why do we trust SSL certificates?
First, some background on strong public/private key cryptography, which SSL is based on:
A key has two parts, the private part and the public part. The public key can be used to encrypt material that requires the private key to decrypt. This allows the use of open communication channels to communicate securely.
One important aspect of public/private key cryptography is that the private key can be used to digitally sign a message which can be verified using the public key. This gives the receiver of a message the ability to verify concretely that the message they received was sent by the sender (the holder of the key).
The key to SSL certificates is that encryption keys themselves can be digitally signed.
A "certificate" is composed of a private/public key pair as well as digitally signed data. When someone buys an SSL certificate they generate a private/public key and submit the public key to a Certification Authority (CA) to be signed. The CA performs an appropriate level of due diligence on the buyer of the SSL certificate and signs the certificate with their private key. The SSL certificate will be bound to a particular website or set of websites and is essentially the CA indicating that they trust the owner of the private key of the certificate to be the proper owner of those websites.
The root certificates (public keys and other meta-data) for trusted CAs are included by default in major shipping browsers and operating systems (in windows, type "certmgr.msc" into a run prompt to see the certificate manager). When you connect to a web server using SSL the server will send you its SSL certificate including the public key and other meta data, all of which is signed by the CA. Your browser is able to verify the validity of the certificate, through the signature and the preloaded root certificates. This creates a chain of trust between the CA and the web server you are connecting to.
Certificates are cryptographically signed by something called a Certificate Authority(CA), and each browser has a list of CAs it implicitly trusts. These CAs are entities that have a set of cryptographic keys that can be used to sign any certificate, often for a fee. Any certificate signed by a CA in the trusted list will give a lock on a browser, because it's proven to be "trusted" and belongs to that domain.
You can self-sign a certificate, but the browser will warn you that the signer is not trusted, either by showing a big error box before allowing you in, or showing a broken lock icon.
In addition, even a trusted certificate will give an error if it's used for the wrong domain, or is modified to include another domain. This is ensured because the certificate includes the domains it is allowed to be used for, and it also has a cryptographic checksum/fingerprint that ensures its integrity.
This is not 100% safe at the moment, as there is the possibility to fake CA certificates that use MD5, see this link: http://www.phreedom.org/research/rogue-ca/. Though it has to be noted that this is pretty hard, as they exploited a weakness in an already existing CA, which may or may not have been closed by now.
In essence, we trust the certificates as much as we trust that our browser providers know how to select "proper" CAs. Those CAs are only trusted on virtue of their reputation, as a single misstep theoretically would be a very heavy blow on their trustworthiness if detected.
The whole CA business is amazing. I've purchased a couple of certificates from rapidssl.com, and all the "proof" they required was:
- I could receive mail to the domain.
- I could answer my phone.
That was it. Keep in mind, when trusting the little locks in the browser.