diff --git a/include/mbedtls/entropy.h b/include/mbedtls/entropy.h
index a99d20d..597f03d 100644
--- a/include/mbedtls/entropy.h
+++ b/include/mbedtls/entropy.h
@@ -53,7 +53,8 @@
 #define MBEDTLS_ERR_ENTROPY_SOURCE_FAILED                 -0x003C  /**< Critical entropy source failure. */
 #define MBEDTLS_ERR_ENTROPY_MAX_SOURCES                   -0x003E  /**< No more sources can be added. */
 #define MBEDTLS_ERR_ENTROPY_NO_SOURCES_DEFINED            -0x0040  /**< No sources have been added to poll. */
-#define MBEDTLS_ERR_ENTROPY_FILE_IO_ERROR                 -0x0058  /**< Read/write error in file. */
+#define MBEDTLS_ERR_ENTROPY_NO_STRONG_SOURCE              -0x003D  /**< No strong sources have been added to poll. */
+#define MBEDTLS_ERR_ENTROPY_FILE_IO_ERROR                 -0x003F  /**< Read/write error in file. */
 
 /**
  * \name SECTION: Module settings
@@ -82,6 +83,9 @@
 #define MBEDTLS_ENTROPY_MAX_SEED_SIZE   1024    /**< Maximum size of seed we read from seed file */
 #define MBEDTLS_ENTROPY_SOURCE_MANUAL   MBEDTLS_ENTROPY_MAX_SOURCES
 
+#define MBEDTLS_ENTROPY_SOURCE_STRONG   1       /**< Entropy source is strong   */
+#define MBEDTLS_ENTROPY_SOURCE_WEAK     0       /**< Entropy source is weak     */
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -109,6 +113,7 @@
     void *          p_source;   /**< The callback data pointer */
     size_t          size;       /**< Amount received in bytes */
     size_t          threshold;  /**< Minimum bytes required before release */
+    int             strong;     /**< Is the source strong? */
 }
 mbedtls_entropy_source_state;
 
@@ -156,12 +161,17 @@
  * \param p_source  Function data
  * \param threshold Minimum required from source before entropy is released
  *                  ( with mbedtls_entropy_func() ) (in bytes)
+ * \param strong    MBEDTLS_ENTROPY_SOURCE_STRONG or
+ *                  MBEDTSL_ENTROPY_SOURCE_WEAK.
+ *                  At least one strong source needs to be added.
+ *                  Weaker sources (such as the cycle counter) can be used as
+ *                  a complement.
  *
  * \return          0 if successful or MBEDTLS_ERR_ENTROPY_MAX_SOURCES
  */
 int mbedtls_entropy_add_source( mbedtls_entropy_context *ctx,
                         mbedtls_entropy_f_source_ptr f_source, void *p_source,
-                        size_t threshold );
+                        size_t threshold, int strong );
 
 /**
  * \brief           Trigger an extra gather poll for the accumulator
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 9244249..4f43018 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -61,9 +61,8 @@
  * PADLOCK   1  0x0030-0x0030
  * DES       1  0x0032-0x0032
  * CTR_DBRG  4  0x0034-0x003A
- * ENTROPY   3  0x003C-0x0040
+ * ENTROPY   3  0x003C-0x0040   0x003D-0x003F
  * NET       9  0x0042-0x0052   0x0043-0x0043
- * ENTROPY   1  0x0058-0x0058
  * ASN1      7  0x0060-0x006C
  * PBKDF2    1  0x007C-0x007C
  * HMAC_DRBG 4  0x0003-0x0009
diff --git a/library/entropy.c b/library/entropy.c
index 4dddb75..92717c8 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -77,18 +77,23 @@
 #if !defined(MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES)
 #if !defined(MBEDTLS_NO_PLATFORM_ENTROPY)
     mbedtls_entropy_add_source( ctx, mbedtls_platform_entropy_poll, NULL,
-                        MBEDTLS_ENTROPY_MIN_PLATFORM );
+                                MBEDTLS_ENTROPY_MIN_PLATFORM,
+                                MBEDTLS_ENTROPY_SOURCE_STRONG );
 #endif
 #if defined(MBEDTLS_TIMING_C)
-    mbedtls_entropy_add_source( ctx, mbedtls_hardclock_poll, NULL, MBEDTLS_ENTROPY_MIN_HARDCLOCK );
+    mbedtls_entropy_add_source( ctx, mbedtls_hardclock_poll, NULL,
+                                MBEDTLS_ENTROPY_MIN_HARDCLOCK,
+                                MBEDTLS_ENTROPY_SOURCE_WEAK );
 #endif
 #if defined(MBEDTLS_HAVEGE_C)
     mbedtls_entropy_add_source( ctx, mbedtls_havege_poll, &ctx->havege_data,
-                        MBEDTLS_ENTROPY_MIN_HAVEGE );
+                                MBEDTLS_ENTROPY_MIN_HAVEGE,
+                                MBEDTLS_ENTROPY_SOURCE_STRONG );
 #endif
 #if defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
     mbedtls_entropy_add_source( ctx, mbedtls_hardware_poll, NULL
-                                MBEDTLS_ENTROPY_MIN_HARDWARE );
+                                MBEDTLS_ENTROPY_MIN_HARDWARE,
+                                MBEDTLS_ENTROPY_SOURCE_STRONG );
 #endif
 #endif /* MBEDTLS_NO_DEFAULT_ENTROPY_SOURCES */
 }
@@ -106,7 +111,7 @@
 
 int mbedtls_entropy_add_source( mbedtls_entropy_context *ctx,
                         mbedtls_entropy_f_source_ptr f_source, void *p_source,
-                        size_t threshold )
+                        size_t threshold, int strong )
 {
     int index, ret = 0;
 
