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I have misconceptions regarding CA (Certificate Authority) certificates. No matter how many things I read, it is still not fully clear.

Let's take an example, Bob accesses a website. In order for trusted and encrypted communication to happen between Bob wand the website, the website first issues Bob with a digital certificate, which contains a public key and other information.

Bob will then use this public key to encrypt the data in which he will send to the website, and the website will use the corresponding private key to decrypt it. (Just considering one-way communication here)

A man in the middle could pretend to be the website and supply Bob with what he believes is a valid digital certificate and then things go horribly wrong.

If the website uses a CA for this problem to validate or generate its own certificate, which one of my statements is correct, or are both partly correct? :

1 ) Bob simply compares the digital certificate received from the website with the one from the CA, so no decryption is performed, just comparison? In this case, Is every single CA certificate in the world stored on Bob's local computer to compare with? How does this happen.

2) Bob just has a special CA certificate which is used to decrypt certificates from sites. The CA has previously encrypted the digital certificate of the website which Bob wants to use with the CA private key. Bob then gets the certificate from the site, decrypts it with the CA's public key from the CA's certificate. If the certificate can't be decrypted, it's obvious that the CA did not encrypt it and so invalid.

Thanks in advance.

Engineer999
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2 Answers2

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You do have a misunderstanding. In fact, both your scenarios are wrong.

First, a digital certificate is issued by a Certificate Authority (CA) and contains the public key for digital signing and specifies the identity of the signer. The certificate is used to confirm that the public key belongs to the signer, where the CA acts as the guarantor.

The certificate is verified against the CA via Internet and not locally, as regarding the validity of the certificate, retrieving from the CA the public key in the process.

The public key is used to verify, using a hash algorithm, that the signed object was truly the one signed and was not changed in any way.

As a picture is worth a thousand words:

enter image description here

Source : Understanding digital signatures.

harrymc
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First of all, there are two steps to verifying a CA-issued certificate:

  1. Is this specific CA trusted to issue certificates at all?
  2. Has this certificate really been issued by the CA that it claims to be?

Both of your descriptions are nearly right for one half of the whole process but completely ignore the other half. Combine the two, and you're kiiiinda on the right track.

But the second point: Encryption is not used at all in this situation, and certificates (containing public keys) may be used to encrypt data, but not to decrypt it. What's performed is signature verification.

(I suspect that some of your confusion is caused by having read somewhere that e.g. "in RSA, encryption and signing are the same". Forget that. Although mostly correct in the mathematical sense, it is incredibly misleading in practice, as the intention is completely opposite between the two. So when you're reading a document that talks about signing something, do not assume it means the same as encrypting it.)

"Has this website's certificate really been issued by the CA that it claims to be?"

Variant #2 is almost correct about this one. There are at least two certificates involved – one representing the website (the "server certificate"), and one representing the CA itself (called the "CA certificate", or the root cert).

The website's own certificate is not encrypted with the CA's private key; it is signed with the CA's private key. If the signature cannot be verified, it is obvious that the CA didn't really sign what's claimed.

(In practice, the chain is usually a little longer, consisting of at least three certificates – but the mechanism is still the same; each one signs the next in chain.)

But where does Bob get this "special CA certificate" in the first place? See below.

"Is this CA trusted to issue certificates in general?"

The received CA certificate is compared against a list of "trusted roots" stored within the local computer.

Now that's similar to what you describe in variant #1, except the computer doesn't store all certificates issued by CAs – instead, it only stores the certificates belonging to CAs themselves, i.e. the "special CA certificates" in your variant #1.

There are roughly 50–100 "root certificates" installed on most systems – not quite "every single CA in the world", but it covers many larger and smaller companies. (It costs quite a bit to be included in the list.)

grawity
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