diff --git a/ch04.asciidoc b/ch04.asciidoc
index 38be5e83..b078369f 100644
--- a/ch04.asciidoc
+++ b/ch04.asciidoc
@@ -555,7 +555,7 @@ include::code/ec-math.py[]
 
 [NOTE]
 ====
-The example above uses +os.urandom+ which reflects a cryptographically secure random number generator (CSRNG) provided by the underlying OS. In the case of UNIX-like operating system such as Linux, it draws from +/dev/urandom+ and in the case of Windows calls +CryptGenRandom()+. If a suitable randomness source is not found, +NotImplementedError+ will be raised. While the random number generator used here is for demonstration purposes, it is not appropriate for generating production-quality bitcoin keys as it is not implemented with sufficient security.
+The example above uses +os.urandom+ which reflects a cryptographically secure random number generator (CSRNG) provided by the underlying operating system. In the case of UNIX-like operating system such as Linux, it draws from +/dev/urandom+ and in the case of Windows calls +CryptGenRandom()+. If a suitable randomness source is not found, +NotImplementedError+ will be raised. While the random number generator used here is for demonstration purposes, it is _not_ appropriate for generating production-quality bitcoin keys as it is not implemented with sufficient security.
 ====
 
 [[ec_math_run]]
@@ -961,6 +961,11 @@ include::code/vanity-miner.cpp[]
 ----
 ====
 
+[NOTE]
+====
+The example above uses +std::random_device+. Depending on the implementation it may reflect a cryptographically secure random number generator (CSRNG) provided by the underlying operating system. In the case of UNIX-like operating system such as Linux, it draws from +/dev/urandom+. While the random number generator used here is for demonstration purposes, it is _not_ appropriate for generating production-quality bitcoin keys as it is not implemented with sufficient security.
+====
+
 The example code must be compiled using a C++ compiler and linked against the libbitcoin library (which must be first installed on that system). To run the example, run the ++vanity-miner++ executable with no parameters (see <<vanity_miner_run>>) and it will attempt to find a vanity address starting with "1kid".
 
 [[vanity_miner_run]]
diff --git a/code/vanity-miner.cpp b/code/vanity-miner.cpp
index 06112918..9b85567e 100644
--- a/code/vanity-miner.cpp
+++ b/code/vanity-miner.cpp
@@ -12,8 +12,12 @@ bool match_found(const std::string& address);
 
 int main()
 {
+    // random_device on Linux uses "/dev/urandom"
+    // CAUTION: Depending on implementation this RNG may not be secure enough!
+    // Do not use vanity keys generated by this example in production
     std::random_device random;
     std::default_random_engine engine(random());
+    
     // Loop continuously...
     while (true)
     {