用c语言代码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

好的，以下是使用C语言实现数字信封加密的代码，代码中使用了OpenSSL库中的RSA算法和OFB模式加解密算法，需要先安装OpenSSL库。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>
#include <openssl/rand.h>
#include <openssl/evp.h>

#define KEY_LENGTH 2048
#define PUB_KEY_FILE "public_key.pem"
#define PRI_KEY_FILE "private_key.pem"

void generate_session_key(unsigned char* key, int keylen) {
    // 利用系统自带的伪随机数生成函数生成会话密钥
    RAND_bytes(key, keylen);
}

int encrypt_session_key(unsigned char* key, int keylen, unsigned char* encrypted_key, RSA* pub_key) {
    // 用接收方的公钥对会话密钥加密
    int encrypted_len = RSA_public_encrypt(keylen, key, encrypted_key, pub_key, RSA_PKCS1_PADDING);
    return encrypted_len;
}

int decrypt_session_key(unsigned char* encrypted_key, int encrypted_len, unsigned char* decrypted_key, RSA* pri_key) {
    // 接收方使用私钥解密会话密钥
    int decrypted_len = RSA_private_decrypt(encrypted_len, encrypted_key, decrypted_key, pri_key, RSA_PKCS1_PADDING);
    return decrypted_len;
}

int encrypt_file(unsigned char* key, int keylen, char* plain_file, char* cipher_file) {
    FILE* fp_plain = fopen(plain_file, "rb");
    FILE* fp_cipher = fopen(cipher_file, "wb");

    // 初始化加密算法
    EVP_CIPHER_CTX* ctx = EVP_CIPHER_CTX_new();
    EVP_EncryptInit_ex(ctx, EVP_aes_256_ofb(), NULL, key, NULL);

    unsigned char inbuf[1024];
    unsigned char outbuf[1024 + EVP_MAX_BLOCK_LENGTH];
    int inlen, outlen, totlen = 0;

    // 用会话密钥以OFB模式对明文文件进行加密
    while ((inlen = fread(inbuf, 1, sizeof(inbuf), fp_plain)) > 0) {
        EVP_EncryptUpdate(ctx, outbuf, &outlen, inbuf, inlen);
        fwrite(outbuf, 1, outlen, fp_cipher);
        totlen += outlen;
    }
    EVP_EncryptFinal_ex(ctx, outbuf, &outlen);
    fwrite(outbuf, 1, outlen, fp_cipher);
    totlen += outlen;

    // 清除加密算法上下文
    EVP_CIPHER_CTX_free(ctx);

    fclose(fp_plain);
    fclose(fp_cipher);

    return totlen;
}

int decrypt_file(unsigned char* key, int keylen, char* cipher_file, char* plain_file) {
    FILE* fp_cipher = fopen(cipher_file, "rb");
    FILE* fp_plain = fopen(plain_file, "wb");

    // 初始化解密算法
    EVP_CIPHER_CTX* ctx = EVP_CIPHER_CTX_new();
    EVP_DecryptInit_ex(ctx, EVP_aes_256_ofb(), NULL, key, NULL);

    unsigned char inbuf[1024 + EVP_MAX_BLOCK_LENGTH];
    unsigned char outbuf[1024];
    int inlen, outlen, totlen = 0;

    // 用会话密钥以OFB模式对密文文件进行解密
    while ((inlen = fread(inbuf, 1, sizeof(inbuf), fp_cipher)) > 0) {
        EVP_DecryptUpdate(ctx, outbuf, &outlen, inbuf, inlen);
        fwrite(outbuf, 1, outlen, fp_plain);
        totlen += outlen;
    }
    EVP_DecryptFinal_ex(ctx, outbuf, &outlen);
    fwrite(outbuf, 1, outlen, fp_plain);
    totlen += outlen;

    // 清除解密算法上下文
    EVP_CIPHER_CTX_free(ctx);

    fclose(fp_cipher);
    fclose(fp_plain);

    return totlen;
}

int main() {
    // 读取接收方的公钥和私钥
    RSA* pub_key = NULL;
    RSA* pri_key = NULL;
    FILE* fp_pub = fopen(PUB_KEY_FILE, "rb");
    FILE* fp_pri = fopen(PRI_KEY_FILE, "rb");
    if (fp_pub == NULL || fp_pri == NULL) {
        printf("Error: cannot open public or private key file.\n");
        return 1;
    }
    pub_key = PEM_read_RSA_PUBKEY(fp_pub, NULL, NULL, NULL);
    pri_key = PEM_read_RSAPrivateKey(fp_pri, NULL, NULL, NULL);
    fclose(fp_pub);
    fclose(fp_pri);

    // 生成会话密钥
    unsigned char session_key[32];
    generate_session_key(session_key, sizeof(session_key));

    // 加密会话密钥
    unsigned char encrypted_key[256];
    int encrypted_len = encrypt_session_key(session_key, sizeof(session_key), encrypted_key, pub_key);

    // 加密明文文件
    int cipher_len = encrypt_file(session_key, sizeof(session_key), "p_text.txt", "c_text.txt");

    // 解密会话密钥
    unsigned char decrypted_key[32];
    int decrypted_len = decrypt_session_key(encrypted_key, encrypted_len, decrypted_key, pri_key);

    // 解密密文文件
    int plain_len = decrypt_file(decrypted_key, sizeof(decrypted_key), "c_text.txt", "p1_text.txt");

    // 对比明文文件和解密后的明文文件
    if (plain_len != cipher_len) {
        printf("Error: decrypted file length is different from original file length.\n");
        return 1;
    }
    FILE* fp_plain = fopen("p_text.txt", "rb");
    FILE* fp_p1 = fopen("p1_text.txt", "rb");
    unsigned char buf1[1024];
    unsigned char buf2[1024];
    while (1) {
        int len1 = fread(buf1, 1, sizeof(buf1), fp_plain);
        int len2 = fread(buf2, 1, sizeof(buf2), fp_p1);
        if (len1 != len2 || memcmp(buf1, buf2, len1) != 0) {
            printf("Error: decrypted file is different from original file.\n");
            fclose(fp_plain);
            fclose(fp_p1);
            return 1;
        }
        if (len1 == 0 || len2 == 0) {
            break;
        }
    }
    fclose(fp_plain);
    fclose(fp_p1);

    printf("Encryption and decryption succeeded.\n");

    return 0;
}

需要注意的是，代码中使用了256位的AES算法和2048位的RSA算法，如果需要改变加密算法的强度，可以修改对应的参数。同时，代码中忽略了密钥管理的部分，实际使用中需要根据具体情况设计密钥管理方案。