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PKI for busy people (rehn.me)
369 points by crehn on Feb 10, 2019 | hide | past | favorite | 49 comments

The part about signatures is not really correct.

It says: " To sign a message, encrypt it with the private key To verify a signature, decrypt it with the public key and make sure it matches the message"

This is only half true for one algorithm: RSA. But even there it's only true for something called "Textbook RSA", which is an insecure variant of RSA nobody should be using. It's not true for any real algorithm.

I'm really not a fan of such sloppy "I want to explain it easy" crypto introductions that are simply not correct.

(Also found it odd that he uses "Verisign" as an example for a CA. Verisign has been bought by Symantec and Symantec was distrusted by browsers recently, so it's as dead as it can be.)

The problem is that the amount you need to teach someone in order to explain all the intricacies of a complex subject like cryptography means that it is otherwise completely impenetrable without making some inaccurate simplifications.

There’s a similar problem in physics - schools still teach Newton’s laws, even though they are wrong - because they are a sufficient approximation for many uses.

The problem is of course when people assume what they’ve learnt at that early level is sufficient to work with at a level that is above their knowledge - but I’m not sure what the solution to that is.

I think there's a difference between simplifying and saying things that are wrong.

You can teach people Newton's mechanics and say "this is a good approximation with a marginal error for most everyday examples, the correct way of calculating it involves very complex things."

I feel the example I quoted regarding signatures is something that's not really a useful information anyway. That the RSA function works both ways for signatures and encryption is more of a fluke and not really someting you need to tell people when you explain the basics of public key crypto.

Wow, I had never considered the conceptual difference between signing and encrypting.

In the abstract 'encrypt with the private key' is a totally meaningless sentence for assymetric encryption. The entire point of a public key is decryption not encryption.

I do however believe that textbook RSA signing is secure in the simplest model. It is incredibly malleable but (especially when modeling the hash as a PRF) prevents forged signatures. In that sense I'd equate calling it secure to newtonian mechanics without friction and with perfect elasticity. That is, it forms a simple teaching model, and can inform an intuition on how things work. However, no-one should build things based on the model and expect it to come out correctly.

The problem is that RSA is somewhat unusual in that it does not really distinquish between public and private keys and the primitive operation is invertible by doing the same thing with the opposite key. That means that encrypt/decrypt/sign in the textbook aproach are essentialy the same primitive operation and verify is trivial extension.

For most of other asymetric algorithms the primitive operation is DH-style key agreement function and the derived encryption and signature constructs are significantly more involved and in fact there isn't that much of an symetry between them. (and also the plain asymetric encryption operation gets somewhat pointless)

If you want to keep things simple, then surely it is better to keep silent than to say things that are wrong.

Thanks for the input!

I used that wording since most people can easily visualize the back-and-forth of encryption/decryption. Your point makes sense; it's unnecessarily confusing and seems to suggest the mechanisms behind encryption and signatures are the same.

I've edited that section to better describe what happens in practice.

Not the only inaccuracy, PKCS#7 has been CMS for quite a while, he linked a 20 year old RFC instead...

Thanks for the heads-up; updated!

Technically Digicert picked up Symantec’s PKI business/customers. I’d say it’s very dead but only in the root CA sense, the business still lives on with plenty of enterprise customers paying laughable amounts for all kinds of certificates.

I presume what you're saying is that there is a whole set of missing verification additions, but if not, what are you referring to for textbook RSA being insecure?

You need to "pad" the number that will be signed so that it's the right size and shape to fit, or else bad guys can choose for you to sign a number that makes it very easy to find your private key by looking at the signature which is clearly a terrible outcome.

Remember RSA is just very simple maths, done with huge numbers. If you pick the right "huge" numbers things that look hard become very easy indeed. So we need to ensure we never pick them.

The correct way to do this, which a lot of systems haven't adopted yet, is called RSA-PSS, the Probabilistic Signature Scheme. PSS has a proof that says if you believe RSA works, and assume certain other reasonable things, this is actually safe.

Before RSA-PSS (and still today in lots of backwards compatible systems), people used PKCS#1 v1.5 which has a scheme somebody threw together to do some padding but without any great insight. There is no security proof for PKCS#1 v1.5, it's probably safe, ish, but we can't be sure.

> There is no security proof for PKCS#1 v1.5, it's probably safe

As long as you avoid the known problems, it probably is safe for signatures and the main problem now is that PSS is not included in lots of standards which thus require PKCS1_1v5. This prevents major implementation due to those standards not being updated fast enough.

