用c ++生成SHA256

Question

I need to generate SHA256 of some data. I found this example is a very good one. Now my question is Can I generate a sha256 by using my own key.

EDIT:

First of all, sorry for wrong question. I don't mean that to change the key used to generate SHA256. I really need is that, to convert the following java code to c++

public static String calculateHMAC(String data, String key) throws Exception {
String result;
try {
    // get an hmac_sha2 key from the raw key bytes
    SecretKeySpec signingKey = new SecretKeySpec(key.getBytes(), HMAC_SHA2_ALGORITHM);

    // get an hmac_sha1 Mac instance and initialize with the signing key
    Mac sha256_HMAC = Mac.getInstance(HMAC_SHA2_ALGORITHM);
    sha256_HMAC.init(signingKey);

    // compute the hmac on input data bytes
    byte[] rawHmac = sha256_HMAC.doFinal(data.getBytes());

    // base64-encode the hmac 
    StringBuilder sb = new StringBuilder();
    char[] charArray = Base64.encode(rawHmac);
        for ( char a : charArray){
            sb.append(a);
            }
        result = sb.toString();
    }
    catch (Exception e) {
        throw new SignatureException("Failed to generate HMAC : " + e.getMessage());
    }
    return result;
}

Answer 1

SHA-256 is a member of SHA-2 cryptographic hash functions family, which usually generates 256 bits or 32 bytes HASH Code from an input message.

It's not an encryption mechanism which implies that from the HASH Code , also known as message digest or simply the digest , you can not regenerate the message.

Hence there's need no key to use SHA-256 to generate message digest .

Moreover, hash functions considered practically impossible to invert, that is, to recreate the input data from its hash value (message digest) alone. So You can't "decrypt" a HASH message/message digest to its input message, means reversing is not possible for Hashing. For example,

SHA256(plainText) -> digest

then there is NO mechanism like inverseSHA256 which can do the following,

inverseSHA256(digest) -> plainText

Answer 2

I would recommend the free Crypto++ library. Here's a sample for HMAC .