diff --git a/ChangeLog.d/make_base64_table_access_constant_flow.txt b/ChangeLog.d/make_base64_table_access_constant_flow.txt
new file mode 100644
index 000000000..733c972d0
--- /dev/null
+++ b/ChangeLog.d/make_base64_table_access_constant_flow.txt
@@ -0,0 +1,4 @@
+Security
+   * Guard against strong local side channel attack against base64 tables by
+     making access aceess to them use constant flow code.
+
diff --git a/library/base64.c b/library/base64.c
index d39474a08..1a05226ef 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -65,6 +65,99 @@ static const unsigned char base64_dec_map[128] =
 
 #define BASE64_SIZE_T_MAX   ( (size_t) -1 ) /* SIZE_T_MAX is not standard */
 
+/*
+ * Constant flow conditional assignment to unsigned char
+ */
+static void mbedtls_base64_cond_assign_uchar( unsigned char * dest, const unsigned char * const src,
+                                       unsigned char condition )
+{
+    /* MSVC has a warning about unary minus on unsigned integer types,
+     * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+
+    /* Generate bitmask from condition, mask will either be 0xFF or 0 */
+    unsigned char mask = ( condition | -condition );
+    mask >>= 7;
+    mask = -mask;
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
+    *dest = ( ( *src ) & mask ) | ( ( *dest ) & ~mask );
+}
+
+/*
+ * Constant flow conditional assignment to uint_32
+ */
+static void mbedtls_base64_cond_assign_uint32( uint32_t * dest, const uint32_t src,
+                                       uint32_t condition )
+{
+    /* MSVC has a warning about unary minus on unsigned integer types,
+     * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+
+    /* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
+    uint32_t mask = ( condition | -condition );
+    mask >>= 31;
+    mask = -mask;
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
+    *dest = ( src & mask ) | ( ( *dest ) & ~mask );
+}
+
+/*
+ * Constant flow check for equality
+ */
+static unsigned char mbedtls_base64_eq( size_t in_a, size_t in_b )
+{
+    size_t difference = in_a ^ in_b;
+
+    /* MSVC has a warning about unary minus on unsigned integer types,
+     * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+
+    difference |= -difference;
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
+    /* cope with the varying size of size_t per platform */
+    difference >>= ( sizeof( difference ) * 8 - 1 );
+
+    return (unsigned char) ( 1 ^ difference );
+}
+
+/*
+ * Constant flow lookup into table.
+ */
+static unsigned char mbedtls_base64_table_lookup( const unsigned char * const table,
+                                                 const size_t table_size, const size_t table_index )
+{
+    size_t i;
+    unsigned char result = 0;
+
+    for( i = 0; i < table_size; ++i )
+    {
+        mbedtls_base64_cond_assign_uchar( &result, &table[i], mbedtls_base64_eq( i, table_index ) );
+    }
+
+    return result;
+}
+
 /*
  * Encode a buffer into base64 format
  */
@@ -105,10 +198,17 @@ int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
         C2 = *src++;
         C3 = *src++;
 
-        *p++ = base64_enc_map[(C1 >> 2) & 0x3F];
-        *p++ = base64_enc_map[(((C1 &  3) << 4) + (C2 >> 4)) & 0x3F];
-        *p++ = base64_enc_map[(((C2 & 15) << 2) + (C3 >> 6)) & 0x3F];
-        *p++ = base64_enc_map[C3 & 0x3F];
+        *p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
+                                            ( ( C1 >> 2 ) & 0x3F ) );
+
+        *p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
+                                            ( ( ( ( C1 &  3 ) << 4 ) + ( C2 >> 4 ) ) & 0x3F ) );
+
+        *p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
+                                            ( ( ( ( C2 & 15 ) << 2 ) + ( C3 >> 6 ) ) & 0x3F ) );
+
+        *p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
+                                            ( C3 & 0x3F ) );
     }
 
     if( i < slen )
@@ -116,11 +216,15 @@ int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
         C1 = *src++;
         C2 = ( ( i + 1 ) < slen ) ? *src++ : 0;
 