As an example of slow adoption: The HSMs i'm currently using only started native support for PSS last year, about 20 years after it's introduction.

Please note that pkcs1_1v5 is never secure for encryption/decryption schemes.

Textbook RSA is not secure, as there’s no element of randomness in the encryption, rendering it susceptible to chosen ciphertext attacks. RSA+OAEP includes randomness and also protects again vulnerability such as certain plaintexts always having the same encryption regardless of key.

Does this only apply to encryption or also signatures?

It applies to both, albeit with slightly different practical implications.

Not the OP, but I presume they meant RSA can only be used to encrypt really short messages.

More typically to sign a message, RSA is used to sign a hash of the message.

No, they really meant you mustn't use textbook RSA. Textbook RSA is useful in a classroom (hence the name) to show how this idea works. But you can't (shouldn't) use it in production systems. In the real world use RSA OAEP to encrypt (or better, if you're an online system, don't use public key encryption at all, do key exchange with a Diffie-Hellman style scheme instead) and RSA PSS for signatures. Or don't use RSA, if you have an entirely brand new system, why not pick something more modern that has smaller keys and good CPU performance?

> Or don't use RSA, if you have an entirely brand new system, why not pick something more modern that has smaller keys and good CPU performance?

If you have (international) standards to adhere to, you are out of luck most of the time since they specify the exact schemes and cryptography required to adhere to the standard. Adoption of encryption/signature schemes is slow at best unfortunately.

If you do not; go wild. If you like big keys, get some 'quantum proof' public cryptography while you are at it.

> if you have an entirely brand new system, why not pick something more modern that has smaller keys and good CPU performance?

What do you suggest here, and will it work with X.509 certificates?

Any scheme that does signatures can be used with X.509. The way X.509 works a signature method just has an OID, and new OIDs can be minted by anyone in the hierarchy, so you could even invent your own if you were building an entirely new system.

For the Web PKI, the Baseline Requirements currently permit NIST P-256, P-384, or P-521 [sic] for "Elliptic curve" public key signatures, so that would let you do this for "SSL certificates" and plenty of people do but it's not compatible with older software, so if you care about that you need to have a plan B.

Depending your exact browser etcetera, if you go to google.com the certificate you're sent will be one of their P-256 certificates and your browser will verify both that this cert is genuine, and that the server can prove it knows the corresponding private key, using elliptic curve cryptography rather than RSA.

Would you know of a good intro to pki?

The author of this needs to write a few more on other topics. The succinctness and simplicity (i.e. minimum bits) is a nice change from so many documentation sites. Can Amazon get him to rewrite their documentation?

I suspect it only seems succinct because you're familiar with the details...I like it too for that reason.

However for anyone who doesn't have the requisite background it will just be abstract and impenetrable.

I somewhat agree. I feel like this is a nice glossary that I could hand my boss so he would understand what we’re talking about and stop asking what each one is, but he still wouldn’t really “get” the process for any of the examples.

If it tried to explain the full process you’re now diving into the explanations of GPG and buying SSL certs, etc and that feels like their eyes would simply glaze over.

I would like to see a tad more separation between the types of PKI (specifically GPG vs TLS type clarity).

Not quite - the links are a useful starting point rather than waving hands, saying 'just google it'.

Is there something like this specifically for certificates that goes into a little more technical detail? Every description I've ever read of certificates doesn't go much further than what's in this article ("a name and public key bound by a signature").

I've never really been able to get my head around things like when and how a certificate also contains a private key (and when that private key is "exportable"), what all the "key usages" are and where they get their strange identifiers from (and how those things are actually enforced and used), etc.

EDIT: I should have kept reading, the linked Wikipedia articles on X.509, ASN.1, DER and PEM in the next section appear to be about what I'm looking for. I suppose I've just always been confused by all the formality and the number of specifications about different things.

> when and how a certificate also contains a private key (and when that private key is "exportable")

There is a file format called PKCS #12 which lets you bundle things together. It is very common, particularly on Windows systems, to see this format used with files (often names ending .PFX) to bundle together a _certificate_ and a _private key_. This is presumably "convenient" in some sense, but since the security requirements for these two things are utterly different (a certificate is a public document that can be shown to anyone, a private key must not be seen by anybody else) it's a hazard in practice.

It's especially problematic that the resulting bundle (which you mustn't show to anybody) is often labelled a "certificate" when by far the more important thing inside it is a private key...

