使用openssl 中的EVP库函数写一个加解密例子程序 c语言的

好的，这是一个关于加解密的技术问题，我可以回答。下面是一个简单的例子程序：

#include <stdio.h>
#include <string.h>
#include <openssl/evp.h>

#define MAX_BUF_SIZE 1024

// 加密函数
int encrypt(unsigned char *plaintext, int plaintext_len, unsigned char *key,
            unsigned char *iv, unsigned char *ciphertext) {
    EVP_CIPHER_CTX *ctx;
    int len;
    int ciphertext_len;

    if(!(ctx = EVP_CIPHER_CTX_new())) return -1;

    if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv))
        return -1;

    if(1 != EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len))
        return -1;
    ciphertext_len = len;

    if(1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len)) return -1;
    ciphertext_len += len;

    EVP_CIPHER_CTX_free(ctx);

    return ciphertext_len;
}

// 解密函数
int decrypt(unsigned char *ciphertext, int ciphertext_len, unsigned char *key,
            unsigned char *iv, unsigned char *plaintext) {
    EVP_CIPHER_CTX *ctx;
    int len;
    int plaintext_len;
    int ret;

    if(!(ctx = EVP_CIPHER_CTX_new())) return -1;

    if(1 != EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv))
        return -1;

    if(1 != EVP_DecryptUpdate(ctx, plaintext, &len, ciphertext, ciphertext_len))
        return -1;
    plaintext_len = len;

    ret = EVP_DecryptFinal_ex(ctx, plaintext + len, &len);
    EVP_CIPHER_CTX_free(ctx);
    if(ret > 0) {
        plaintext_len += len;
        return plaintext_len;
    } else {
        return -1;
    }
}

int main() {
    unsigned char plaintext[MAX_BUF_SIZE] = "Hello, world!";
    unsigned char key[EVP_MAX_KEY_LENGTH] = "01234567890123456789012345678901";
    unsigned char iv[EVP_MAX_IV_LENGTH] = "0123456789012345";
    unsigned char ciphertext[MAX_BUF_SIZE] = {0};
    unsigned char decryptedtext[MAX_BUF_SIZE] = {0};
    int decryptedtext_len, ciphertext_len;

    ciphertext_len = encrypt(plaintext, strlen((char *)plaintext), key, iv, ciphertext);
    printf("Ciphertext is:\n");
    BIO_dump_fp(stdout, (char *)ciphertext, ciphertext_len);

    decryptedtext_len = decrypt(ciphertext, ciphertext_len, key, iv, decryptedtext);
    decryptedtext[decryptedtext_len] = '\0';
    printf("Decrypted text is:\n");
    printf("%s\n", decryptedtext);

    return 0;
}

这个程序使用了 OpenSSL 的 EVP 库函数，用 AES-256 CBC 算法加密和解密数据。注意，这个程序中的密钥和 IV 都是硬编码的，实际应用中应该使用更安全的方式来保存和传输密钥和 IV。