-        *p++ = base64_enc_map[(C1 >> 2) & 0x3F];
-        *p++ = base64_enc_map[(((C1 & 3) << 4) + (C2 >> 4)) & 0x3F];
+        *p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
+                                            ( ( C1 >> 2 ) & 0x3F ) );
+
+        *p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
+                                            ( ( ( ( C1 & 3 ) << 4 ) + ( C2 >> 4 ) ) & 0x3F ) );
 
         if( ( i + 1 ) < slen )
-             *p++ = base64_enc_map[((C2 & 15) << 2) & 0x3F];
+             *p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
+                                                 ( ( ( C2 & 15 ) << 2 ) & 0x3F ) );
         else *p++ = '=';
 
         *p++ = '=';
@@ -141,6 +245,7 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
     size_t i, n;
     uint32_t j, x;
     unsigned char *p;
+    unsigned char dec_map_lookup;
 
     /* First pass: check for validity and get output length */
     for( i = n = j = 0; i < slen; i++ )
@@ -171,10 +276,12 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
         if( src[i] == '=' && ++j > 2 )
             return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
 
-        if( src[i] > 127 || base64_dec_map[src[i]] == 127 )
+        dec_map_lookup = mbedtls_base64_table_lookup( base64_dec_map, sizeof( base64_dec_map ), src[i] );
+
+        if( src[i] > 127 || dec_map_lookup == 127 )
             return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
 
-        if( base64_dec_map[src[i]] < 64 && j != 0 )
+        if( dec_map_lookup < 64 && j != 0 )
             return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
 
         n++;
@@ -204,8 +311,10 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
         if( *src == '\r' || *src == '\n' || *src == ' ' )
             continue;
 
-        j -= ( base64_dec_map[*src] == 64 );
-        x  = ( x << 6 ) | ( base64_dec_map[*src] & 0x3F );
+        dec_map_lookup = mbedtls_base64_table_lookup( base64_dec_map, sizeof( base64_dec_map ), *src );
+
+        mbedtls_base64_cond_assign_uint32( &j, j - 1, mbedtls_base64_eq( dec_map_lookup, 64 ) );
+        x  = ( x << 6 ) | ( dec_map_lookup & 0x3F );
 
         if( ++n == 4 )
         {
diff --git a/tests/suites/test_suite_base64.data b/tests/suites/test_suite_base64.data
index da99ffa87..3a892f479 100644
--- a/tests/suites/test_suite_base64.data
+++ b/tests/suites/test_suite_base64.data
@@ -151,6 +151,20 @@ base64_encode_hex:"0102030405060708":"AQIDBAUGBwg=":13:0
 Base64 encode hex #4
 base64_encode_hex:"01020304050607":"AQIDBAUGBw==":13:0
 
