diff --git a/ChangeLog.d/issue3819.txt b/ChangeLog.d/issue3819.txt
new file mode 100644
index 0000000..e41520f
--- /dev/null
+++ b/ChangeLog.d/issue3819.txt
@@ -0,0 +1,10 @@
+Security
+   * Fix a security reduction in CTR_DRBG when the initial seeding obtained a
+     nonce from entropy. Applications were affected if they called
+     mbedtls_ctr_drbg_set_nonce_len(), if they called
+     mbedtls_ctr_drbg_set_entropy_len() with a size that was 3/2 times the key
+     length, or when the entropy module uses SHA-256 and CTR_DRBG uses AES-256.
+     In such cases, a random nonce was necessary to achieve the advertised
+     security strength, but the code incorrectly used a constant instead of
+     entropy from the nonce.
+     Found by John Stroebel in #3819 and fixed in #3973.
diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index 023aac5..3815dc7 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -394,7 +394,7 @@
     /* Gather entropy for a nonce if requested. */
     if( nonce_len != 0 )
     {
-        if( 0 != ctx->f_entropy( ctx->p_entropy, seed, nonce_len ) )
+        if( 0 != ctx->f_entropy( ctx->p_entropy, seed + seedlen, nonce_len ) )
         {
             return( MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED );
         }
@@ -684,54 +684,134 @@
 
 #if defined(MBEDTLS_SELF_TEST)
 
-static const unsigned char entropy_source_pr[96] =
-    { 0xc1, 0x80, 0x81, 0xa6, 0x5d, 0x44, 0x02, 0x16,
-      0x19, 0xb3, 0xf1, 0x80, 0xb1, 0xc9, 0x20, 0x02,
-      0x6a, 0x54, 0x6f, 0x0c, 0x70, 0x81, 0x49, 0x8b,
-      0x6e, 0xa6, 0x62, 0x52, 0x6d, 0x51, 0xb1, 0xcb,
-      0x58, 0x3b, 0xfa, 0xd5, 0x37, 0x5f, 0xfb, 0xc9,
-      0xff, 0x46, 0xd2, 0x19, 0xc7, 0x22, 0x3e, 0x95,
-      0x45, 0x9d, 0x82, 0xe1, 0xe7, 0x22, 0x9f, 0x63,
-      0x31, 0x69, 0xd2, 0x6b, 0x57, 0x47, 0x4f, 0xa3,
-      0x37, 0xc9, 0x98, 0x1c, 0x0b, 0xfb, 0x91, 0x31,
-      0x4d, 0x55, 0xb9, 0xe9, 0x1c, 0x5a, 0x5e, 0xe4,
-      0x93, 0x92, 0xcf, 0xc5, 0x23, 0x12, 0xd5, 0x56,
-      0x2c, 0x4a, 0x6e, 0xff, 0xdc, 0x10, 0xd0, 0x68 };
-
-static const unsigned char entropy_source_nopr[64] =
-    { 0x5a, 0x19, 0x4d, 0x5e, 0x2b, 0x31, 0x58, 0x14,
-      0x54, 0xde, 0xf6, 0x75, 0xfb, 0x79, 0x58, 0xfe,
-      0xc7, 0xdb, 0x87, 0x3e, 0x56, 0x89, 0xfc, 0x9d,
-      0x03, 0x21, 0x7c, 0x68, 0xd8, 0x03, 0x38, 0x20,
-      0xf9, 0xe6, 0x5e, 0x04, 0xd8, 0x56, 0xf3, 0xa9,
-      0xc4, 0x4a, 0x4c, 0xbd, 0xc1, 0xd0, 0x08, 0x46,
-      0xf5, 0x98, 0x3d, 0x77, 0x1c, 0x1b, 0x13, 0x7e,
-      0x4e, 0x0f, 0x9d, 0x8e, 0xf4, 0x09, 0xf9, 0x2e };
-
-static const unsigned char nonce_pers_pr[16] =
-    { 0xd2, 0x54, 0xfc, 0xff, 0x02, 0x1e, 0x69, 0xd2,
-      0x29, 0xc9, 0xcf, 0xad, 0x85, 0xfa, 0x48, 0x6c };
-
-static const unsigned char nonce_pers_nopr[16] =
-    { 0x1b, 0x54, 0xb8, 0xff, 0x06, 0x42, 0xbf, 0xf5,
-      0x21, 0xf1, 0x5c, 0x1c, 0x0b, 0x66, 0x5f, 0x3f };
+/* The CTR_DRBG NIST test vectors used here are available at
+ * https://csrc.nist.gov/CSRC/media/Projects/Cryptographic-Algorithm-Validation-Program/documents/drbg/drbgtestvectors.zip
+ *
+ * The parameters used to derive the test data are:
+ *
+ * [AES-128 use df]
+ * [PredictionResistance = True/False]
+ * [EntropyInputLen = 128]
+ * [NonceLen = 64]
+ * [PersonalizationStringLen = 128]
+ * [AdditionalInputLen = 0]
+ * [ReturnedBitsLen = 512]
+ *
+ * [AES-256 use df]
+ * [PredictionResistance = True/False]
+ * [EntropyInputLen = 256]
+ * [NonceLen = 128]
+ * [PersonalizationStringLen = 256]
+ * [AdditionalInputLen = 0]
+ * [ReturnedBitsLen = 512]
+ *
+ */
 
