[Backport 2.x] Move base64 constant-time functions to the new module

library/base64.c

@@ -22,7 +22,7 @@
 #if defined(MBEDTLS_BASE64_C)
 
 #include "mbedtls/base64.h"
-#include "base64_invasive.h"
+#include "constant_time_internal.h"
 
 #include <stdint.h>
 
@@ -38,41 +38,6 @@
 
 #define BASE64_SIZE_T_MAX   ( (size_t) -1 ) /* SIZE_T_MAX is not standard */
 
-/* Return 0xff if low <= c <= high, 0 otherwise.
- *
- * Constant flow with respect to c.
- */
-MBEDTLS_STATIC_TESTABLE
-unsigned char mbedtls_base64_mask_of_range( unsigned char low,
-                                            unsigned char high,
-                                            unsigned char c )
-{
-    /* low_mask is: 0 if low <= c, 0x...ff if low > c */
-    unsigned low_mask = ( (unsigned) c - low ) >> 8;
-    /* high_mask is: 0 if c <= high, 0x...ff if c > high */
-    unsigned high_mask = ( (unsigned) high - c ) >> 8;
-    return( ~( low_mask | high_mask ) & 0xff );
-}
-
-/* Given a value in the range 0..63, return the corresponding Base64 digit.
- * The implementation assumes that letters are consecutive (e.g. ASCII
- * but not EBCDIC).
- */
-MBEDTLS_STATIC_TESTABLE
-unsigned char mbedtls_base64_enc_char( unsigned char val )
-{
-    unsigned char digit = 0;
-    /* For each range of values, if val is in that range, mask digit with
-     * the corresponding value. Since val can only be in a single range,
-     * only at most one masking will change digit. */
-    digit |= mbedtls_base64_mask_of_range(  0, 25, val ) & ( 'A' + val );
-    digit |= mbedtls_base64_mask_of_range( 26, 51, val ) & ( 'a' + val - 26 );
-    digit |= mbedtls_base64_mask_of_range( 52, 61, val ) & ( '0' + val - 52 );
-    digit |= mbedtls_base64_mask_of_range( 62, 62, val ) & '+';
-    digit |= mbedtls_base64_mask_of_range( 63, 63, val ) & '/';
-    return( digit );
-}
-
 /*
  * Encode a buffer into base64 format
  */
@@ -113,25 +78,25 @@ int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
         C2 = *src++;
         C3 = *src++;
 
-        *p++ = mbedtls_base64_enc_char( ( C1 >> 2 ) & 0x3F );
-        *p++ = mbedtls_base64_enc_char( ( ( ( C1 &  3 ) << 4 ) + ( C2 >> 4 ) )
+        *p++ = mbedtls_ct_base64_enc_char( ( C1 >> 2 ) & 0x3F );
+        *p++ = mbedtls_ct_base64_enc_char( ( ( ( C1 &  3 ) << 4 ) + ( C2 >> 4 ) )
                                         & 0x3F );
-        *p++ = mbedtls_base64_enc_char( ( ( ( C2 & 15 ) << 2 ) + ( C3 >> 6 ) )
+        *p++ = mbedtls_ct_base64_enc_char( ( ( ( C2 & 15 ) << 2 ) + ( C3 >> 6 ) )
                                         & 0x3F );
-        *p++ = mbedtls_base64_enc_char( C3 & 0x3F );
+        *p++ = mbedtls_ct_base64_enc_char( C3 & 0x3F );
     }
 
     if( i < slen )
     {
         C1 = *src++;
         C2 = ( ( i + 1 ) < slen ) ? *src++ : 0;
 
-        *p++ = mbedtls_base64_enc_char( ( C1 >> 2 ) & 0x3F );
-        *p++ = mbedtls_base64_enc_char( ( ( ( C1 & 3 ) << 4 ) + ( C2 >> 4 ) )
+        *p++ = mbedtls_ct_base64_enc_char( ( C1 >> 2 ) & 0x3F );
+        *p++ = mbedtls_ct_base64_enc_char( ( ( ( C1 & 3 ) << 4 ) + ( C2 >> 4 ) )
                                         & 0x3F );
 
         if( ( i + 1 ) < slen )
-             *p++ = mbedtls_base64_enc_char( ( ( C2 & 15 ) << 2 ) & 0x3F );
+             *p++ = mbedtls_ct_base64_enc_char( ( ( C2 & 15 ) << 2 ) & 0x3F );
         else *p++ = '=';
 
         *p++ = '=';
@@ -143,35 +108,6 @@ int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
     return( 0 );
 }
 
