写一个简单的C语言程序用RSA实现以数字信封的形式对明文文件进行加密。要求如下： 1、 发送方利用系统自带的伪随机数生成函数生成会话密钥 2、 用接收方的公钥对会话密钥加密 3、 用会话密钥以OFB的模式对明文文件p_text.txt（文件大于1KB）进行加密，结果存于密文文件c_text.txt 4、 接收方利用私钥解密会话密钥，然后用会话密钥对密文文件解密，结果存于明文文件p1_text.txt，最后对比p_text.txt和p1_text.txt

由于RSA算法的实现比较复杂，我提供一个简化版的代码，仅供参考。需要注意的是，本代码仅用于演示RSA加密和解密的基本流程，安全性和实用性有待进一步提升。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <math.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>

#define KEY_LENGTH 2048
#define PUB_KEY_FILE "pubkey.pem"
#define PRI_KEY_FILE "prikey.pem"
#define BLOCK_SIZE 16

/* 生成会话密钥 */
void gen_session_key(unsigned char *key, size_t key_len) {
    srand(time(NULL));
    for (int i = 0; i < key_len; i++) {
        key[i] = rand() % 256;
    }
}

/* 使用公钥加密会话密钥 */
int rsa_encrypt_session_key(unsigned char *session_key, size_t session_key_len, unsigned char *encrypted_key, size_t *encrypted_key_len) {
    RSA *rsa = NULL;
    FILE *fp = NULL;
    int ret = -1;

    fp = fopen(PUB_KEY_FILE, "rb");
    if (!fp) {
        printf("Error: cannot open public key file.\n");
        return -1;
    }

    rsa = PEM_read_RSA_PUBKEY(fp, NULL, NULL, NULL);
    if (!rsa) {
        printf("Error: cannot read public key.\n");
        fclose(fp);
        return -1;
    }

    *encrypted_key_len = RSA_public_encrypt(session_key_len, session_key, encrypted_key, rsa, RSA_PKCS1_PADDING);
    if (*encrypted_key_len == -1) {
        printf("Error: RSA encryption failed.\n");
        RSA_free(rsa);
        fclose(fp);
        return -1;
    }

    RSA_free(rsa);
    fclose(fp);
    return 0;
}

/* 使用私钥解密会话密钥 */
int rsa_decrypt_session_key(unsigned char *encrypted_key, size_t encrypted_key_len, unsigned char *session_key, size_t *session_key_len) {
    RSA *rsa = NULL;
    FILE *fp = NULL;
    int ret = -1;

    fp = fopen(PRI_KEY_FILE, "rb");
    if (!fp) {
        printf("Error: cannot open private key file.\n");
        return -1;
    }

    rsa = PEM_read_RSAPrivateKey(fp, NULL, NULL, NULL);
    if (!rsa) {
        printf("Error: cannot read private key.\n");
        fclose(fp);
        return -1;
    }

    *session_key_len = RSA_private_decrypt(encrypted_key_len, encrypted_key, session_key, rsa, RSA_PKCS1_PADDING);
    if (*session_key_len == -1) {
        printf("Error: RSA decryption failed.\n");
        RSA_free(rsa);
        fclose(fp);
        return -1;
    }

    RSA_free(rsa);
    fclose(fp);
    return 0;
}

/* 使用会话密钥对明文文件进行加密 */
int ofb_encrypt_file(unsigned char *session_key, size_t session_key_len, char *plain_file, char *cipher_file) {
    FILE *fp_plain = NULL;
    FILE *fp_cipher = NULL;
    unsigned char iv[BLOCK_SIZE] = {0};
    unsigned char block[BLOCK_SIZE] = {0};
    unsigned char encrypted_block[BLOCK_SIZE] = {0};
    int read_len = 0;
    int total_len = 0;
    int ret = -1;

    fp_plain = fopen(plain_file, "rb");
    if (!fp_plain) {
        printf("Error: cannot open plain file.\n");
        return -1;
    }

    fp_cipher = fopen(cipher_file, "wb");
    if (!fp_cipher) {
        printf("Error: cannot open cipher file.\n");
        fclose(fp_plain);
        return -1;
    }

    /* 生成初始向量 */
    srand(time(NULL));
    for (int i = 0; i < BLOCK_SIZE; i++) {
        iv[i] = rand() % 256;
    }

    /* 写入初始向量 */
    fwrite(iv, sizeof(unsigned char), BLOCK_SIZE, fp_cipher);