+# Rotate the bytes around so that they end up at each offset modulo 3 in
+# successive test cases.
+Base64 encode hex all valid input bytes #0
+base64_encode_hex:"000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f404142434445464748494a4b4c4d4e4f505152535455565758595a5b5c5d5e5f606162636465666768696a6b6c6d6e6f707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f909192939495969798999a9b9c9d9e9fa0a1a2a3a4a5a6a7a8a9aaabacadaeafb0b1b2b3b4b5b6b7b8b9babbbcbdbebfc0c1c2c3c4c5c6c7c8c9cacbcccdcecfd0d1d2d3d4d5d6d7d8d9dadbdcdddedfe0e1e2e3e4e5e6e7e8e9eaebecedeeeff0f1f2f3f4f5f6f7f8f9fafbfcfdfeff":"AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PD0+P0BBQkNERUZHSElKS0xNTk9QUVJTVFVWV1hZWltcXV5fYGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6e3x9fn+AgYKDhIWGh4iJiouMjY6PkJGSk5SVlpeYmZqbnJ2en6ChoqOkpaanqKmqq6ytrq+wsbKztLW2t7i5uru8vb6/wMHCw8TFxsfIycrLzM3Oz9DR0tPU1dbX2Nna29zd3t/g4eLj5OXm5+jp6uvs7e7v8PHy8/T19vf4+fr7/P3+/w==":345:0
+
+Base64 encode hex all valid input bytes #1
+base64_encode_hex:"0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f404142434445464748494a4b4c4d4e4f505152535455565758595a5b5c5d5e5f606162636465666768696a6b6c6d6e6f707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f909192939495969798999a9b9c9d9e9fa0a1a2a3a4a5a6a7a8a9aaabacadaeafb0b1b2b3b4b5b6b7b8b9babbbcbdbebfc0c1c2c3c4c5c6c7c8c9cacbcccdcecfd0d1d2d3d4d5d6d7d8d9dadbdcdddedfe0e1e2e3e4e5e6e7e8e9eaebecedeeeff0f1f2f3f4f5f6f7f8f9fafbfcfdfeff00":"AQIDBAUGBwgJCgsMDQ4PEBESExQVFhcYGRobHB0eHyAhIiMkJSYnKCkqKywtLi8wMTIzNDU2Nzg5Ojs8PT4/QEFCQ0RFRkdISUpLTE1OT1BRUlNUVVZXWFlaW1xdXl9gYWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXp7fH1+f4CBgoOEhYaHiImKi4yNjo+QkZKTlJWWl5iZmpucnZ6foKGio6SlpqeoqaqrrK2ur7CxsrO0tba3uLm6u7y9vr/AwcLDxMXGx8jJysvMzc7P0NHS09TV1tfY2drb3N3e3+Dh4uPk5ebn6Onq6+zt7u/w8fLz9PX29/j5+vv8/f7/AA==":345:0
+
+Base64 encode hex all valid input bytes #2
+base64_encode_hex:"02030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f404142434445464748494a4b4c4d4e4f505152535455565758595a5b5c5d5e5f606162636465666768696a6b6c6d6e6f707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f909192939495969798999a9b9c9d9e9fa0a1a2a3a4a5a6a7a8a9aaabacadaeafb0b1b2b3b4b5b6b7b8b9babbbcbdbebfc0c1c2c3c4c5c6c7c8c9cacbcccdcecfd0d1d2d3d4d5d6d7d8d9dadbdcdddedfe0e1e2e3e4e5e6e7e8e9eaebecedeeeff0f1f2f3f4f5f6f7f8f9fafbfcfdfeff0001":"AgMEBQYHCAkKCwwNDg8QERITFBUWFxgZGhscHR4fICEiIyQlJicoKSorLC0uLzAxMjM0NTY3ODk6Ozw9Pj9AQUJDREVGR0hJSktMTU5PUFFSU1RVVldYWVpbXF1eX2BhYmNkZWZnaGlqa2xtbm9wcXJzdHV2d3h5ent8fX5/gIGCg4SFhoeIiYqLjI2Oj5CRkpOUlZaXmJmam5ydnp+goaKjpKWmp6ipqqusra6vsLGys7S1tre4ubq7vL2+v8DBwsPExcbHyMnKy8zNzs/Q0dLT1NXW19jZ2tvc3d7f4OHi4+Tl5ufo6err7O3u7/Dx8vP09fb3+Pn6+/z9/v8AAQ==":345:0
+
+Base64 encode all valid output characters at all offsets
+base64_encode_hex:"00108310518720928b30d38f41149351559761969b71d79f8218a39259a7a29aabb2dbafc31cb3d35db7e39ebbf3dfbff800420c41461c824a2cc34e3d04524d45565d865a6dc75e7e08628e49669e8a6aaecb6ebf0c72cf4d76df8e7aefcf7effe00108310518720928b30d38f41149351559761969b71d79f8218a39259a7a29aabb2dbafc31cb3d35db7e39ebbf3dfbff800420c41461c824a2cc34e3d04524d45565d865a6dc75e7e08628e49669e8a6aaecb6ebf0c72cf4d76df8e7aefcf7efd0":"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/+ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/+ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/+ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/Q":261:0
+
 Base64 decode hex #1
 base64_decode_hex:"AQIDBAUGBwgJ":"010203040506070809":9:0
 
