diff --git a/library/hmac_drbg.c b/library/hmac_drbg.c
index c50330e..50ac660 100644
--- a/library/hmac_drbg.c
+++ b/library/hmac_drbg.c
@@ -148,21 +148,28 @@
     return( 0 );
 }
 
-/*
- * HMAC_DRBG reseeding: 10.1.2.4 (arabic) + 9.2 (Roman)
- */
-int mbedtls_hmac_drbg_reseed( mbedtls_hmac_drbg_context *ctx,
-                      const unsigned char *additional, size_t len )
+static int hmac_drbg_reseed_internal( mbedtls_hmac_drbg_context *ctx,
+                                      const unsigned char *additional, size_t len,
+                                      int use_nonce )
 {
     unsigned char seed[MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT];
-    size_t seedlen;
+    size_t seedlen = 0;
     int ret;
 
-    /* III. Check input length */
-    if( len > MBEDTLS_HMAC_DRBG_MAX_INPUT ||
-        ctx->entropy_len + len > MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT )
     {
-        return( MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG );
+        size_t total_entropy_len;
+
+        if( use_nonce == 0 )
+            total_entropy_len = ctx->entropy_len;
+        else
+            total_entropy_len = ctx->entropy_len * 3 / 2;
+
+        /* III. Check input length */
+        if( len > MBEDTLS_HMAC_DRBG_MAX_INPUT ||
+            total_entropy_len + len > MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT )
+        {
+            return( MBEDTLS_ERR_HMAC_DRBG_INPUT_TOO_BIG );
+        }
     }
 
     memset( seed, 0, MBEDTLS_HMAC_DRBG_MAX_SEED_INPUT );
@@ -170,9 +177,32 @@
     /* IV. Gather entropy_len bytes of entropy for the seed */
     if( ( ret = ctx->f_entropy( ctx->p_entropy,
                                 seed, ctx->entropy_len ) ) != 0 )
+    {
         return( MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED );
+    }
+    seedlen += ctx->entropy_len;
 
-    seedlen = ctx->entropy_len;
+    /* IV'. For initial seeding, allow adding of nonce generated
+     *      from the entropy source. See Sect 8.6.7 in SP800-90A. */
+    if( use_nonce )
+    {
+        /* Note: We don't merge the two calls to f_entropy() in order
+         *       to avoid requesting too much entropy from f_entropy()
+         *       at once. Specifically, if the underlying digest is not
+         *       SHA-1, 3 / 2 * entropy_len is at least 36 Bytes, which
+         *       is larger than the maximum of 32 Bytes that our own
+         *       entropy source implementation can emit in a single
+         *       call in configurations disabling SHA-512. */
+        if( ( ret = ctx->f_entropy( ctx->p_entropy,
+                                    seed + seedlen,
+                                    ctx->entropy_len / 2 ) ) != 0 )
+        {
+            return( MBEDTLS_ERR_HMAC_DRBG_ENTROPY_SOURCE_FAILED );
+        }
+
+        seedlen += ctx->entropy_len / 2;
+    }
+
 
     /* 1. Concatenate entropy and additional data if any */
     if( additional != NULL && len != 0 )
@@ -195,6 +225,15 @@
 }
 
 /*
+ * HMAC_DRBG reseeding: 10.1.2.4 (arabic) + 9.2 (Roman)
+ */
+int mbedtls_hmac_drbg_reseed( mbedtls_hmac_drbg_context *ctx,
+                      const unsigned char *additional, size_t len )
+{
+    return( hmac_drbg_reseed_internal( ctx, additional, len, 0 ) );
+}
+
+/*
  * HMAC_DRBG initialisation (10.1.2.3 + 9.1)
  */
 int mbedtls_hmac_drbg_seed( mbedtls_hmac_drbg_context *ctx,
@@ -205,7 +244,7 @@
                     size_t len )
 {
     int ret;
-    size_t entropy_len, md_size;
+    size_t md_size;
 
     if( ( ret = mbedtls_md_setup( &ctx->md_ctx, md_info, 1 ) ) != 0 )
         return( ret );
@@ -233,20 +272,15 @@
      *
      * (This also matches the sizes used in the NIST test vectors.)
      */
-    entropy_len = md_size <= 20 ? 16 : /* 160-bits hash -> 128 bits */
-                  md_size <= 28 ? 24 : /* 224-bits hash -> 192 bits */
-                                  32;  /* better (256+) -> 256 bits */
+    ctx->entropy_len = md_size <= 20 ? 16 : /* 160-bits hash -> 128 bits */
+                       md_size <= 28 ? 24 : /* 224-bits hash -> 192 bits */
+                       32;  /* better (256+) -> 256 bits */
 
-    /*
-     * For initialisation, use more entropy to emulate a nonce
-     * (Again, matches test vectors.)
-     */
-    ctx->entropy_len = entropy_len * 3 / 2;
-
-    if( ( ret = mbedtls_hmac_drbg_reseed( ctx, custom, len ) ) != 0 )
+    if( ( ret = hmac_drbg_reseed_internal( ctx, custom, len,
+                                           1 /* add nonce */ ) ) != 0 )
+    {
         return( ret );
-
-    ctx->entropy_len = entropy_len;
+    }
 
     return( 0 );
 }