I'm new to asymmetric cryptography (public-private keys) and has a fundamental confusion:
I understand that once a person has broadcast his public key to the network, the following communications become safe using the public-private key paradigm.
However, how do you make sure the public key received is valid at the first place? For example, if Bob wants to establish a secure communication with Alice, with Eve eavesdropping in the middle, how do you make sure that Bob receives the real public key from Alice, instead of the fake public key forged by Eve?
Thanks!
Cheers,
M.
Public Key Infrastructure (PKI).
You have a third-party trusted authority that issues certificates to different people/companies. You don't only trust the people, but also the Certification Authority (CA).
The Certification Authority must ensure and validate that the people/company that is buying a certificate is in fact who it claims to be.
The verification is done by signing a the certificate using a private key (from this CA), then both peers in the connection will check that the certificate was signed by a trusted CA and validate the connection.
In you example, Bob will sign the certificate in a CA that Alice trusts.
If Eve tries to sign a certificate in this CA, claiming to be Bob, this will be rejected, because the CA will validate its authenticity. Try to buy a client certificate here to see: https://www.verisign.com/
Another example is your browser, it only shows that a trusted connection is done with StackOverflow, because it trust on Let´s Encrypt, which issued the StackOverflow.com certificate.
Every browser has some default CAs that are already trusted.
The public key would be signed by another authority. Check if this authority is genuine and renowned. If it is unknown then try to further check its signing authority and see if its a renowned one.
The concept of trusting this public key is based on basing your inherent trust in authority which signs this certificate. Signatures are mathematically verifiable so they cannot be forged unless there's an issue with implementation or mathematical algorithm itself.
So in a nutshell, try to see the signing authority of the certificate and if it is something you trust.
The certificate authorities(CA) or web of trust(WOT) model shall help you in ensuring the authenticity of public key.
The Certificate authority is a centralized entity that issues digital certificate that certifies the ownership of a public key by the named subject of the certificate. It acts as a trusted 3rd party, trusted both by the subject (owner) of the certificate and by the party relying upon the certificate.
The web of trust is a decentralized trust model to establish the authenticity of the binding between a public key and its owner . In a Web of Trust, everybody is a kind of CA. Every user signs certificates for whoever he wants. It relies on value of trust or trust relationships between regular users.
Web Of Trust model operates such that if we verify that A's certificate is actually for A and we verify that B's certificate is from B, A and B can both verify that C is in fact C. Now, when C want's to interact with us, he can provide his certificate which is trusted by both A and B. Since we trust A and B , we can conclude that it is highly probable that C is actually C. This works since we expect that A and B weren't both working together and C had to have convinced them both that he was ok.
Related
I am searching the algorithm about how SSL validation process is performed, but almost everywhere, they explain the certificate validation step as "certificate is checked by client" or something like that, but I wonder what is the scenario behind this.
What I know is:
When the client receives a copy of the certificate that belongs to which website/server you wanna attempt to handshake, there are some indicators that shows the public information of webserver (I think this is for matching the entries in your cached certificate entries, which your browser has installed.)
Once the client matchs a cached-certificate with the webserver's one, it starts validating it. It uses the public key of cached-certificate to decrypt the signature of webserver's one.(? [Not sure this because public keys are used to "encrypt" the data, not decrypt. Also this step may be totally wrong.])
Once it decrypts, it compares the signature between cached one and webserver's one. If it is same, certificate is valid.
I also heard about chains. I really wonder how a chain is known, and how to determine if the webserver's certificate just belongs to a chain.(?)
How SSL certificates are checked by client? I need the answer as step by step and clarified. Thanks :)
Edit:
I think the public key here is used for "decrypting" instead of "encrypting". So a public key is not responsible for encrypting everytime, it can also decrypt and don't encrypt some data. The magic here is that since the public key decrypts here, if you want to fake the certificate, you should have that CA's private key to apply the changes as encrypted (because only private key can encrypt the data). But now, another question comes... What if we decrypt it using webserver's public key, then change the entries in the signature, then encrypt it again using our own private key (we generate it manually, it doesn't belong to server.), which actually make us behave like a CA; and finally overwrite the certificate to hold our own public key which is able to decrypt the data encrypted with our own private key?
There is differences between Encryption and Signature.
Public Key is used by the client to encrypt data that only the server can decrypt with the Private Key of the server.
Public Key is used by the client to verify the signature of the data send by the Server that can only be signed by the Private Key of the server.
When the client wants to access a server, the server send you a certificate containing a public key. The client usually the web browser will check in his integrated CA's list certificates if it contains it.
