ASP.Net MVC cookies - tamper resistant?
Simply set the forms authentication protection method to Encryption and validation.
This will protect against bitflipping attacks as the signature will not match once a bit is flipped. Validation means the message is signed.
How does validation work?
The validation algorithm can be set in web.config in the validationAlgorithm attribute of the machineKey element.
By default it uses HMACSHA256 which is a SHA-256 hash applied using the HMAC construct.
The hash signs the cookie - if the cookie value changes, the hash will no longer match. As an end user does not know the secret, they cannot change the cookie value.
e.g. Try on here to generate a SHA-256 HMAC for message foo with the secret secret.
逻辑类似于HMAC加密，通过明文和密钥进行加密，会得到一个摘要，其他人不知道密钥，是无法进行数据篡改的。否则无法通过服务端的校验
This should give you: 773ba44693c7553d6ee20f61ea5d2757a9a4f4a44d2841ae4e95b52e4cd62db4
Note that if you change the foo to something else, the hash will be different. This is how a cookie can be protected from tampering. This cookie would be set as follows.
Set-Cookie: id=foo&hash=773ba44693c7553d6ee20f61ea5d2757a9a4f4a44d2841ae4e95b52e4cd62db4
Notice that the value is in the clear, but the hash signature prevents any tampering. This is validation only.
With the encryption and validation option foo would be encrypted first and the signature would prevent any bit flipping. This enables values to be stored in private from the end user.
If you're implementing this outside of Forms Authentication, remember though to store some type of expiry date in the cookie and include this in the signature. This date can be checked server side. Forms authentication does this by default.
Otherwise, a user could make note of a valid cookie value. Say they are given administrator access for one day and the following cookie is issued:
username=admin&hash=71b3ba92493e92ce3c60042988e9de428f44b35a6be61c8da99fa43f950d3056
The next day when the administrator access is revoked, all the user would need to do is use a cookie editor to set their cookie to the above value and they would have administrator access to the system again.
To fix this you would issue a cookie like so:
username=admin&expiry=20150508100000&hash=e38a3a003b30ceb9060165d19bb8d2a2bca6c7c531a37e888448ca417166db3a
This has the expiry date in the cookie, which is signed in the hash. Any attempt to modify the expiry date will result in a HMAC mismatch, and the user will not have access. This will prevent any tampering with the cookie expiry date or any recreation client-side.
What are our other don't-mess-with-my-cookie options if not using forms authentication?
Another method is to store a completely random, cryptographically secure generated string and set that as the cookie value. On your server, store it hashed with SHA-2 (no need for salt) and lookup this value to retrieve details about the user's session. Of course this has some more overhead. The advantage is that sessions can be killed server side by deleting the entry.
This can be done with Forms Authentication too, however you would need to implement a custom provider.