/**
 * @file se050_rng.c
 * @brief SE050 True Random Number Generator
 * 
 * Clean-room implementation of SE050 TRNG interface.
 * 
 * License: MIT (Clean-room implementation)
 */

#define _POSIX_C_SOURCE 200809L
#include "se050_i2c_hal.h"
#include "se050_wireguard.h"
#include "se050_crypto_utils.h"
#include "se050_session_internal.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>

/**
 * @brief RNG context structure
 */
struct se050_rng_ctx {
    se050_session_ctx_t *session;       /**< Associated session */
    uint32_t rng_counter;               /**< RNG generation counter */
    uint8_t entropy_pool[64];           /**< Entropy pool for mixing */
    size_t entropy_len;                 /**< Current entropy pool size */
};

/* ============================================================================
 * RNG Management
 * ============================================================================ */

se050_status_t se050_rng_init(se050_rng_ctx_t **ctx, se050_session_ctx_t *session)
{
    se050_rng_ctx_t *rng;
    
    if (!ctx || !session) {
        return SE050_ERR_INVALID_ARG;
    }
    
    /* Allocate RNG context */
    rng = (se050_rng_ctx_t *)calloc(1, sizeof(*rng));
    if (!rng) {
        return SE050_ERR_FAIL;
    }
    
    rng->session = session;
    rng->rng_counter = 0;
    rng->entropy_len = 0;
    
    /* Zeroize entropy pool */
    memzero_explicit(rng->entropy_pool, sizeof(rng->entropy_pool));
    
    *ctx = rng;
    return SE050_OK;
}

void se050_rng_free(se050_rng_ctx_t *ctx)
{
    if (!ctx) {
        return;
    }
    
    /* Securely zeroize entropy pool */
    if (ctx->entropy_len > 0) {
        memzero_explicit(ctx->entropy_pool, ctx->entropy_len);
        ctx->entropy_len = 0;
    }
    
    /* Free RNG context */
    free(ctx);
}

/* ============================================================================
 * Random Number Generation
 * ============================================================================ */

se050_status_t se050_rng_generate(se050_rng_ctx_t *ctx, uint8_t *output, size_t length)
{
    uint8_t cmd[64];
    uint8_t rsp[256];
    int ret;
    size_t cmd_len, rsp_len;
    size_t offset = 0;
    
    if (!ctx || !output) {
        return SE050_ERR_INVALID_ARG;
    }
    
    if (length == 0 || length > sizeof(rsp) - 2) {
        return SE050_ERR_INVALID_ARG;
    }
    
    while (offset < length) {
        size_t chunk_len = length - offset;
        if (chunk_len > 240) {
            chunk_len = 240;  /* Maximum chunk size */
        }
        
        /* Build GET_RANDOM APDU */
        cmd[0] = 0x80;              /* CLA */
        cmd[1] = 0xC0;              /* INS - GET_RANDOM */
        cmd[2] = 0x00;              /* P1 */
        cmd[3] = 0x00;              /* P2 */
        cmd[4] = 0x00;              /* LC */
        cmd[5] = (uint8_t)chunk_len; /* LE - requested length */
        
        cmd_len = 6;
        
        /* Send command */
        ret = se050_i2c_write(ctx->session->hal, cmd, (int)cmd_len);
        if (ret < 0) {
            return SE050_ERR_I2C;
        }
        
        /* Receive response */
        rsp_len = sizeof(rsp);
        ret = se050_i2c_read(ctx->session->hal, rsp, (int)rsp_len);
        if (ret < 0) {
            return SE050_ERR_I2C;
        }
        rsp_len = (size_t)ret;
        
        /* Check response status */
        if (rsp_len < 2) {
            return SE050_ERR_RNG;
        }
        
        uint16_t sw = (uint16_t)((rsp[rsp_len - 2] << 8) | rsp[rsp_len - 1]);
        if (sw != 0x9000) {
            return SE050_ERR_RNG;
        }
        
        /* Copy random data (excluding status word) */
        size_t data_len = rsp_len - 2;
        if (data_len > chunk_len) {
            data_len = chunk_len;
        }
        
        memcpy(output + offset, rsp, data_len);
        offset += data_len;
        
        /* Increment counter */
        ctx->rng_counter++;
    }
    
    return SE050_OK;
}

/**
 * @brief Generate random bytes using host crypto fallback
 * This is a fallback implementation if SE050 TRNG is not available.
 */
se050_status_t se050_rng_generate_fallback(uint8_t *output, size_t length)
{
#ifdef __linux__
    int fd;
    ssize_t ret;
    
    /* Use /dev/urandom as fallback */
    fd = open("/dev/urandom", O_RDONLY);
    if (fd < 0) {
        return SE050_ERR_RNG;
    }
    
    ret = read(fd, output, length);
    close(fd);
    
    if (ret != (ssize_t)length) {
        return SE050_ERR_RNG;
    }
    
    return SE050_OK;
#else
    /* Simple LCG-based fallback (NOT SECURE - for testing only) */
    static uint32_t lcg_state = 0x12345678;
    size_t i;
    
    for (i = 0; i < length; i++) {
        lcg_state = lcg_state * 1103515245 + 12345;
        output[i] = (uint8_t)((lcg_state >> 16) & 0xFF);
    }
    
    return SE050_OK;
#endif
}

/**
 * @brief Generate a random 32-byte key
 */
se050_status_t se050_rng_generate_key(se050_rng_ctx_t *ctx, uint8_t *key)
{
    return se050_rng_generate(ctx, key, 32);
}

/**
 * @brief Generate a random nonce
 */
se050_status_t se050_rng_generate_nonce(se050_rng_ctx_t *ctx, uint8_t *nonce, size_t length)
{
    return se050_rng_generate(ctx, nonce, length);
}