[gcrypt-devel] gcry_pk_genkey function is extremely slow in libgcrypt

Karl Magdsick kmagnum at gmail.com
Tue Sep 6 03:20:47 CEST 2016

Virtual machines may gather entropy incredibly slowly.  While you're stuck,
in another terminal try

prompt> time dd bs=128 count=1 if=/dev/random/ | uuencode -

My guess is this alone will take several minutes for your vm.


On Sep 6, 2016 2:51 AM, "Shuai Wang" <wangshuai901 at gmail.com> wrote:

I am a newbie to libgcrypt(version 1.6.1), and right now I am trying to
produce a public/private key pair for rsa algorithm.

I list the code I am using below. What I am trapped into is the
gcry_pk_genkey function, in which it can take over 1.5 hours but never

int main(int argc, char** argv)
if (argc != 2) {
    fprintf(stderr, "Usage: %s <rsa-keypair.sp>\n", argv[0]);
    xerr1("Invalid arguments.");


gcry_error_t err = 0;
gcry_sexp_t rsa_parms;
gcry_sexp_t rsa_keypair;

err &= gcry_sexp_build(&rsa_parms, NULL, "(genkey (rsa (nbits 4:2048)))");
if (err) {
    xerr1("gcrypt: failed to create rsa params");

err &= gcry_pk_genkey(&rsa_keypair, rsa_parms); <------- This function call
if (err) {
    xerr1("gcrypt: failed to create rsa key pair");

char* fname = argv[1];
err = gcrypt_sexp_to_file(fname, rsa_keypair, 1 << 16);

    printf("i am here3\n");

return err;

I am aware that this function can take a few minutes. Your computer needs
to gather random entropy.. However, I can hardly believe it could take
almost 2 hours without return/throw exception...

I am using a 32-bit Ubuntu 14.04, inside a virtualbox VM instance. Am I
doing anything wrong here?

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