diff --git a/ChangeLog.d/ecdsa-random-leading-zeros.txt b/ChangeLog.d/ecdsa-random-leading-zeros.txt
new file mode 100644
index 0000000..793b371
--- /dev/null
+++ b/ChangeLog.d/ecdsa-random-leading-zeros.txt
@@ -0,0 +1,7 @@
+Security
+* Fix a potential side channel vulnerability in ECDSA ephemeral key generation.
+  An adversary who is capable of very precise timing measurements could
+  learn partial information about the leading bits of the nonce used for the
+  signature, allowing the recovery of the private key after observing a
+  large number of signature operations. This completes a partial fix in
+  Mbed TLS 2.16.4.
diff --git a/library/ecp.c b/library/ecp.c
index 2168981..9db6988 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -3059,6 +3059,11 @@
 {
     int ret = MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
     size_t n_size;
+#if defined(ECP_SHORTWEIERSTRASS)
+    mbedtls_mpi one;
+
+    mbedtls_mpi_init( &one );
+#endif
 
     ECP_VALIDATE_RET( grp   != NULL );
     ECP_VALIDATE_RET( d     != NULL );
@@ -3099,7 +3104,10 @@
     {
         /* SEC1 3.2.1: Generate d such that 1 <= n < N */
         int count = 0;
-        unsigned cmp = 0;
+        unsigned lt_lower = 1, lt_upper = 0;
+
+        MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &one, grp->N.n ) );
+        MBEDTLS_MPI_CHK( mbedtls_mpi_lset( &one, 1 ) );
 
         /*
          * Match the procedure given in RFC 6979 (deterministic ECDSA):
@@ -3123,19 +3131,22 @@
              * such as secp224k1 are actually very close to the worst case.
              */
             if( ++count > 30 )
-                return( MBEDTLS_ERR_ECP_RANDOM_FAILED );
-
-            ret = mbedtls_mpi_lt_mpi_ct( d, &grp->N, &cmp );
-            if( ret != 0 )
             {
+                ret = MBEDTLS_ERR_ECP_RANDOM_FAILED;
                 goto cleanup;
             }
+
+            MBEDTLS_MPI_CHK( mbedtls_mpi_lt_mpi_ct( d, &grp->N, &lt_upper ) );
+            MBEDTLS_MPI_CHK( mbedtls_mpi_lt_mpi_ct( d, &one, &lt_lower ) );
         }
-        while( mbedtls_mpi_cmp_int( d, 1 ) < 0 || cmp != 1 );
+        while( lt_lower != 0 || lt_upper == 0 );
     }
 #endif /* ECP_SHORTWEIERSTRASS */
 
 cleanup:
+#if defined(ECP_SHORTWEIERSTRASS)
+    mbedtls_mpi_free( &one );
+#endif
     return( ret );
 }