HMAC signer for http req

HttpSignatureUtils.java

/*Request
        1. Date Format RFC1123 (Sun, 06 Nov 1994 08:49:37 UTC)
        2. Request Line (POST /v1/account/check HTTP/1.1)
        Signature
        signing_string = ‘date: Sun, 06 Nov 1994 08:49:37 UTC\nPOST /v1/account/check HTTP/1.1’
        digest=HMAC-SHA256(signing_string)
        signature=BASE64(digest)
        Body
        {
        “bank_id”: ”45”,
        “account_number”: “234398045983”
        }
        value=SHA-256(body)
        Digest=BASE64(value)
        Final Header
        Date: Sun, 06 Nov 1994 08:49:37 UTC
        Authorization: "hmac username=\"myun\", algorithm=\"hmac-sha256\", headers=\"date request-line\", signature=\"%s\"", signature
        Digest: SHA-256=Digest

 */

/**
 * example usage:
 * try {
 * HttpSignatureUtils httpSignatureUtils = new HttpSignatureUtils();
 * request =  httpSignatureUtils.mergeToRequest(request, requestObject.toString());
 *
 * }catch (Exception e){
 * e.printStackTrace();
 * }
 */
public class HttpSignatureUtils {
    final static String SECRET_HMAC = BuildConfig.SECRET_HMAC;
    final static String DATE_FORMAT = "EEE, dd MMM yyyy HH:mm:ss z";
    final static String AUTH_FORMAT = "\"hmac username=\"myun\", algorithm=\"hmac-sha256\", headers=\"date request-line\", signature=\"%s\"\"";
    final static String DIGEST_FORMAT = "SHA-256=%s";

    Date mDate = new Date();

    public Request mergeToRequest(Request request, String json) throws Exception {
        String auth = String.format(AUTH_FORMAT, getSignatureEncodedBase64(request));
        String date = getFormattedDateFormat();
        String digest = String.format(DIGEST_FORMAT, getEncodedBase64(json));

        return request.newBuilder()
                .addHeader("Date", date)
                .addHeader("Digest", digest)
                .addHeader("Authorization", auth).build();
    }

    /**
     * expected format: Sun, 06 Nov 1994 08:49:37 UTC
     *
     * @return
     */
    public String getFormattedDateFormat() {
        SimpleDateFormat dateFormat = new SimpleDateFormat(DATE_FORMAT, Locale.US);
        dateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
        return dateFormat.format(mDate.getTime());
    }

    /**
     * Base64(HMAC-sha256)
     * // expected: ‘date: Sun, 06 Nov 1994 08:49:37 UTC\nPOST /v1/account/check HTTP/1.1’
     *
     * @return
     */
    public String getSignature(@NonNull String httpMethod, @NonNull String httpUrl) {
        String signatureFormated = String.format("date: %s\n%s %s HTTP/1.1",
                getFormattedDateFormat(),
                httpMethod.toUpperCase(),
                httpUrl);
        return signatureFormated;
    }

    public String getSignatureEncodedBase64(Request httpRequest) throws Exception {
        return getSignatureEncodedBase64(httpRequest.method(), httpRequest.url().encodedPath());
    }

    public String getSignatureEncodedBase64(@NonNull String httpMethod, @NonNull String httpUrl)
            throws Exception {
        String hash = Base64.encodeToString(
                getHMAC_SHA256Digest(SECRET_HMAC, getSignature(httpMethod, httpUrl)), Base64.NO_WRAP);
        return hash;
    }


    public byte[] getHMAC_SHA256Digest(String secret, String data)
            throws Exception {
        String algoritm = "HmacSHA256";
        Mac sha256_HMAC = Mac.getInstance(algoritm);
        SecretKeySpec secretKeySpec = new SecretKeySpec(secret.getBytes(), algoritm);
        sha256_HMAC.init(secretKeySpec);
        return sha256_HMAC.doFinal(data.getBytes());
    }

    public String getEncodedBase64(String data) {
        byte[] sha256 = DigestUtils.getSha256Digest().digest(data.getBytes());
        String hash = Base64.encodeToString(sha256, Base64.NO_WRAP);
        return hash;
    }

}