@@ -122,9 +127,10 @@
         goto exit;
     }
 
-    ctx->source[index].f_source = f_source;
-    ctx->source[index].p_source = p_source;
+    ctx->source[index].f_source  = f_source;
+    ctx->source[index].p_source  = p_source;
     ctx->source[index].threshold = threshold;
+    ctx->source[index].strong    = strong;
 
     ctx->source_count++;
 
@@ -198,7 +204,7 @@
  */
 static int entropy_gather_internal( mbedtls_entropy_context *ctx )
 {
-    int ret, i;
+    int ret, i, have_one_strong = 0;
     unsigned char buf[MBEDTLS_ENTROPY_MAX_GATHER];
     size_t olen;
 
@@ -210,6 +216,9 @@
      */
     for( i = 0; i < ctx->source_count; i++ )
     {
+        if( ctx->source[i].strong == MBEDTLS_ENTROPY_SOURCE_STRONG )
+            have_one_strong = 1;
+
         olen = 0;
         if( ( ret = ctx->source[i].f_source( ctx->source[i].p_source,
                         buf, MBEDTLS_ENTROPY_MAX_GATHER, &olen ) ) != 0 )
@@ -227,6 +236,9 @@
         }
     }
 
+    if( have_one_strong == 0 )
+        return( MBEDTLS_ERR_ENTROPY_NO_STRONG_SOURCE );
+
     return( 0 );
 }
 
@@ -424,11 +436,12 @@
 
     mbedtls_entropy_init( &ctx );
 
-    /* First do a gather to mek sure we have default sources */
+    /* First do a gather to make sure we have default sources */
     if( ( ret = mbedtls_entropy_gather( &ctx ) ) != 0 )
         goto cleanup;
 
-    ret = mbedtls_entropy_add_source( &ctx, entropy_dummy_source, NULL, 16 );
+    ret = mbedtls_entropy_add_source( &ctx, entropy_dummy_source, NULL, 16,
+                                      MBEDTLS_ENTROPY_SOURCE_WEAK );
     if( ret != 0 )
         goto cleanup;
 
diff --git a/programs/pkey/gen_key.c b/programs/pkey/gen_key.c
index 13d23fc..a58a80e 100644
--- a/programs/pkey/gen_key.c
+++ b/programs/pkey/gen_key.c
@@ -286,7 +286,8 @@
     if( opt.use_dev_random )
     {
         if( ( ret = mbedtls_entropy_add_source( &entropy, dev_random_entropy_poll,
-                                        NULL, DEV_RANDOM_THRESHOLD ) ) != 0 )
+                                        NULL, DEV_RANDOM_THRESHOLD,
+                                        MBEDTLS_ENTROPY_SOURCE_STRONG ) ) != 0 )
         {
             mbedtls_printf( " failed\n  ! mbedtls_entropy_add_source returned -0x%04x\n", -ret );
             goto exit;
diff --git a/tests/suites/test_suite_entropy.function b/tests/suites/test_suite_entropy.function
index 83584e1..3b739cc 100644
--- a/tests/suites/test_suite_entropy.function
+++ b/tests/suites/test_suite_entropy.function
@@ -68,9 +68,11 @@
      * since we don't know how many sources were automatically added.
      */
     for( i = 0; i < MBEDTLS_ENTROPY_MAX_SOURCES; i++ )
-        (void) mbedtls_entropy_add_source( &ctx, entropy_dummy_source, NULL, 16 );
+        (void) mbedtls_entropy_add_source( &ctx, entropy_dummy_source, NULL,
+                                           16, MBEDTLS_ENTROPY_SOURCE_WEAK );
 
-    TEST_ASSERT( mbedtls_entropy_add_source( &ctx, entropy_dummy_source, NULL, 16 )
+    TEST_ASSERT( mbedtls_entropy_add_source( &ctx, entropy_dummy_source, NULL,
+                                             16, MBEDTLS_ENTROPY_SOURCE_WEAK )
                  == MBEDTLS_ERR_ENTROPY_MAX_SOURCES );
 
 exit:
@@ -116,7 +118,9 @@
 
     mbedtls_entropy_init( &ctx );
 
-    TEST_ASSERT( mbedtls_entropy_add_source( &ctx, entropy_dummy_source, &fail, 16 )
+    TEST_ASSERT( mbedtls_entropy_add_source( &ctx, entropy_dummy_source,
+                                             &fail, 16,
+                                             MBEDTLS_ENTROPY_SOURCE_WEAK )
                  == 0 );
 
     TEST_ASSERT( mbedtls_entropy_func( &ctx, buf, sizeof( buf ) )
@@ -147,7 +151,8 @@
     mbedtls_entropy_init( &ctx );
 
     TEST_ASSERT( mbedtls_entropy_add_source( &ctx, entropy_dummy_source,
-                                     &chunk_size, threshold ) == 0 );
+                                     &chunk_size, threshold,
+                                     MBEDTLS_ENTROPY_SOURCE_WEAK ) == 0 );
 
     entropy_dummy_calls = 0;
     ret = mbedtls_entropy_func( &ctx, buf, sizeof( buf ) );