[openssl-users] beginner needs advice on data signature/verification

[Michael Wojcik]

Response inline below, prefixed with "MW". (Unfortunately Outlook is incapable of replying to HTML messages properly, so you'll have to excuse the formatting.)


Michael Wojcik
Technology Specialist, Micro Focus


From: openssl-users [mailto:openssl-users-bounces at openssl.org] On Behalf Of Marco Warga
Sent: Saturday, June 20, 2015 04:48
To: openssl-users at openssl.org
Subject: [openssl-users] beginner needs advice on data signature/verification

Hi,

I hope some of you could give me advice on my project using openssl.

MW: Why are you using OpenSSL for this application? You want to create a file on a trusted system, pass it through an untrusted intermediary, and process it on another trusted system. Why not simply use an existing mechanism like secure email? (GPG is the obvious choice, unless there are licensing issues.) If you are determined to create your own protocol from primitives, then really all you appear to need here is an HMAC. Don't involve the horrific mess that is X.509 PKI unless it actually provides some benefit.

Lets say I have a server/service on a machine processing a file a corresponding client sends. That file is usually created by me on a clean third machine. The server side is assumed to be uncompromised (no hacker). The client side may be compromised. Now I need to make sure that the service only accepts those files that are created by me. I believe that is a very common requirement and has been done alot of times - I just can't find tutorials on how to implement it. Know any ?

MW: No, but that's probably because what you've described isn't "a very common requirement". It's too vague. We don't know what problem you're actually trying to solve. It may be that you just need to send a file with a verifier, which - as I noted above - is commonly done, generally using something like GPG or (for roll-your-own protocols where both ends are controlled by the same party) an HMAC.


Lets assume I have an x509 cert together with its private key signed by a ca owned by me. The trusted ca cert will be present on the server side. This is what I plan to do:

1.) Create the data files/blobs and sign them using the priv key of the cert. Distribute the cert and the signature along with (or inside) the data file.
2.) Have the client send that data file to the server (cert/sig first)
3.) Service receives the cert, builds a cert store with the local ca cert in it and verifies the client's cert with X509_verify_cert()
4.) if cert verifies ok, service compares the signature against the one calculated from the incoming data using the public key that came inside the cert just verified


Would this be the right approach considering that anything the client sends may be forged (cert, sig, data...) ?

MW: It's safe from malicious behavior by the client, under a threat model where an attacker is not able to forge client certificates or client signatures. In other words, it's safe as long as the private keys are neither leaked nor forced.

Or would it be safer to have the cert used for signing stored on the server side and not send with the data (instead just its subject protected by the signature) ?

MW: Irrelevant to the security of the scheme. Simpler from a development and operations standpoint. But using something like PGP/GPG or S/MIME would be simpler yet. There are any number of examples online for signing a file and verifying its signature.


Thanks alot,
Marco
X509_verify_cert
X509_verify_cert


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