-/* Given a Base64 digit, return its value.
- * If c is not a Base64 digit ('A'..'Z', 'a'..'z', '0'..'9', '+' or '/'),
- * return -1.
- *
- * The implementation assumes that letters are consecutive (e.g. ASCII
- * but not EBCDIC).
- *
- * The implementation is constant-flow (no branch or memory access depending
- * on the value of c) unless the compiler inlines and optimizes a specific
- * access.
- */
-MBEDTLS_STATIC_TESTABLE
-signed char mbedtls_base64_dec_value( unsigned char c )
-{
-    unsigned char val = 0;
-    /* For each range of digits, if c is in that range, mask val with
-     * the corresponding value. Since c can only be in a single range,
-     * only at most one masking will change val. Set val to one plus
-     * the desired value so that it stays 0 if c is in none of the ranges. */
-    val |= mbedtls_base64_mask_of_range( 'A', 'Z', c ) & ( c - 'A' +  0 + 1 );
-    val |= mbedtls_base64_mask_of_range( 'a', 'z', c ) & ( c - 'a' + 26 + 1 );
-    val |= mbedtls_base64_mask_of_range( '0', '9', c ) & ( c - '0' + 52 + 1 );
-    val |= mbedtls_base64_mask_of_range( '+', '+', c ) & ( c - '+' + 62 + 1 );
-    val |= mbedtls_base64_mask_of_range( '/', '/', c ) & ( c - '/' + 63 + 1 );
-    /* At this point, val is 0 if c is an invalid digit and v+1 if c is
-     * a digit with the value v. */
-    return( val - 1 );
-}
-
 /*
  * Decode a base64-formatted buffer
  */
@@ -224,7 +160,7 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
         {
             if( equals != 0 )
                 return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
-            if( mbedtls_base64_dec_value( src[i] ) < 0 )
+            if( mbedtls_ct_base64_dec_value( src[i] ) < 0 )
                 return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
         }
         n++;
@@ -259,7 +195,7 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
         if( *src == '=' )
             ++equals;
         else
-            x |= mbedtls_base64_dec_value( *src );
+            x |= mbedtls_ct_base64_dec_value( *src );
 
         if( ++accumulated_digits == 4 )
         {

library/constant_time.c

@@ -40,6 +40,10 @@
 #include "mbedtls/rsa.h"
 #endif
 
+#if defined(MBEDTLS_BASE64_C)
+#include "constant_time_invasive.h"
+#endif
+
 #include <string.h>
 
 int mbedtls_ct_memcmp( const void *a,
@@ -150,6 +154,26 @@ size_t mbedtls_ct_size_mask_ge( size_t x,
 
 #endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
 
+#if defined(MBEDTLS_BASE64_C)
+
+/* Return 0xff if low <= c <= high, 0 otherwise.
+ *
+ * Constant flow with respect to c.
+ */
+MBEDTLS_STATIC_TESTABLE
+unsigned char mbedtls_ct_uchar_mask_of_range( unsigned char low,
+                                              unsigned char high,
+                                              unsigned char c )
+{
+    /* low_mask is: 0 if low <= c, 0x...ff if low > c */
+    unsigned low_mask = ( (unsigned) c - low ) >> 8;
+    /* high_mask is: 0 if c <= high, 0x...ff if c > high */
+    unsigned high_mask = ( (unsigned) high - c ) >> 8;
+    return( ~( low_mask | high_mask ) & 0xff );
+}
+
+#endif /* MBEDTLS_BASE64_C */
+
 unsigned mbedtls_ct_size_bool_eq( size_t x,
                                   size_t y )
 {
@@ -301,6 +325,41 @@ void mbedtls_ct_mpi_uint_cond_assign( size_t n,
 
 #endif /* MBEDTLS_BIGNUM_C */
 
+#if defined(MBEDTLS_BASE64_C)
+
+unsigned char mbedtls_ct_base64_enc_char( unsigned char value )
+{
+    unsigned char digit = 0;
+    /* For each range of values, if value is in that range, mask digit with
+     * the corresponding value. Since value can only be in a single range,
+     * only at most one masking will change digit. */
+    digit |= mbedtls_ct_uchar_mask_of_range(  0, 25, value ) & ( 'A' + value );
+    digit |= mbedtls_ct_uchar_mask_of_range( 26, 51, value ) & ( 'a' + value - 26 );
+    digit |= mbedtls_ct_uchar_mask_of_range( 52, 61, value ) & ( '0' + value - 52 );
+    digit |= mbedtls_ct_uchar_mask_of_range( 62, 62, value ) & '+';
+    digit |= mbedtls_ct_uchar_mask_of_range( 63, 63, value ) & '/';
+    return( digit );
+}
+
+signed char mbedtls_ct_base64_dec_value( unsigned char c )
+{
+    unsigned char val = 0;
+    /* For each range of digits, if c is in that range, mask val with
+     * the corresponding value. Since c can only be in a single range,
+     * only at most one masking will change val. Set val to one plus
+     * the desired value so that it stays 0 if c is in none of the ranges. */
+    val |= mbedtls_ct_uchar_mask_of_range( 'A', 'Z', c ) & ( c - 'A' +  0 + 1 );
+    val |= mbedtls_ct_uchar_mask_of_range( 'a', 'z', c ) & ( c - 'a' + 26 + 1 );
+    val |= mbedtls_ct_uchar_mask_of_range( '0', '9', c ) & ( c - '0' + 52 + 1 );
+    val |= mbedtls_ct_uchar_mask_of_range( '+', '+', c ) & ( c - '+' + 62 + 1 );
+    val |= mbedtls_ct_uchar_mask_of_range( '/', '/', c ) & ( c - '/' + 63 + 1 );
+    /* At this point, val is 0 if c is an invalid digit and v+1 if c is
+     * a digit with the value v. */
+    return( val - 1 );
+}
+
+#endif /* MBEDTLS_BASE64_C */
+
 #if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
 
