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A digital signature vs digital certificate — these are two very different things. You can find one on the other, but it’s important to understand the differences to get a better idea about SSL/TLS and public key infrastructure (PKI) in general.
When discussing the difference between a digital signature vs a digital certificate, you need to know what each is and how it functions. A digital signature is a unique cryptographic code that’s affixed to documents, email, software and digital certificates. It can be used to prove ownership of the certificate and the accompanying key pair.
The way this works is that there’s a mathematical relationship between the private key that made the signature and the public keys whose job it is to decrypt that signature. The public key can’t be used to decrypt everything the private key encrypts, but because of that mathematical relationship, it can verify the private key’s unique signature.
When a signed file is presented to a client, it can use the key pair associated with that signature to verify it. There’s also a hash function performed while signing that serves as a checksum. Hashing is a cryptographic function that can most easily be described as one-way encryption. When you hash something, you’re taking data that can be of any length and mapping it to a fixed length output. No two disparate inputs can create the same output, or hash value. So, when a client receives a signed file, in addition to verifying the signature, it also runs the same hash function on the file as was performed when it was signed. Then it compares the values. If they match, the file can be trusted as it’s arrived in the same state it was in when it was signed, and the signature is valid.
Part of what facilitates these signatures is the digital certificate that’s presented alongside them.
A digital certificate, such as an SSL certificate, is a cryptographic file that binds a key pair to a validated entity. It both facilitates signatures and is facilitated by signatures. Kind of like a snake eating its own tail. When a digital certificate is issued, it’s signed by the certificate authority (CA) that’s issuing it. When a client receives that certificate, it performs the aforementioned function to ensure that the certificate was signed by a trusted entity. It’s the CA that’s vouching for you here — you’re not trusted, it is. So, assuming the certificate is signed by a trusted CA, it can be trusted.
That means that when the signed certificate is presented to a client, it can verify the signature on the certificate, as well as the signature left by the certificate’s private key. It works this way in all forms of PKI. Whether it’s document signing, S/MIME, code signing or SSL, digital signatures are the building blocks of PKI.
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A digital signature is a cryptographic code that gets affixed to other files to assert identity and authenticity. A digital certificate is a cryptographic file that contains a digital signature. The certificate is what binds the signing key to the entity. And the digital signature is what lets clients know that entity is trusted, and that what’s signed is authentic.
Essentially, digital signatures and digital certificates work together, hand-in-hand.