library/base64.c

/*
 *  RFC 1521 base64 encoding/decoding
 *
 *  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.
 */

#include "common.h"

#if defined(MBEDTLS_BASE64_C)

#include "mbedtls/base64.h"

#include <stdint.h>

#if defined(MBEDTLS_SELF_TEST)
#include <string.h>
#if defined(MBEDTLS_PLATFORM_C)
#include "mbedtls/platform.h"
#else
#include <stdio.h>
#define mbedtls_printf printf
#endif /* MBEDTLS_PLATFORM_C */
#endif /* MBEDTLS_SELF_TEST */

static const unsigned char base64_enc_map[64] =
{
    'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
    'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
    'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd',
    'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
    'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x',
    'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7',
    '8', '9', '+', '/'
};

#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 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;
}

/* Return 0xff if low <= c <= high, 0 otherwise.
 *
 * Constant flow with respect to c.
 */
static unsigned char mask_of_range( unsigned char low, unsigned char high,
                                    unsigned char c )
{
    unsigned low_mask = ( c - low ) >> 8;
    unsigned high_mask = ( c - high - 1 ) >> 8;
    return( ~low_mask & high_mask & 0xff );
}

/*
 * Encode a buffer into base64 format
 */
int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
                   const unsigned char *src, size_t slen )
{
    size_t i, n;
    int C1, C2, C3;
    unsigned char *p;

    if( slen == 0 )
    {
        *olen = 0;
        return( 0 );
    }

    n = slen / 3 + ( slen % 3 != 0 );

    if( n > ( BASE64_SIZE_T_MAX - 1 ) / 4 )
    {
        *olen = BASE64_SIZE_T_MAX;
        return( MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL );
    }

    n *= 4;

    if( ( dlen < n + 1 ) || ( NULL == dst ) )
    {
        *olen = n + 1;
        return( MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL );
    }

    n = ( slen / 3 ) * 3;

    for( i = 0, p = dst; i < n; i += 3 )
    {
        C1 = *src++;
        C2 = *src++;
        C3 = *src++;

        *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 )
    {
        C1 = *src++;
        C2 = ( ( i + 1 ) < slen ) ? *src++ : 0;

        *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++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
                                                 ( ( ( C2 & 15 ) << 2 ) & 0x3F ) );
        else *p++ = '=';

        *p++ = '=';
    }

    *olen = p - dst;
    *p = 0;

    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.
 */
static signed char 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 |= mask_of_range( 'A', 'Z', c ) & ( c - 'A' +  0 + 1 );
    val |= mask_of_range( 'a', 'z', c ) & ( c - 'a' + 26 + 1 );
    val |= mask_of_range( '0', '9', c ) & ( c - '0' + 52 + 1 );
    val |= mask_of_range( '+', '+', c ) & ( c - '+' + 62 + 1 );
    val |= 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
 */
int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
                   const unsigned char *src, size_t slen )
{
    size_t i; /* index in source */
    size_t n; /* number of digits or trailing = in source */
    uint32_t x; /* value accumulator */
    unsigned equals = 0;
    int spaces_present = 0;
    unsigned char *p;

    /* First pass: check for validity and get output length */
    for( i = n = 0; i < slen; i++ )
    {
        /* Skip spaces before checking for EOL */
        spaces_present = 0;
        while( i < slen && src[i] == ' ' )
        {
            ++i;
            spaces_present = 1;
        }

        /* Spaces at end of buffer are OK */
        if( i == slen )
            break;

        if( ( slen - i ) >= 2 &&
            src[i] == '\r' && src[i + 1] == '\n' )
            continue;

        if( src[i] == '\n' )
            continue;

        /* Space inside a line is an error */
        if( spaces_present )
            return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );

        if( src[i] > 127 )
            return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );

        if( src[i] == '=' )
        {
            if( ++equals > 2 )
                return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
        }
        else
        {
            if( equals != 0 )
                return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
            if( dec_value( src[i] ) < 0 )
                return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
        }
        n++;
    }

    if( n == 0 )
    {
        *olen = 0;
        return( 0 );
    }

    /* The following expression is to calculate the following formula without
     * risk of integer overflow in n:
     *     n = ( ( n * 6 ) + 7 ) >> 3;
     */
    n = ( 6 * ( n >> 3 ) ) + ( ( 6 * ( n & 0x7 ) + 7 ) >> 3 );
    n -= equals;

    if( dst == NULL || dlen < n )
    {
        *olen = n;
        return( MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL );
    }

    equals = 0;
    for( n = x = 0, p = dst; i > 0; i--, src++ )
    {
        if( *src == '\r' || *src == '\n' || *src == ' ' )
            continue;

        x = x << 6;
        if( *src == '=' )
            ++equals;
        else
            x |= dec_value( *src );

        if( ++n == 4 )
        {
            n = 0;
            *p++ = MBEDTLS_BYTE_2( x );
            if( equals <= 1 ) *p++ = MBEDTLS_BYTE_1( x );
            if( equals <= 0 ) *p++ = MBEDTLS_BYTE_0( x );
        }
    }

    *olen = p - dst;

    return( 0 );
}

#if defined(MBEDTLS_SELF_TEST)

static const unsigned char base64_test_dec[64] =
{
    0x24, 0x48, 0x6E, 0x56, 0x87, 0x62, 0x5A, 0xBD,
    0xBF, 0x17, 0xD9, 0xA2, 0xC4, 0x17, 0x1A, 0x01,
    0x94, 0xED, 0x8F, 0x1E, 0x11, 0xB3, 0xD7, 0x09,
    0x0C, 0xB6, 0xE9, 0x10, 0x6F, 0x22, 0xEE, 0x13,
    0xCA, 0xB3, 0x07, 0x05, 0x76, 0xC9, 0xFA, 0x31,
    0x6C, 0x08, 0x34, 0xFF, 0x8D, 0xC2, 0x6C, 0x38,
    0x00, 0x43, 0xE9, 0x54, 0x97, 0xAF, 0x50, 0x4B,
    0xD1, 0x41, 0xBA, 0x95, 0x31, 0x5A, 0x0B, 0x97
};

static const unsigned char base64_test_enc[] =
    "JEhuVodiWr2/F9mixBcaAZTtjx4Rs9cJDLbpEG8i7hPK"
    "swcFdsn6MWwINP+Nwmw4AEPpVJevUEvRQbqVMVoLlw==";

/*
 * Checkup routine
 */
int mbedtls_base64_self_test( int verbose )
{
    size_t len;
    const unsigned char *src;
    unsigned char buffer[128];

    if( verbose != 0 )
        mbedtls_printf( "  Base64 encoding test: " );

    src = base64_test_dec;

    if( mbedtls_base64_encode( buffer, sizeof( buffer ), &len, src, 64 ) != 0 ||
         memcmp( base64_test_enc, buffer, 88 ) != 0 )
    {
        if( verbose != 0 )
            mbedtls_printf( "failed\n" );

        return( 1 );
    }

    if( verbose != 0 )
        mbedtls_printf( "passed\n  Base64 decoding test: " );

    src = base64_test_enc;

    if( mbedtls_base64_decode( buffer, sizeof( buffer ), &len, src, 88 ) != 0 ||
         memcmp( base64_test_dec, buffer, 64 ) != 0 )
    {
        if( verbose != 0 )
            mbedtls_printf( "failed\n" );

        return( 1 );
    }

    if( verbose != 0 )
        mbedtls_printf( "passed\n\n" );

    return( 0 );
}

#endif /* MBEDTLS_SELF_TEST */

#endif /* MBEDTLS_BASE64_C */