 /** Shift some data towards the left inside a buffer.

library/constant_time_internal.h

@@ -167,6 +167,35 @@ void mbedtls_ct_mpi_uint_cond_assign( size_t n,
 
 #endif /* MBEDTLS_BIGNUM_C */
 
+#if defined(MBEDTLS_BASE64_C)
+
+/** Given a value in the range 0..63, return the corresponding Base64 digit.
+ *
+ * The implementation assumes that letters are consecutive (e.g. ASCII
+ * but not EBCDIC).
+ *
+ * \param value     A value in the range 0..63.
+ *
+ * \return          A base64 digit converted from \p value.
+ */
+unsigned char mbedtls_ct_base64_enc_char( unsigned char value );
+
+/** Given a Base64 digit, return its value.
+ *
+ * If c is not a Base64 digit ('A'..'Z', 'a'..'z', '0'..'9', '+' or '/'),
+ * return -1.
+ *
+ * The implementation assumes that letters are consecutive (e.g. ASCII
+ * but not EBCDIC).
+ *
+ * \param c     A base64 digit.
+ *
+ * \return      The value of the base64 digit \p c.
+ */
+signed char mbedtls_ct_base64_dec_value( unsigned char c );
+
+#endif /* MBEDTLS_BASE64_C */
+
 #if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
 
 /** Conditional memcpy without branches.

library/constant_time_invasive.h

@@ -0,0 +1,51 @@
+/**
+ * \file constant_time_invasive.h
+ *
+ * \brief Constant-time module: interfaces for invasive testing only.
+ *
+ * The interfaces in this file are intended for testing purposes only.
+ * They SHOULD NOT be made available in library integrations except when
+ * building the library for testing.
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#ifndef MBEDTLS_CONSTANT_TIME_INVASIVE_H
+#define MBEDTLS_CONSTANT_TIME_INVASIVE_H
+
+#include "common.h"
+
+#if defined(MBEDTLS_TEST_HOOKS)
+
+/** Turn a value into a mask:
+ * - if \p low <= \p c <= \p high,
+ *   return the all-bits 1 mask, aka (unsigned) -1
+ * - otherwise, return the all-bits 0 mask, aka 0
+ *
+ * \param low   The value to analyze.
+ * \param high  The value to analyze.
+ * \param c     The value to analyze.
+ *
+ * \return      All-bits-one if \p low <= \p c <= \p high, otherwise zero.
+ */
+unsigned char mbedtls_ct_uchar_mask_of_range( unsigned char low,
+                                              unsigned char high,
+                                              unsigned char c );
+
+#endif /* MBEDTLS_TEST_HOOKS */
+
+#endif /* MBEDTLS_CONSTANT_TIME_INVASIVE_H */

tests/suites/test_suite_base64.function

@@ -1,6 +1,7 @@
 /* BEGIN_HEADER */
 #include "mbedtls/base64.h"
-#include "base64_invasive.h"
+#include "constant_time_internal.h"
+#include "constant_time_invasive.h"
 #include <test/constant_flow.h>
 
 #if defined(MBEDTLS_TEST_HOOKS)
@@ -24,7 +25,7 @@ void mask_of_range( int low_arg, int high_arg )
     {
         mbedtls_test_set_step( c );
         TEST_CF_SECRET( &c, sizeof( c ) );
-        unsigned char m = mbedtls_base64_mask_of_range( low, high, c );
+        unsigned char m = mbedtls_ct_uchar_mask_of_range( low, high, c );
         TEST_CF_PUBLIC( &c, sizeof( c ) );
         TEST_CF_PUBLIC( &m, sizeof( m ) );
         if( low <= c && c <= high )
@@ -42,7 +43,7 @@ void enc_chars( )
     {
         mbedtls_test_set_step( value );
         TEST_CF_SECRET( &value, sizeof( value ) );
-        unsigned char digit = mbedtls_base64_enc_char( value );
+        unsigned char digit = mbedtls_ct_base64_enc_char( value );
         TEST_CF_PUBLIC( &value, sizeof( value ) );
         TEST_CF_PUBLIC( &digit, sizeof( digit ) );
         TEST_EQUAL( digit, base64_digits[value] );
@@ -66,7 +67,7 @@ void dec_chars( )
         else
             expected = p - base64_digits;
         TEST_CF_SECRET( &c, sizeof( c ) );
-        signed char actual = mbedtls_base64_dec_value( c );
+        signed char actual = mbedtls_ct_base64_dec_value( c );
         TEST_CF_PUBLIC( &c, sizeof( c ) );
         TEST_CF_PUBLIC( &actual, sizeof( actual ) );
         TEST_EQUAL( actual, expected );