This allow switching the hashing algorithm (between a crypt(3)-based
one and MD5) at run-time, rather than at compile-time.

	-mi

--- dht-example.c	2018-02-09 16:44:37.000000000 -0500
+++ dht-example.c	2018-12-02 17:47:54.158580000 -0500
@@ -12,4 +12,5 @@
 #include <fcntl.h>
 #include <sys/time.h>
+#include <netinet/in.h>
 #include <arpa/inet.h>
 #include <sys/types.h>
@@ -18,4 +19,7 @@
 #include <signal.h>
 #include <sys/signal.h>
+#include <signal.h>
+#include <unistd.h>
+#include <md5.h>
 
 #include "dht.h"
@@ -115,4 +119,11 @@
 
 
+typedef void (hashing_method)(void *, int,
+	const void *, int,
+	const void *, int,
+	const void *, int);
+
+static hashing_method *hasher, crypt_hash, md5_hash;
+
 int
 main(int argc, char **argv)
@@ -137,6 +148,8 @@
     sin6.sin6_family = AF_INET6;
 
+    hasher = crypt_hash;
+
     while(1) {
-        opt = getopt(argc, argv, "q46b:i:");
+        opt = getopt(argc, argv, "q46b:i:m");
         if(opt < 0)
             break;
@@ -165,4 +178,6 @@
             id_file = optarg;
             break;
+	case 'm':
+            hasher = md5_hash;
         default:
             goto usage;
@@ -443,6 +458,7 @@
 
  usage:
-    printf("Usage: dht-example [-q] [-4] [-6] [-i filename] [-b address]...\n"
-           "                   port [address port]...\n");
+    printf("Usage: dht-example [-q] [-4] [-6] [-i filename] [-b address] [-m]\n"
+           "                   port [address port]...\n"
+           "(Use -m to use MD5 digest instead of crypt()-based one)\n");
     exit(1);
 }
@@ -465,25 +481,30 @@
 /* We need to provide a reasonably strong cryptographic hashing function.
    Here's how we'd do it if we had RSA's MD5 code. */
-#if 0
+
 void
-dht_hash(void *hash_return, int hash_size,
+md5_hash(void *hash_return, int hash_size,
          const void *v1, int len1,
          const void *v2, int len2,
          const void *v3, int len3)
 {
-    static MD5_CTX ctx;
+    MD5_CTX ctx;
     MD5Init(&ctx);
     MD5Update(&ctx, v1, len1);
     MD5Update(&ctx, v2, len2);
     MD5Update(&ctx, v3, len3);
-    MD5Final(&ctx);
-    if(hash_size > 16)
-        memset((char*)hash_return + 16, 0, hash_size - 16);
-    memcpy(hash_return, ctx.digest, hash_size > 16 ? 16 : hash_size);
+    if (hash_size >= 16) {
+        MD5Final(hash_return, &ctx);
+        if(hash_size > 16)
+            memset((char*)hash_return + 16, 0, hash_size - 16);
+    } else {
+        unsigned char digest[16];
+        MD5Final(digest, &ctx);
+        memcpy(hash_return, digest, hash_size);
+    }
 }
-#else
+
 /* But for this toy example, we might as well use something weaker. */
 void
-dht_hash(void *hash_return, int hash_size,
+crypt_hash(void *hash_return, int hash_size,
          const void *v1, int len1,
          const void *v2, int len2,
@@ -505,5 +526,14 @@
     strncpy(hash_return, crypt(key, "jc"), hash_size);
 }
-#endif
+
+void
+dht_hash(void *hash_return, int hash_size,
+         const void *v1, int len1,
+         const void *v2, int len2,
+         const void *v3, int len3)
+{
+	hasher(hash_return, hash_size, v1, len1,
+	    v2, len2, v3, len3);
+}
 
 int