Rust based AWS Cognito secret_hash function for rusoto_cogntio_idp to enable AdminInitiateAuthRequest

AdminInitiateAuthRequest.rs

use sha2::Sha256;
use hmac::{Hmac, Mac, NewMac};
use openssl::base64;

let user = "username_here"

// Given that your .env is configured and you have initialized CognitoIdentityProviderClient
// and defined your "Auth Flow" to "ADMIN_NO_SRP_AUTH" which is currently not supported in rusoto_cognito_rdp

type HmacSha256 = Hmac<Sha256>;
fn secret_hash(user: &str) -> String {
    let mut mac =
        HmacSha256::new_varkey(String::from(&env::var("COGNITO_CLIENTSECRET").unwrap()).as_bytes())
            .expect("HMAC can take key of any size");
    let mut message_string = user.to_owned();
    message_string.push_str(&env::var("COGNITO_CLIENTID").unwrap());
    mac.update(&message_string.as_bytes());
    let res = mac.finalize();
    let code_bytes = res.into_bytes();
    base64::encode_block(&code_bytes)
}