Generate salt using 128bits from arc4random_buf

Dockerfile

@@ -2,7 +2,7 @@ FROM ubuntu:latest
 
 WORKDIR .
 
-RUN apt-get update && apt-get install -y curl gcc make git libpcre3-dev zlib1g-dev
+RUN apt-get update && apt-get install -y curl gcc make git libpcre3-dev zlib1g-dev libbsd-dev
 
 ADD ipscrub /ipscrub/
 ADD Makefile /

README.md

@@ -15,7 +15,7 @@
 
 ## Security Model
 
-1. On initialization, and again every `PERIOD`, generate `salt` as `HASH(ngx_random() ++ timestamp)`.
+1. On initialization, and again every `PERIOD`, generate `salt` using 128bits from `arc4random_buf()`.
 2. On each request, generate masked IP address as `HASH(salt ++ IP address)`.
 3. Log masked IP address.
 

ipscrub/config

@@ -4,7 +4,13 @@ ngx_module_type=HTTP
 ngx_module_name=ngx_ipscrub_module
 ngx_module_deps="$ngx_addon_dir/src/ngx_ipscrub_support.h $ngx_addon_dir/src/ngx_ipscrub_debug.h"
 ngx_module_srcs="$ngx_addon_dir/src/ngx_ipscrub_module.c $ngx_addon_dir/src/ngx_ipscrub_support.c $ngx_addon_dir/src/ngx_ipscrub_debug.c"
-ngx_module_libs=SHA1
+
+# On Linux, link with libbsd, to get arc4random.
+if [ `uname` = Linux ]; then
+    ngx_module_libs="SHA1 -lbsd"
+else
+    ngx_module_libs="SHA1"
+fi
 
 . auto/module
 

ipscrub/src/ngx_ipscrub_module.c

@@ -42,7 +42,8 @@ static ngx_command_t  ngx_ipscrub_commands[] = {
 // Globals.
 const int default_period_seconds = 10 * 60; // Period between salt changes.
 time_t period_start = -1;
-long nonce = -1; // Input to salt generation.
+#define num_nonce_bytes 16
+u_char nonce[num_nonce_bytes]; // Input to salt generation.
 
 
 static ngx_http_module_t  ngx_ipscrub_module_ctx = {
@@ -146,7 +147,7 @@ ngx_http_variable_remote_addr_ipscrub(ngx_http_request_t *r, ngx_http_variable_v
   // Regenerate salt if past end of period.
   time_t now = time(NULL);
   if (period_start == -1 || now - period_start > icf->period_seconds) {
-    rc = randlong(&nonce);
+    rc = randbytes((u_char *) &nonce, num_nonce_bytes);
     if (rc != NGX_OK) {
         return NGX_HTTP_INTERNAL_SERVER_ERROR;
     }
@@ -156,7 +157,7 @@ ngx_http_variable_remote_addr_ipscrub(ngx_http_request_t *r, ngx_http_variable_v
   }
 
   salt.data = (u_char *) &nonce;
-  salt.len = sizeof(nonce);
+  salt.len = num_nonce_bytes;
 
   // Although ngx_crypt provides a salted SHA function, specified by a salt beginning with {SSHA}, that function exposes the salt in its result. For our security model, this is inappropriate. Instead, we use the regular nginx SHA function specified by {SHA}, and manually combine the nonce and plaintext.
   rc = concat(r->pool, r->connection->addr_text, salt, &combined);

ipscrub/src/ngx_ipscrub_support.c

@@ -3,6 +3,15 @@
 
 #include "ngx_ipscrub_support.h"
 
+#if (NGX_FREEBSD || NGX_SOLARIS || NGX_DARWIN)
+// arc4random is built-in on these platforms.
+#elif (NGX_LINUX)
+#include <bsd/stdlib.h>
+#else
+// TODO: test using libbsd on Windows.
+#error ipscrub requires arc4random_buf.
+#endif
+
 // null_terminate allocates a new, null-terminated string based on input.
 ngx_int_t null_terminate(ngx_pool_t *pool, ngx_str_t input, u_char **out)
 {
@@ -37,23 +46,20 @@ ngx_int_t concat(ngx_pool_t *pool, ngx_str_t prefix, ngx_str_t suffix, u_char **
   return NGX_OK;
 }
 
-// randlong fills out with secure random bytes and returns NGX_OK iff successful.
+// randbytes fills out with secure random bytes.
-ngx_int_t randlong(long *out) {
+// Return value of NGX_OK indicates success.
-  #if !(NGX_DARWIN || NGX_SOLARIS || NGX_FREEBSD || NGX_LINUX)
+// Return value of NGX_ERROR indicates error.
-    // Windows not supported a.t.m.
+ngx_int_t randbytes(u_char *out, int num_bytes) {
-    // TODO: support Windows (https://msdn.microsoft.com/en-us/library/sxtz2fa8.aspx).
+  if (out == NULL) {
-    return -1;
+    return NGX_ERROR;
-  #endif
+  }
-
+  if (num_bytes < 1 || num_bytes > 64) {
-  int rand = open("/dev/urandom", O_RDONLY);
+    // Values outside these bounds may indicate parameter usage mistake.
-  if (rand < 0) {
+    return NGX_ERROR;
-      return -1;
   }
 
-  ssize_t ret = read(rand, out, sizeof(long));
+
-  if (ret != sizeof(long)) {
+  arc4random_buf(out, num_bytes);
-      return -1;
-  }
 
   return NGX_OK;
 }

ipscrub/src/ngx_ipscrub_support.h

@@ -8,6 +8,6 @@
 
 ngx_int_t null_terminate(ngx_pool_t *pool, ngx_str_t input, u_char **hashed);
 ngx_int_t concat(ngx_pool_t *pool, ngx_str_t prefix, ngx_str_t suffix, u_char **out);
-ngx_int_t randlong(long *out);
+ngx_int_t randbytes(u_char *out, int num_bytes);
 
 #endif /* _IPSCRUB_SUPPORT_H_INCLUDED_ */