 #if defined(MBEDTLS_CTR_DRBG_USE_128_BIT_KEY)
-static const unsigned char result_pr[16] =
-    { 0x95, 0x3c, 0xa5, 0xbd, 0x44, 0x1, 0x34, 0xb7,
-      0x13, 0x58, 0x3e, 0x6a, 0x6c, 0x7e, 0x88, 0x8a };
+static const unsigned char entropy_source_pr[] =
+    { 0x04, 0xd9, 0x49, 0xa6, 0xdc, 0xe8, 0x6e, 0xbb,
+      0xf1, 0x08, 0x77, 0x2b, 0x9e, 0x08, 0xca, 0x92,
+      0x65, 0x16, 0xda, 0x99, 0xa2, 0x59, 0xf3, 0xe8,
+      0x38, 0x7e, 0x3f, 0x6b, 0x51, 0x70, 0x7b, 0x20,
+      0xec, 0x53, 0xd0, 0x66, 0xc3, 0x0f, 0xe3, 0xb0,
+      0xe0, 0x86, 0xa6, 0xaa, 0x5f, 0x72, 0x2f, 0xad,
+      0xf7, 0xef, 0x06, 0xb8, 0xd6, 0x9c, 0x9d, 0xe8 };
 
-static const unsigned char result_nopr[16] =
-    { 0x6c, 0x25, 0x27, 0x95, 0xa3, 0x62, 0xd6, 0xdb,
-      0x90, 0xfd, 0x69, 0xb5, 0x42, 0x9, 0x4b, 0x84 };
+static const unsigned char entropy_source_nopr[] =
+    { 0x07, 0x0d, 0x59, 0x63, 0x98, 0x73, 0xa5, 0x45,
+      0x27, 0x38, 0x22, 0x7b, 0x76, 0x85, 0xd1, 0xa9,
+      0x74, 0x18, 0x1f, 0x3c, 0x22, 0xf6, 0x49, 0x20,
+      0x4a, 0x47, 0xc2, 0xf3, 0x85, 0x16, 0xb4, 0x6f,
+      0x00, 0x2e, 0x71, 0xda, 0xed, 0x16, 0x9b, 0x5c };
+
+static const unsigned char pers_pr[] =
+    { 0xbf, 0xa4, 0x9a, 0x8f, 0x7b, 0xd8, 0xb1, 0x7a,
+      0x9d, 0xfa, 0x45, 0xed, 0x21, 0x52, 0xb3, 0xad };
+
+static const unsigned char pers_nopr[] =
+    { 0x4e, 0x61, 0x79, 0xd4, 0xc2, 0x72, 0xa1, 0x4c,
+      0xf1, 0x3d, 0xf6, 0x5e, 0xa3, 0xa6, 0xe5, 0x0f };
+
+static const unsigned char result_pr[] =
+    { 0xc9, 0x0a, 0xaf, 0x85, 0x89, 0x71, 0x44, 0x66,
+      0x4f, 0x25, 0x0b, 0x2b, 0xde, 0xd8, 0xfa, 0xff,
+      0x52, 0x5a, 0x1b, 0x32, 0x5e, 0x41, 0x7a, 0x10,
+      0x1f, 0xef, 0x1e, 0x62, 0x23, 0xe9, 0x20, 0x30,
+      0xc9, 0x0d, 0xad, 0x69, 0xb4, 0x9c, 0x5b, 0xf4,
+      0x87, 0x42, 0xd5, 0xae, 0x5e, 0x5e, 0x43, 0xcc,
+      0xd9, 0xfd, 0x0b, 0x93, 0x4a, 0xe3, 0xd4, 0x06,
+      0x37, 0x36, 0x0f, 0x3f, 0x72, 0x82, 0x0c, 0xcf };
+
+static const unsigned char result_nopr[] =
+    { 0x31, 0xc9, 0x91, 0x09, 0xf8, 0xc5, 0x10, 0x13,
+      0x3c, 0xd3, 0x96, 0xf9, 0xbc, 0x2c, 0x12, 0xc0,
+      0x7c, 0xc1, 0x61, 0x5f, 0xa3, 0x09, 0x99, 0xaf,
+      0xd7, 0xf2, 0x36, 0xfd, 0x40, 0x1a, 0x8b, 0xf2,
+      0x33, 0x38, 0xee, 0x1d, 0x03, 0x5f, 0x83, 0xb7,
+      0xa2, 0x53, 0xdc, 0xee, 0x18, 0xfc, 0xa7, 0xf2,
+      0xee, 0x96, 0xc6, 0xc2, 0xcd, 0x0c, 0xff, 0x02,
+      0x76, 0x70, 0x69, 0xaa, 0x69, 0xd1, 0x3b, 0xe8 };
 #else /* MBEDTLS_CTR_DRBG_USE_128_BIT_KEY */
-static const unsigned char result_pr[16] =
-    { 0x34, 0x01, 0x16, 0x56, 0xb4, 0x29, 0x00, 0x8f,
-      0x35, 0x63, 0xec, 0xb5, 0xf2, 0x59, 0x07, 0x23 };
 
