How does Public Key Cryptography work with FIDO?

[Callum Brankin]

From my understanding is that, after registration, the private key is stored on the users device and the public key is stored on the database server (normally it's the other way round). So when a user comes to do the first part of the UAF authentication, the app will make a call to the server to 'grab' the public key to "match" it with the private key on the users device, if the keys "match", it authenticates the users device. Is my understanding on this part of FIDO authentication correct? 

[Satish KARRY]

Callum,

Here is a simple video that explains this concept. Let me know if this is helpful.

 

https://www.youtube.com/watch?v=BXl6npgDk8o

 

Thanks,

Satish

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Satish KARRY

650 804 3824   | ska...@noknok.com |  Palo Alto, CA

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[Ron Garret]

On Oct 21, 2016, at 8:10 AM, Callum Brankin <cal...@pixelpin.co.uk> wrote:

> From my understanding is that, after registration, the private key is stored on the users device and the public key is stored on the database server

That’s right.

> (normally it's the other way round).

No, that’s not right. Private keys should *never* be stored on a server. You may be confusing a private key with a password hash. Not the same thing at all.

> So when a user comes to do the first part of the UAF authentication, the app will make a call to the server to 'grab' the public key to "match" it with the private key on the users device, if the keys “match"

No. A U2F token produces a digital signature of a random string generated by the server (the “challenge string”). The server verifies this signature against the public key. It’s not checking to see if the keys “match”, it’s checking to see if the signature is valid. It needs to know which public key to use to check the validity of the signature, but it does this by associating public keys with user names. The server only queries the token for its public key during registration, not during authentication.

rg

[Arshad Noor]

One of the best introductions to Public Key Cryptography was written by
someone at Netscape almost two decades ago. We've permanently linked to
this article on our own website and use it to train every employee we
hire. You can find the article at:

https://developer.mozilla.org/en-US/docs/Archive/Security/Introduction_to_Public-Key_Cryptography

Apply the concepts you read in this paper - except substitute Public Key
wherever you see "digital certificate" - and you'll understand.
However, FIDO neither uses Certification Authorities nor any artifact
associated with Public Key Infrastructure (PKI) - save for the
Attestation Certificate - in its protocols.

Arshad Noor
StrongAuth, Inc.

[Tiểu Hảo Lương]

Hi all,

Let me ask some question. If I have a private key, can I generate a public key to combine with private key and make a key pair?

Thanks!

[Arshad Noor]

Depends on the programming language and cryptographic library you are
using.

Speaking for the Java programming language, if you already have a
Private Key, you generally already have the Public Key too, since the
Java Cryptographic Extensions (JCE) API generates a KeyPair in a single
call to the library. From the KeyPair, you can choose to extract the
PublicKey and/or the PrivateKey (if your cryptographic module supports
extracting the PrivateKey; some will give you a "handle", but you do
not actually get the PrivateKey).

I do recall that BouncyCastle provides some API methods that allow you
to derive the PublicKey from the PrivateKey (but it has been sometime
since I last recall using that capability so I could be foggy on it).

In any case, with Object Oriented Programming languages, you generate
KeyPairs in one call and can extract either key from that as needed

Arshad Noor
StrongAuth, Inc.

[Fred Le Tamanoir]

Arshad answered with tools consideration in mind, but to be clear :
FIDO U2F specifications make it mandatory to use Elliptic curve cryptography (ECC) with this specific curve NIST P-256 (also called SECG secp256r1 or ANSI prime256v1).
If you know an ECC private key and the used curve name, you can generate the public key from the private key.
If you are used to work with RSA private keys, you can't do that with RSA, meaning you always have to store the full RSA key pair. With ECC, you don't have to have a full stored key pair if you want. It is still asymmetric cryptography but you can generate the public key from the private one.

[Callum Brankin]

Great video (link is dead now). How are the public and private keys created exactly? And in the FIDO UAF documentations,are they referred to as Attestation keys?

[Ron Garret]

The answer to your question as stated is: yes, of course you can.

But a more constructive answer is: why are you asking?  Because the only circumstances I can imagine under which you would need to do this are ones in which you would have already known not only that you can do it, but exactly how to do it.

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