EVP_DigestUpdate 和 "invalid conversion from ‘unsigned char’ to ‘const char’"

Question

以下是从 OpenSSL wiki 上的 EVP Message Digests 获取的示例：

void digest_message(unsigned char *message, unsigned char **digest, unsigned int *digest_len)
{
    EVP_MD_CTX *mdctx;

    if((mdctx = EVP_MD_CTX_create()) == NULL)
        handleErrors();

    if(1 != EVP_DigestInit_ex(mdctx, EVP_sha256(), NULL))
        handleErrors();

    if(1 != EVP_DigestUpdate(mdctx, message, strlen(message)))
        handleErrors();

    if((*digest = (unsigned char *)OPENSSL_malloc(EVP_MD_size(EVP_sha256()))) == NULL)
        handleErrors();

    if(1 != EVP_DigestFinal_ex(mdctx, *digest, digest_len))
        handleErrors();

    EVP_MD_CTX_destroy(mdctx);
}

问题是当我尝试编译它时出现以下错误：

evp_test.cpp:18:60: error: invalid conversion from ‘unsigned char*’ to ‘const char*’ [-fpermissive]
 if(1 != EVP_DigestUpdate(mdctx, message, strlen(message)))

In file included from /usr/include/c++/5/cstring:42:0

所以 strlen 在消息上调用会导致错误，因为 strlen 需要 const char？这是因为字符串是 const chars 的数组吗？

我应该做些什么来解决这个问题？

Answer 1

已修复：

void digest_message(const unsigned char *message, size_t message_len,
                    unsigned char **digest, unsigned int *digest_len)
{
    EVP_MD_CTX *mdctx;

    if((mdctx = EVP_MD_CTX_create()) == NULL)
        handleErrors();

    if(1 != EVP_DigestInit_ex(mdctx, EVP_sha256(), NULL))
        handleErrors();

    if(1 != EVP_DigestUpdate(mdctx, message, message_len))
        handleErrors();

    if((*digest = (unsigned char *)OPENSSL_malloc(EVP_MD_size(EVP_sha256()))) == NULL)
        handleErrors();

    if(1 != EVP_DigestFinal_ex(mdctx, *digest, digest_len))
        handleErrors();

    EVP_MD_CTX_destroy(mdctx);
}

你是对的，strlen 需要一个 char 数组，而不是 unsigned char 或 byte。

更大的问题是 design/engineering。需要一个明确的长度参数，因为不能保证字节数组像 C 字符串一样以 NULL 结尾。事实上，字节数组中可能嵌入了一些 NULL。

EVP_DigestUpdate 和 "invalid conversion from ‘unsigned char’ to ‘const char’"

EVP_DigestUpdate and "invalid conversion from ‘unsigned char’ to ‘const char’"

c++

hash

openssl

sha256

EVP_DigestUpdate 和 "invalid conversion from ‘unsigned char*’ to ‘const char*’"

EVP_DigestUpdate and "invalid conversion from ‘unsigned char*’ to ‘const char*’"

c++

hash

openssl

sha256

EVP_DigestUpdate 和 "invalid conversion from ‘unsigned char’ to ‘const char’"

EVP_DigestUpdate and "invalid conversion from ‘unsigned char’ to ‘const char’"