-static const unsigned char result_nopr[16] =
-    { 0xa0, 0x54, 0x30, 0x3d, 0x8a, 0x7e, 0xa9, 0x88,
-      0x9d, 0x90, 0x3e, 0x07, 0x7c, 0x6f, 0x21, 0x8f };
+static const unsigned char entropy_source_pr[] =
+    { 0xca, 0x58, 0xfd, 0xf2, 0xb9, 0x77, 0xcb, 0x49,
+      0xd4, 0xe0, 0x5b, 0xe2, 0x39, 0x50, 0xd9, 0x8a,
+      0x6a, 0xb3, 0xc5, 0x2f, 0xdf, 0x74, 0xd5, 0x85,
+      0x8f, 0xd1, 0xba, 0x64, 0x54, 0x7b, 0xdb, 0x1e,
+      0xc5, 0xea, 0x24, 0xc0, 0xfa, 0x0c, 0x90, 0x15,
+      0x09, 0x20, 0x92, 0x42, 0x32, 0x36, 0x45, 0x45,
+      0x7d, 0x20, 0x76, 0x6b, 0xcf, 0xa2, 0x15, 0xc8,
+      0x2f, 0x9f, 0xbc, 0x88, 0x3f, 0x80, 0xd1, 0x2c,
+      0xb7, 0x16, 0xd1, 0x80, 0x9e, 0xe1, 0xc9, 0xb3,
+      0x88, 0x1b, 0x21, 0x45, 0xef, 0xa1, 0x7f, 0xce,
+      0xc8, 0x92, 0x35, 0x55, 0x2a, 0xd9, 0x1d, 0x8e,
+      0x12, 0x38, 0xac, 0x01, 0x4e, 0x38, 0x18, 0x76,
+      0x9c, 0xf2, 0xb6, 0xd4, 0x13, 0xb6, 0x2c, 0x77,
+      0xc0, 0xe7, 0xe6, 0x0c, 0x47, 0x44, 0x95, 0xbe };
+
+static const unsigned char entropy_source_nopr[] =
+    { 0x4c, 0xfb, 0x21, 0x86, 0x73, 0x34, 0x6d, 0x9d,
+      0x50, 0xc9, 0x22, 0xe4, 0x9b, 0x0d, 0xfc, 0xd0,
+      0x90, 0xad, 0xf0, 0x4f, 0x5c, 0x3b, 0xa4, 0x73,
+      0x27, 0xdf, 0xcd, 0x6f, 0xa6, 0x3a, 0x78, 0x5c,
+      0x01, 0x69, 0x62, 0xa7, 0xfd, 0x27, 0x87, 0xa2,
+      0x4b, 0xf6, 0xbe, 0x47, 0xef, 0x37, 0x83, 0xf1,
+      0xb7, 0xec, 0x46, 0x07, 0x23, 0x63, 0x83, 0x4a,
+      0x1b, 0x01, 0x33, 0xf2, 0xc2, 0x38, 0x91, 0xdb,
+      0x4f, 0x11, 0xa6, 0x86, 0x51, 0xf2, 0x3e, 0x3a,
+      0x8b, 0x1f, 0xdc, 0x03, 0xb1, 0x92, 0xc7, 0xe7 };
+
+static const unsigned char pers_pr[] =
