How To Detect Hidden Field Tampering?

On a form of my web app, I've got a hidden field that I need to protect from tampering for security reasons. I'm trying to come up with a solution whereby I can detect if the valu

Solution 1:

It would be better to store the hash on the server-side. It is conceivable that the attacker can change the value and generate his/her own SHA-1 hash and add the random string (they can easily figure this out from accessing the page repeatedly). If the hash is on the server-side (maybe in some sort of cache), you can recalculate the hash and check it to make sure that the value wasn't tampered with in any way.

EDIT

I read the question wrong regarding the random string (constant salt). But I guess the original point still stands. The attacker can build up a list of hash values that correspond to the hidden value.

Solution 2:

Digital Signature

Its probably overkill, but this sounds no different than when you digitally sign an outgoing email so the recipient can verify its origin and contents are authentic. The tamper-sensitive field's signature can be released into the wild with your tamper-sensitive field with little fear of undetectable tampering, as long as you protect the private key and verify the data and the signature with the public key on return.

This scheme even has the nifty property that you can limit "signing" to very protected set of servers/processes with access to the private key, but use a larger set of servers/processes provided with the public key to process form submissions.

If you have a really sensitive "do-not-tamper" field and can't maintain the hash signature of it on the server, then this is the method I would consider.

Although I suspect most are familiar with digital signing, here's some Wikipedia for any of the uninitiated:

Public Key Cryptography - Security

... Another type of application in public-key cryptography is that of digital signature schemes. Digital signature schemes can be used for sender authentication and non-repudiation. In such a scheme a user who wants to send a message computes a digital signature of this message and then sends this digital signature together with the message to the intended receiver. Digital signature schemes have the property that signatures can only be computed with the knowledge of a private key. To verify that a message has been signed by a user and has not been modified the receiver only needs to know the corresponding public key. In some cases (e.g. RSA) there exist digital signature schemes with many similarities to encryption schemes. In other cases (e.g. DSA) the algorithm does not resemble any encryption scheme. ...

Solution 3:

If you can't handle the session on the server, consider encrypting the data with your private key and generating an HMAC for it, send the results as the hidden field(s). You can then verify that what is returned matches what was sent because, since no-one else knows your private key, no-one else can generate the valid information. But it would be much better to handle the 'must not be changed' data on the server side.

You have to recognize that anyone sufficiently determined can send an HTTP request to you (your form) that contains the information they want, which may or may not bear any relation to what you last sent them.

Solution 4:

If you can't/won't store the hash server side, you need to be able to re-generate it server-side in order to verify it.

For what it's worth, you should also salt your hashes. This might be what you meant when you said:

concatenated with a constant long random string (i.e. the same string will be used every time)

Know that if that value is not different per user/login/sesison, it's not actually a salt value.

Solution 5:

As long as you guard the "constant long random string" with your life then your method is relatively strong.

To go further, you can generate one time / unique "constant long random string"'s.