AWS身份验证中，计算HMAC SHA 256用于生成签名。签名用于验证请求是否来自已验证的AWS用户，并用于授权请求访问资源。以下是计算HMAC SHA 256的示例代码：

import hashlib
import hmac
import base64

def sign(key, msg):
    return hmac.new(key, msg.encode('utf-8'), hashlib.sha256).digest()

def get_signature_key(key, datestamp, region_name, service_name):
    kDate = sign(('AWS4' + key).encode('utf-8'), datestamp)
    kRegion = sign(kDate, region_name)
    kService = sign(kRegion, service_name)
    kSigning = sign(kService, 'aws4_request')
    return kSigning

def get_aws_signature(secret_key, message, region_name, service_name):
    datestamp = '20201224' # 格式为yyyymmdd
    kSigning = get_signature_key(secret_key, datestamp, region_name, service_name)
    signature = hmac.new(kSigning, message.encode('utf-8'), hashlib.sha256).hexdigest()
    return signature

上述代码使用Python编写，其中secret_key为AWS用户的秘密访问密钥，message为要签名的消息，region_name和service_name为AWS服务的名称和区域。该方法将返回一个签名字符串，用于身份验证和授权。