decrypt.c

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/*****************************************************************************
 * ugBASIC - an isomorphic BASIC language compiler for retrocomputers        *
 *****************************************************************************
 * Copyright 2021-2025 Marco Spedaletti (asimov@mclink.it)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
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/****************************************************************************
 * INCLUDE SECTION 
 ****************************************************************************/
 
#include "../../ugbc.h"
 
/****************************************************************************
 * CODE SECTION 
 ****************************************************************************/

/* <usermanual>
@keyword DECRYPT
 
@english
 
The ''DECRYPT'' command applies a symmetric decryption algorithm to a 
structured variable or memory area, checking for security checksum
present at the end, to prevent data corruption or accidental tampering.
 
When the command is executed, the destination buffer is filled with the 
original data decrypted by the encryption operation. A computed value 
a checksum is recalculated and checked with the one on the end of the 
buffer, to serve as a trust seal.
 
Note that the size of the key must be the same as the size of the data.
 
@italian
 
Il comando ''DECRYPT'' applica un algoritmo di decrittazione simmetrico a una
variabile strutturata o a un'area di memoria, verificando la presenza di un checksum di sicurezza
alla fine, per prevenire il danneggiamento dei dati o manomissioni accidentali.
 
Quando il comando viene eseguito, il buffer di destinazione viene riempito con i
dati originali decrittografati dall'operazione di crittografia. Un valore calcolato
un checksum viene ricalcolato e confrontato con quello alla fine del
buffer, per fungere da sigillo di attendibilità.
 
Si noti che la dimensione della chiave deve essere uguale alla dimensione dei dati.
 
@syntax = DECRYPT( data, key TO var )
 
@example DIM params(2) AS INTEGER
@example IF DECRYPT( ENCRYPT( params, "OKOK" ), "OKOK" TO params) THEN: PRINT "OK!": ENDIF
 
</usermanual> */
Variable * decrypt( Environment * _environment, char * _data, char * _key, char * _var ) {
 
    Variable * data = variable_retrieve( _environment, _data );
    Variable * key = variable_retrieve( _environment, _key );
    Variable * output = variable_retrieve( _environment, _var );
    Variable * result = variable_temporary( _environment, VT_SBYTE, "(result)");
 
    Variable * dataAddress = variable_temporary( _environment, VT_ADDRESS, "(address of data)");
    Variable * dataSize = variable_temporary( _environment, VT_BYTE, "(size of data)");
 
    Variable * keyAddress = variable_temporary( _environment, VT_ADDRESS, "(address of data)");
    Variable * keySize = variable_temporary( _environment, VT_BYTE, "(size of data)");
 
    Variable * outputAddress = variable_temporary( _environment, VT_ADDRESS, "(address of output)");
 
    switch( data->type ) {
        case VT_STRING: {
            cpu_move_8bit( _environment, data->realName, dataSize->realName );
            cpu_addressof_16bit( _environment, data->realName, dataAddress->realName );
            cpu_inc_16bit( _environment, dataAddress->realName );
            break;
        };
        case VT_DSTRING: {
            cpu_dsdescriptor( _environment, data->realName, dataAddress->realName, dataSize->realName );
            break;
        };
        case VT_TYPE:
        case VT_TARRAY:
        case VT_BUFFER: {
            cpu_store_8bit( _environment, dataSize->realName, data->size );
            cpu_addressof_16bit( _environment, data->realName, dataAddress->realName );
            break;
        };
    }
 
    switch( key->type ) {
        case VT_STRING: {
            cpu_move_8bit( _environment, key->realName, keySize->realName );
            cpu_addressof_16bit( _environment, key->realName, keyAddress->realName );
            cpu_inc_16bit( _environment, keyAddress->realName );
            break;
        };
        case VT_TARRAY:
        case VT_TYPE:
        case VT_BUFFER: {
            cpu_store_8bit( _environment, key->realName, key->size );
            cpu_addressof_16bit( _environment, key->realName, keyAddress->realName );
            break;
        };
        case VT_DSTRING: {
            cpu_dsdescriptor( _environment, key->realName, keyAddress->realName, keySize->realName );
            break;
        };
    }
 
    if ( VT_BITWIDTH( output->type ) ) {
        cpu_store_8bit( _environment, dataSize->realName, output->size );
        cpu_addressof_16bit( _environment, output->realName, outputAddress->realName );
    } else {
        switch( output->type ) {
            case VT_TARRAY:
            case VT_TYPE:
            case VT_BUFFER: {
                cpu_store_8bit( _environment, dataSize->realName, output->size );
                cpu_addressof_16bit( _environment, output->realName, outputAddress->realName );
                break;
            };
            case VT_DSTRING: {
                cpu_dsfree( _environment, output->realName );
                cpu_dsalloc( _environment, keySize->realName, output->realName );
                cpu_dsdescriptor( _environment, output->realName, outputAddress->realName, NULL );
                break;
            };
            default:
                CRITICAL_CANNOT_DECRYPT_TO_DATATYPE( _var );
        }
    }
    
    cpu_decrypt( _environment, dataAddress->realName, dataSize->realName, keyAddress->realName, keySize->realName, outputAddress->realName, result->realName );
 
    return result;
    
}