/usr/sbin/rebuild-security-providers is in libgcj-common 1:4.8.2-1ubuntu6.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | #! /bin/bash
# Rebuild the list of security providers in classpath.security
secfiles="/var/lib/security/classpath.security"
for secfile in $secfiles; do
  # check if this classpath.security file exists
  #[ -f "$secfile" ] || continue
  sed '/^security\.provider\./d' /etc/java/security/classpath.security \
      > $secfile
  count=0
  for provider in $(ls /etc/java/security/security.d)
  do
    count=$((count + 1))
    echo "security.provider.${count}=${provider#*-}" >> "$secfile"
  done
done
 |