Any perl-monks crypto savvy ?
I'd like to have an opinion on how to choose good primes for DH agreement :
here is an except of my perl code, I have mixed feeling about it, please comment !
(link for downloading the file directly : http://gateway.ipfs.io/ipfs/QmRi8Ag9ApvgXYhHybvdY9RXRwxG1gkUTuihN6qq2MBjeq )

#!/usr/bin/perl

our $dbug = 0;
use YAML::Syck qw(DumpFile);
# 0) initialization get (g,p)
# 1) Bob to create keypair

# 2) Alice to get Bob's public key
# 3) Alice to compute a shared-secret with Bob's public key...
# 4) Alice to send her public-key to Bob

# 5) Bob to generate his side of the shared-secret
# 6) compare !

my $user = 'Bob';

# ---------------------------------------------------
$PKI = {
  anon => ['anonymous', 'pubkey' => '123', secret => undef ]
};
# ---------------------------------------------------
my $seed = srand(); printf "seed: %s\n",$seed;
my $iv_key = rand(4125415);

my ($g,$p) = &get_IV($iv_key);

# keys agreement :
use Crypt::DH;

my $bob; # Bob's DH
# ---------------------------------------------------------
if ($user eq 'Bob') {
   my $I = uc substr($user,0,1);
   $bob = Crypt::DH->new( p => "$p", g => $g );
   if ($dbug) {
      printf "g: %s\n",$bob->g();
      printf "p: %s\n",$bob->p();
   }

   my $pub =  $bob->generate_keys;
   my $keypair = {
      user => $user, pubkey => $pub, seckey => $bob->priv_key()
   };
   printf "%s:\n",$user;
   printf "pub(%s): %s\n",$I,substr($pub,0,32);
   printf "priv(%s): %s...\n",$I,substr($bob->priv_key(),0,32);
   DumpFile("DH_$user.key",$keypair);
   $PKI->{bob}{pubkey} = $bob->pub_key();
   print ".\n";
}
# ---------------------------------------------------------
{ # Alice's 
   my $dh = Crypt::DH->new( p => "$p", g => $g );
   my $pub =  $dh->generate_keys;
   my $keypair = {
      user => 'Alice', pubkey => $pub, seckey => $dh->priv_key()
   };
   printf "%s:\n",'Alice';
   printf "pub(A): %s...\n",substr($pub,0,32);
   printf "priv(A): %s...\n",substr($dh->priv_key(),0,32);
   DumpFile("DH_Alice.key",$keypair);
   print ".\n";


   my $secret = $dh->compute_secret( $PKI->{bob}{pubkey} );
   printf "Alice's secret : %s\n",$secret;

   $PKI->{alice}{pubkey} = $dh->pub_key();
   $PKI->{alice}{secret} = $secret;

}
# ---------------------------------------------------------
{ # BOB again ...
   my $secret = $bob->compute_secret( $PKI->{alice}{pubkey} );
   printf "Bob's secret : %s\n",$secret;
   die if ($secret ne $PKI->{alice}{secret});
}

exit $?;


# --------------------------------
sub get_IV { # set initialization vector for keypair generation
   my $key = shift;

   my ($g,$p) = (2,undef);
   if ($dbug == 1) {
      ($g,$p) = (3,107); # very bad choice but simple case to verify t
+he maths !
   } else {
      $p = &get_prime($key);
   }
   return ($g,$p);
}
# --------------------------------


# ------------------------------------------
sub get_prime {
  my $key = shift;
  local $/ = "\n";
  my $buf = '';
  while (<DATA>) { # get prime from the bottom of this file !
     next if /^#/o; # skip comments
     next if /^__\w+__/; # & markers
     chomp;
     y/ //d;
     $buf .= $_;
  }
  printf "buf: %s.\n",$buf if $dbug > 1;
  use Math::BigInt;
  my $bint = Math::BigInt->from_hex($buf);
  if (defined $key) {
     use Math::Primality qw(is_prime next_prime);
     my $digest = &digest('RIPEMD-160',$key); $digest =~ s/ //go;
     my $offset = Math::BigInt->from_hex($digest);
     my $candidate = $bint + $offset;
     if ($dbug > 1) { 
        print "hash: $digest\n";
        print "cand: $candidate\n";
     }
     if (! &is_prime($candidate)) {
        print " p is not a prime ... will find an other one ! \n" if $
+dbug;
        $bint = &next_prime($candidate);
        if (&is_prime($bint)) {
           printf "%s is now a prime\n",$bint if $dbug;
        }
     } else {
        $bint = $candidate;
        printf "%s is prime!\n",$bint;
     }
  }
  return $bint;
}
# ------------------------------------------
sub digest {
   use Digest qw();
   my $alg = shift;
   my $msg = Digest->new($alg) or die $!;
   $msg->add(join'',@_);
   my $digest = lc( $msg->hexdigest() );
   return $digest; # hex form !
}
# ------------------------------------------
1;
__DATA__
# SSH2 prime:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381
FFFFFFFF FFFFFFFF
__END__
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