+    { 0x5a, 0x70, 0x95, 0xe9, 0x81, 0x40, 0x52, 0x33,
+      0x91, 0x53, 0x7e, 0x75, 0xd6, 0x19, 0x9d, 0x1e,
+      0xad, 0x0d, 0xc6, 0xa7, 0xde, 0x6c, 0x1f, 0xe0,
+      0xea, 0x18, 0x33, 0xa8, 0x7e, 0x06, 0x20, 0xe9 };
+
+static const unsigned char pers_nopr[] =
+    { 0x88, 0xee, 0xb8, 0xe0, 0xe8, 0x3b, 0xf3, 0x29,
+      0x4b, 0xda, 0xcd, 0x60, 0x99, 0xeb, 0xe4, 0xbf,
+      0x55, 0xec, 0xd9, 0x11, 0x3f, 0x71, 0xe5, 0xeb,
+      0xcb, 0x45, 0x75, 0xf3, 0xd6, 0xa6, 0x8a, 0x6b };
+
+static const unsigned char result_pr[] =
+    { 0xce, 0x2f, 0xdb, 0xb6, 0xd9, 0xb7, 0x39, 0x85,
+      0x04, 0xc5, 0xc0, 0x42, 0xc2, 0x31, 0xc6, 0x1d,
+      0x9b, 0x5a, 0x59, 0xf8, 0x7e, 0x0d, 0xcc, 0x62,
+      0x7b, 0x65, 0x11, 0x55, 0x10, 0xeb, 0x9e, 0x3d,
+      0xa4, 0xfb, 0x1c, 0x6a, 0x18, 0xc0, 0x74, 0xdb,
+      0xdd, 0xe7, 0x02, 0x23, 0x63, 0x21, 0xd0, 0x39,
+      0xf9, 0xa7, 0xc4, 0x52, 0x84, 0x3b, 0x49, 0x40,
+      0x72, 0x2b, 0xb0, 0x6c, 0x9c, 0xdb, 0xc3, 0x43 };
+
+static const unsigned char result_nopr[] =
+    { 0xa5, 0x51, 0x80, 0xa1, 0x90, 0xbe, 0xf3, 0xad,
+      0xaf, 0x28, 0xf6, 0xb7, 0x95, 0xe9, 0xf1, 0xf3,
+      0xd6, 0xdf, 0xa1, 0xb2, 0x7d, 0xd0, 0x46, 0x7b,
+      0x0c, 0x75, 0xf5, 0xfa, 0x93, 0x1e, 0x97, 0x14,
+      0x75, 0xb2, 0x7c, 0xae, 0x03, 0xa2, 0x96, 0x54,
+      0xe2, 0xf4, 0x09, 0x66, 0xea, 0x33, 0x64, 0x30,
+      0x40, 0xd1, 0x40, 0x0f, 0xe6, 0x77, 0x87, 0x3a,
+      0xf8, 0x09, 0x7c, 0x1f, 0xe9, 0xf0, 0x02, 0x98 };
 #endif /* MBEDTLS_CTR_DRBG_USE_128_BIT_KEY */
 
 static size_t test_offset;
@@ -751,13 +831,15 @@
                         return( 1 );                        \
                     }
 