    /* 加密明文文件 */
    while ((read_len = fread(block, sizeof(unsigned char), BLOCK_SIZE, fp_plain)) > 0) {
        for (int i = 0; i < BLOCK_SIZE; i++) {
            block[i] ^= iv[i];
        }
        AES_encrypt(block, encrypted_block, session_key);
        fwrite(encrypted_block, sizeof(unsigned char), BLOCK_SIZE, fp_cipher);
        memcpy(iv, encrypted_block, BLOCK_SIZE);
        total_len += read_len;
    }

    printf("Total plain text length: %d bytes.\n", total_len);

    fclose(fp_plain);
    fclose(fp_cipher);
    return 0;
}

/* 使用会话密钥对密文文件进行解密 */
int ofb_decrypt_file(unsigned char *session_key, size_t session_key_len, char *cipher_file, char *plain_file) {
    FILE *fp_plain = NULL;
    FILE *fp_cipher = NULL;
    unsigned char iv[BLOCK_SIZE] = {0};
    unsigned char block[BLOCK_SIZE] = {0};
    unsigned char decrypted_block[BLOCK_SIZE] = {0};
    int read_len = 0;
    int total_len = 0;
    int ret = -1;

    fp_cipher = fopen(cipher_file, "rb");
    if (!fp_cipher) {
        printf("Error: cannot open cipher file.\n");
        return -1;
    }

    fp_plain = fopen(plain_file, "wb");
    if (!fp_plain) {
        printf("Error: cannot open plain file.\n");
        fclose(fp_cipher);
        return -1;
    }

    /* 读取初始向量 */
    fread(iv, sizeof(unsigned char), BLOCK_SIZE, fp_cipher);

    /* 解密密文文件 */
    while ((read_len = fread(block, sizeof(unsigned char), BLOCK_SIZE, fp_cipher)) > 0) {
        AES_decrypt(block, decrypted_block, session_key);
        for (int i = 0; i < BLOCK_SIZE; i++) {
            decrypted_block[i] ^= iv[i];
        }
        fwrite(decrypted_block, sizeof(unsigned char), read_len, fp_plain);
        memcpy(iv, block, BLOCK_SIZE);
        total_len += read_len;
    }

    printf("Total cipher text length: %d bytes.\n", total_len);

    fclose(fp_plain);
    fclose(fp_cipher);
    return 0;
}

int main() {
    unsigned char session_key[KEY_LENGTH/8] = {0};
    unsigned char encrypted_key[KEY_LENGTH/8] = {0};
    size_t encrypted_key_len = 0;
    unsigned char decrypted_key[KEY_LENGTH/8] = {0};
    size_t decrypted_key_len = 0;
    int ret = -1;

    /* 生成会话密钥 */
    gen_session_key(session_key, sizeof(session_key));

    /* 使用公钥加密会话密钥 */
    ret = rsa_encrypt_session_key(session_key, sizeof(session_key), encrypted_key, &encrypted_key_len);
    if (ret != 0) {
        printf("Error: RSA encryption failed.\n");
        return -1;
    }

    /* 使用私钥解密会话密钥 */
    ret = rsa_decrypt_session_key(encrypted_key, encrypted_key_len, decrypted_key, &decrypted_key_len);
    if (ret != 0) {
        printf("Error: RSA decryption failed.\n");
        return -1;
    }

    /* 对明文文件进行加密 */
    ret = ofb_encrypt_file(session_key, sizeof(session_key), "p_text.txt", "c_text.txt");
    if (ret != 0) {
        printf("Error: OFB encryption failed.\n");
        return -1;
    }

    /* 对密文文件进行解密 */
    ret = ofb_decrypt_file(decrypted_key, decrypted_key_len, "c_text.txt", "p1_text.txt");
    if (ret != 0) {
        printf("Error: OFB decryption failed.\n");
        return -1;
    }

    /* 对比明文和解密后的明文 */
    ret = system("diff p_text.txt p1_text.txt");
    if (ret != 0) {
        printf("Error: decryption result is not correct.\n");
        return -1;
    }

    printf("RSA encryption and decryption succeeded.\n");
    return 0;
}

需要注意的是，本代码使用了OpenSSL库来实现RSA加密和解密，需要在编译时链接该库，命令如下：

gcc -o rsa_encrypt rsa_encrypt.c -lcrypto -lssl

其中，`-lcrypto`和`-lssl`分别表示链接`libcrypto`和`libssl`库。

同时，本代码使用了AES算法以OFB模式对明文文件进行加密和解密，需要在编译时链接该库，命令如下：

gcc -o ofb_encrypt ofb_encrypt.c -lcrypto -lssl
gcc -o ofb_decrypt ofb_decrypt.c -lcrypto -lssl

其中，`ofb_encrypt`和`ofb_decrypt`分别表示加密和解密程序的名称。