
SODIUM_STATIC=1 and SODIUM_EXPORT=. This is not required on other platforms.Projects using CMake can include the Findsodium.cmake file from the Facebook Fizz project to detect and link the library.
sodium_init() initializes the library and should be called before any other function provided by Sodium. It is safe to call this function more than once and from different threads – subsequent calls won’t have any effects.After this function returns, all of the other functions provided by Sodium will be thread-safe.
sodium_init() doesn’t perform any memory allocations. However, on Unix systems, it may open /dev/urandom and keep the descriptor open so that the device remains accessible after a chroot() call.Multiple calls to
sodium_init() do not cause additional descriptors to be opened.sodium_init() returns 0 on success, -1 on failure, and 1 if the library had already been initialized.Before returning, the function ensures that the system’s random number generator has been properly seeded.
On some Linux systems, this may take some time, especially when called right after a reboot of the system. This issue has been reported on Digital Ocean virtual machines, Scaleway ARM instances, and AWS Nitro Enclaves.
This can be confirmed with the following command:
cat /proc/sys/kernel/random/entropy_avail
If the command returns
0 or a very low number (< 160), and you are not running an obsolete kernel, this is very likely to be the case.In a virtualized environment, make sure that the
virtio-rng interface is available. If this is a cloud service and the hypervisor settings are out of your reach, consider switching to a different service.Current Linux kernels (>= 5.4) include the
haveged algorithm in order to mitigate that problem. So, before trying the last-resort solutions below, try using a recent kernel.If you have to use a kernel before version 5.4, a possible workaround is to install
haveged:apt-get install haveged
An alternative is
rng-tools:apt-get install rng-tools
Jitterentropy is a better alternative, but most Linux distributions don’t offer it as an installable package yet.
After installating these tools, check the value of
/proc/sys/kernel/random/entropy_avail again.On AWS Nitro Enclaves, workarounds include:
- Calling the
aws_nitro_enclaves_library_seed_entropy()function beforesodium_init(), and occasionally afterwards. - Using the
RDSEEDCPU instruction to seed the kernel RNG (not recommended as a unique entropy source). - Setting
random.trust_cpu=onin the kernel command line (requires Linux kernel > 4.19).
Applications can warn users about the Linux RNG not being seeded before calling
sodium_init() using code similar to the following:#if defined(__linux__)
# include <fcntl.h>
# include <unistd.h>
# include <sys/ioctl.h>
# include <linux/random.h>
#endif
// ...
#if defined(__linux__) && defined(RNDGETENTCNT)
int fd;
int c;
if ((fd = open("/dev/random", O_RDONLY)) != -1) {
if (ioctl(fd, RNDGETENTCNT, &c) == 0 && c < 160) {
fputs("This system doesn't provide enough entropy to quickly generate high-quality random numbers.\n"
"Upgrading the kernel, installing the rng-utils/rng-tools, jitterentropy-rngd or haveged packages may help.\n"
"On virtualized Linux environments, also consider using virtio-rng.\n"
"The service will not start until enough entropy has been collected.\n", stderr);
}
(void) close(fd);
}
#endif
Congrats, you’re all set up!
Last modified 2mo ago