+#define SELF_TEST_OUPUT_DISCARD_LENGTH 64
+
 /*
  * Checkup routine
  */
 int mbedtls_ctr_drbg_self_test( int verbose )
 {
     mbedtls_ctr_drbg_context ctx;
-    unsigned char buf[16];
+    unsigned char buf[ sizeof( result_pr ) ];
 
     mbedtls_ctr_drbg_init( &ctx );
 
@@ -768,16 +850,16 @@
         mbedtls_printf( "  CTR_DRBG (PR = TRUE) : " );
 
     test_offset = 0;
-    mbedtls_ctr_drbg_set_entropy_len( &ctx, 32 );
-    mbedtls_ctr_drbg_set_nonce_len( &ctx, 0 );
+    mbedtls_ctr_drbg_set_entropy_len( &ctx, MBEDTLS_CTR_DRBG_KEYSIZE );
+    mbedtls_ctr_drbg_set_nonce_len( &ctx, MBEDTLS_CTR_DRBG_KEYSIZE / 2 );
     CHK( mbedtls_ctr_drbg_seed( &ctx,
                                 ctr_drbg_self_test_entropy,
                                 (void *) entropy_source_pr,
-                                nonce_pers_pr, 16 ) );
+                                pers_pr, MBEDTLS_CTR_DRBG_KEYSIZE ) );
     mbedtls_ctr_drbg_set_prediction_resistance( &ctx, MBEDTLS_CTR_DRBG_PR_ON );
-    CHK( mbedtls_ctr_drbg_random( &ctx, buf, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );
-    CHK( mbedtls_ctr_drbg_random( &ctx, buf, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );
-    CHK( memcmp( buf, result_pr, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );
+    CHK( mbedtls_ctr_drbg_random( &ctx, buf, SELF_TEST_OUPUT_DISCARD_LENGTH ) );
+    CHK( mbedtls_ctr_drbg_random( &ctx, buf, sizeof( result_pr ) ) );
+    CHK( memcmp( buf, result_pr, sizeof( result_pr ) ) );
 
     mbedtls_ctr_drbg_free( &ctx );
 
@@ -793,16 +875,16 @@
     mbedtls_ctr_drbg_init( &ctx );
 
     test_offset = 0;
-    mbedtls_ctr_drbg_set_entropy_len( &ctx, 32 );
-    mbedtls_ctr_drbg_set_nonce_len( &ctx, 0 );
+    mbedtls_ctr_drbg_set_entropy_len( &ctx, MBEDTLS_CTR_DRBG_KEYSIZE);
+    mbedtls_ctr_drbg_set_nonce_len( &ctx, MBEDTLS_CTR_DRBG_KEYSIZE / 2 );
     CHK( mbedtls_ctr_drbg_seed( &ctx,
                                 ctr_drbg_self_test_entropy,
                                 (void *) entropy_source_nopr,
-                                nonce_pers_nopr, 16 ) );
-    CHK( mbedtls_ctr_drbg_random( &ctx, buf, 16 ) );
+                                pers_nopr, MBEDTLS_CTR_DRBG_KEYSIZE ) );
     CHK( mbedtls_ctr_drbg_reseed( &ctx, NULL, 0 ) );
-    CHK( mbedtls_ctr_drbg_random( &ctx, buf, 16 ) );
-    CHK( memcmp( buf, result_nopr, 16 ) );
+    CHK( mbedtls_ctr_drbg_random( &ctx, buf, SELF_TEST_OUPUT_DISCARD_LENGTH ) );
+    CHK( mbedtls_ctr_drbg_random( &ctx, buf, sizeof( result_nopr ) ) );
+    CHK( memcmp( buf, result_nopr, sizeof( result_nopr ) ) );
 
     mbedtls_ctr_drbg_free( &ctx );