@@ -166,6 +180,9 @@ base64_decode_hex:"AQIDBAUGBw==":"01020304050607":7:0
 Base64 decode hex #5 (buffer too small)
 base64_decode_hex:"AQIDBAUGBw==":"01020304050607":6:MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL
 
+Base64 decode all valid input characters at all offsets
+base64_decode_hex:"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/+ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/+ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/+ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/Q":"00108310518720928b30d38f41149351559761969b71d79f8218a39259a7a29aabb2dbafc31cb3d35db7e39ebbf3dfbff800420c41461c824a2cc34e3d04524d45565d865a6dc75e7e08628e49669e8a6aaecb6ebf0c72cf4d76df8e7aefcf7effe00108310518720928b30d38f41149351559761969b71d79f8218a39259a7a29aabb2dbafc31cb3d35db7e39ebbf3dfbff800420c41461c824a2cc34e3d04524d45565d865a6dc75e7e08628e49669e8a6aaecb6ebf0c72cf4d76df8e7aefcf7efd0":195:0
+
 Base64 Selftest
 depends_on:MBEDTLS_SELF_TEST
 base64_selftest:
diff --git a/tests/suites/test_suite_base64.function b/tests/suites/test_suite_base64.function
index dc6ec153b..be9b6e8c3 100644
--- a/tests/suites/test_suite_base64.function
+++ b/tests/suites/test_suite_base64.function
@@ -1,5 +1,6 @@
 /* BEGIN_HEADER */
 #include "mbedtls/base64.h"
+#include <test/constant_flow.h>
 /* END_HEADER */
 
 /* BEGIN_DEPENDENCIES
@@ -13,13 +14,22 @@ void mbedtls_base64_encode( char * src_string, char * dst_string,
 {
     unsigned char src_str[1000];
     unsigned char dst_str[1000];
-    size_t len;
+    size_t len, src_len;
 
     memset(src_str, 0x00, 1000);
     memset(dst_str, 0x00, 1000);
 
     strncpy( (char *) src_str, src_string, sizeof(src_str) - 1 );
-    TEST_ASSERT( mbedtls_base64_encode( dst_str, dst_buf_size, &len, src_str, strlen( (char *) src_str ) ) == result );
+    src_len = strlen( (char *) src_str );
+
+    TEST_CF_SECRET( src_str, sizeof( src_str ) );
+    TEST_ASSERT( mbedtls_base64_encode( dst_str, dst_buf_size, &len, src_str, src_len) == result );
+    TEST_CF_PUBLIC( src_str, sizeof( src_str ) );
+
+    /* dest_str will have had tainted data copied to it, prevent the TEST_ASSERT below from triggering
+       CF failures by unmarking it. */
+    TEST_CF_PUBLIC( dst_str, len );
+
     if( result == 0 )
     {
         TEST_ASSERT( strcmp( (char *) dst_str, dst_string ) == 0 );
@@ -57,7 +67,14 @@ void base64_encode_hex( data_t * src, char * dst, int dst_buf_size,
 
     res = mbedtls_test_zero_alloc( dst_buf_size );
 
+    TEST_CF_SECRET( src->x, src->len );
     TEST_ASSERT( mbedtls_base64_encode( res, dst_buf_size, &len, src->x, src->len ) == result );
+    TEST_CF_PUBLIC( src->x, src->len );
+
+    /* res will have had tainted data copied to it, prevent the TEST_ASSERT below from triggering
+       CF failures by unmarking it. */
+    TEST_CF_PUBLIC( res, len );
+
     if( result == 0 )
     {
         TEST_ASSERT( len == strlen( dst ) );