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if a self-signed certificate is trusted by the client's certificate store, then it should be fine to be used, since the client trusts the issuer of the certificate as it does any other trusted CA (certificate issuer).

However, I believe that the problem with self-signed certificates is that you have a distributed certificate issuer, meaning that "the CA" is not secured like a CA should be, and the risk of it being compromised is higher than that of a sensitively taken care of CA.

Here's where I get confused. The last paragraph is wrong. Since the certificate is self-signed and the public key of the key pair is trusted, then the certificate issuer is not trusted; just the key pair is trusted.

Is this correct?

Is it safe to assume that self-signed certificates are trustworthy 100% of the time?

Thanks,

Matt

P.S. This is in relation to the issuance of a code signing for Local Update Publisher to be used with a WSUS install. Not a web site.

P.P.S. Did I not just explain how a "root CA" is trusted/trustworthy?

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2 Answers 2

up vote 1 down vote accepted

A Certificate Authority is just one component of the Public Key Infrastructure. The PKI is "a set of hardware, software, people, policies, and procedures needed to create, manage, distribute, use, store, and revoke digital certificates." A CA alone isn't very helpful...

You can use self-singed certificates securely by using Certificate Pinning. This is where you know what certificate to expect, so you don't need a PKI to verify its authenticity.

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Thanks very much. Addition link from the article: thoughtcrime.org/blog/… –  mbrownnyc Dec 29 '12 at 19:17

A self signed certificate is completely fine if you are willing to explicitly trust it. The reason a certificate is signed is that an entity more trusted than the certificate holder is adding authority to the data with a digital signature. As you say in your question, a CA is only as good as the ways in which it is secured - an untrustworthy CA creates untrustworthy certificates.

For the most part, a certificate is required when two parts of a system must communicate. When a computer is simply processing information with not transaction between components, there's no need for any certificate at all. Certificates come into play when you need to: - store data and retrieve it later - send data or instructions from one point to another

In all cases, there's a risk that an unknown, untrusted party could exploit either the data or the channel. Thus the certificate provides identification, encryption, or integrity verification (or all of the above). It's the key pair itself that provides integrity and encryption - the signing of a certificate is largely concerned with the identification/non-repudiation security concern.

Self signing creates two basic concerns:

  • Who are you? How does the system know that this self signed certificate is OK, but another one isn't.
  • Are you still OK? When a self signed certificate is explicitly trusted, how do I know if the private key has been compromised.

Both concerns are usually handled by explicitly trusting the certificate - that tells the system that the certificate is the certificate of the permitted party, and if the private key should ever be compromised, the configuration setting can be changed, removing the trust.

That's manual in most systems, and it will totally work for any cases where you can trust that the administrator will keep on top of any compromises and be able to reconfigure the system in a timely manner. In practice, for a handful of simple systems, that's workable.

When the systems become more certificate dependent or when there are multiple points that rely on the connection to a self-signed certificate, the dependency on human error becomes too great, and most security designers will look for a way to do an automated certificate status check and provide a common entity (a CA) for the creation of all certificates. Then all dependant systems will explicitly trust the CA.

Either way will work. In either case, the security will depend on the security of the private key - loose the private and you've lost control of the identity of the system you're securing. After that, it's mostly a matter of what type of risk you are willing to accept.

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