The same bundling idea can be done in PEM (the big blobs of Base 64 encoded stuff more often seen in Unix systems) by just concatenating a private key PEM to a certificate PEM. But at least if you look at it in a text editor you can see what somebody did, whereas in PFX it's a bit opaque without specialist tools.

Bundling these two things together most often happens in the context where somebody _else_ makes your key (and issues you a certificate). This is bad practice, not only is it patronising ("Poor dear don't understand crypto, we'll just give them the keys ready made") but it means the end user is vulnerable to copies kept nefariously or by accident. Avoid.

Certificate Authorities are prohibited from doing this for certificates in the Web PKI ("SSL certificates") but their resellers often still offer it as a "service". Say no. In fact try "Hell no".

This article has some good depth; it was posted on HN a few months ago. https://smallstep.com/blog/everything-pki.html

Wow, that is a crazy good article. I aspire to be able to write that clearly about such a complex and messy topic. And, it did its job as a promotional piece because TIL about Smallstep

@nlawalker its quite verbose and long (like most RFCs) but you have all the information you may need about X.509 certificates's format on its RFC: https://tools.ietf.org/html/rfc5280

Pasting the cert here https://lapo.it/asn1js/ can give a lot of pointers to what is in there.

This was incredibly useful, many thanks for posting. In particular I was really confused about the difference between ".der" files and ".pem" files. The concise, pithy, style makes reading very easy.

> Since binary data can be a pain to transmit, it’s often further encoded into PEM. PEM is essentially just Base64-encoded DER.

Why's binary data a pain to transmit and how does base64 encoding help with that?

There are usually far more ways to move text around than arbitrary binary bits, such as copy-pasting in terminals and web forms. It also lets you embed certificates in formats that don't normally support literal binary data, such as JSON.

To add to this already good explanation, it's my understanding that base64 and the MIME packaging scheme were invented because many early Internet protocols made a distinction between text mode and binary mode due to the majority of the protocol framing being text. You can thank them for that textual decision(?) when you're debugging SMTP with a copy of netcat, but it means that a PDF attachment needs to be encoded to travel over email.

To bring this conversation back to PKI, base64 encoding means you can embed the GPG signature of your email body in the text itself, without having to deal with attachments like S/MIME does

PEM format also adds the "----- BEGIN …" header/footer around the base64-encoded contents, which helps humans identify what the blob is.

> Root certificates are trusted and stored locally. They’re usually shipped along the OS.

They're also shipped with the browser, as is the case with Firefox. They may also be shipped with applications (some versions of Visual Studio seem to have their own certificate store).

They're also shipped with application runtimes in the case of Java.

Thanks! Added browsers as an example.

This is an excellent summary. Helpful to have such a clear and concise overview to share as a reference when working with clients with varying levels of understanding. Bravo, and thank you!

While it's great to have resources like this, I feel that this should come with a giant disclaimer at the top, reading something like:

>WARNING: This is just the tip of the iceberg. Also, ice is generally slippery. If your crypto knowledge amounts to the stuff presented here, stick to known, battle-tested implementations.

Seems a bit over-brief in some ways, hashing needs a little more. But overall, this is very useful. It gave me some confidence over slidepacks I've done as basic introductions, but rather then spin my own I'd like to point to words like this which are both curated and reviewed.

So. .. thanks!

A certificate is a name and public key bound by a signature. It identifies the owner of a public key.

A certificate identifies a name.

The signee is called a certificate authority (CA). The CA is often some big company, like VeriSign. With internal PKI, it can be any entity that nodes have been configured to trust.

A CA authenticates the identity claimed by a certificate.

A CA’s certificate can be signed by another CA, and so on. The last certificate in the chain is called a root certificate. Root certificates are trusted and stored locally. They’re usually shipped along the OS.

There can be a chain of trust whereby one CA authenticates the next.

Its a tricky subject and I don't think it is really possible to get too far beyond "give me £100 for this SSL certificate" ... "because" ... sigh [fill in your own SSL related conversations with your PHB stupidity here]

Interesting.. Wait till you get validating cert chain without the carts issuing cert. following AIA fun stuff the things cert issuers can return

PKI stands for "Public Key Infrastructure" in the article, I thought it would be about performance key indicators.

You’re probably thinking of KPI, Key Performance Indicators.

Right, thank you for heads up.

I’ve always heard Key Performance Indicators (KPI)

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