If it contains it, the client continue and get the CA(Certificate authority)'s that authorized this certificate.
If it not found but the verification of the signature pass, the client will get a warning saying that the certificate is probably self signed and can be malicious.
If the client can't verify the signature, it means the certificate is not valid.
for the chain of trust you can check wikipedia :
https://en.wikipedia.org/wiki/Chain_of_trust
you need to be in this chain of trust to be register in the web broswer integrated "database" of CA's.
Hope it answers your question, best regards,
M
Put it simple:
Private Key -> Decrypt and Sign
Public Key -> Encrypt and Verify
Certification Authority
Is the authority that signs and guarantees the authenticity of the certification. If you trust the CA, then you trust also its certificates.
It's the same for friends: if you have a friend that you trust and this tells you "I have a friend that I trust", then you also trust the friend of your friend.
Chain of Certificate Authority.
You can have multiple intermediate CA, for example, you can have
Root Certification Authority
Intermediate Certification Authority for WebServer signed by Root Certification Authority
Web Server Certificate signed by Intermediate Certification Authority for WebServer
Intermediate Certification Authority for signing code, signed by Root Certification Authority
Etc
Why?
Because in case one of the intermediate authorities is compromised you can revoke only its child certificates.
At the enterprise level, each CA has a different level of security, for example, the Root Certification Authority can be stored inside a safe and used only when there are two o more admins.
For the intermediate, instead, maybe only one Admin can manage it.
References
How SSL Works
How HTTPS Works
Wht digital certificates
Newbie question: some vendors propose solution like generating dynamic certificates to allow user who haven't classic certificate to sign documents. But why not just generate private/public keys alone instead of bothering with certificate format ?
The purpose of the (public key) certificate is to bind the public key to the identity of its subject (i.e. the owner/entity associated with the key pair), and possibly various attributes telling you what the certificate may be used for. (You may be interested in this question on Security.SE.)
You always sign with the private key (not the public key or the certificate), but the public key or certificate are often attached with the signed document.
If you have an explicit list of public keys you know and can link independently to a user, you don't need a certificate.
The certificate allows third parties (who have signed the certificate) to assert the binding between the identifier and the public key. Even if you don't know that identity in advance, you can link the signature to the signer's identity as long as you trust the entity that signed the certificate.
Dynamically generated certificates may not be very useful in this case, unless you trust the party that generates the certificate dynamically (I'm not sure if you meant the tool itself or perhaps a website that you would also know).
Often, X.509 certificates will be used just to attach to that signature, because the tooling requires it, whereas you may be able to match the public key against an identity you know directly in the tool with which you verify the signature. Sometimes, it's also just done in anticipation of a case where it will be useful one day.
For example, if you publish your own artifacts to the central Maven repository, you will be required to sign it with your PGP certificate (often only referred to as the PGP public key). Yet, no verification of the certificate is made at all during the process (PGP certificate with only its self-signed signature is good enough). This makes this process relatively pointless in this case, but makes it possible to be stricter in which artifacts you want to use, if you're able to verify those certificates later on.
It's the same but you need a third party to consent that private key belongs to whom ever you think it belongs to.
Signing proves first of all authorship (or approval) of the document by some person. And the key alone won't prove anything. This is what the certificate is needed for - some certificate authority signs the certificate of the user and certifies that the keypair belongs to the person (or legal entity) to which the certificate is issued. The reader of the document can ensure that the signature is valid not by just computing the signature itself, but also by validating the certificate and seeing the name of the certificate owner.
I don't quite understand what vendors can issue certificates dynamically - issuing certificate in such way that they are not self-signed (and self-signed certificates make little sense in context of document signing) requires that the private key, used for signing the certificate, should be embedded into software of those vendors, and as such it's also prone to misuse.
I read an article about digital signature (link) and have question as follows.
Let's say Alice wants to send a message to Bob. She need to let Bob know the message is from her. So she encrypted the hashed message with her private key into a certificate. Then Bob can decrypted the message with public key when receiving it. Bob can know it is from Alice if the hash code of the message matches the hash code which is decrypted from certificate. Here we have the assumption that Bob already knows the public key. What if the transmission of public key is already attacked? Bob might use the wrong public key to decrypt the wrong message and get that the message if from Alice. Is there any protocal or policy to avoid the attack against public key? And shall we？
Yes, the authenticity of public keys is a key component of applied cryptography. I can issue a public key that says "I am the website of your bank, trust me", but you shouldn't really trust it. Different schemes have been developed to establish authentication of public keys. One approach is the web of trust model in PGP and GnuPG, others are PKI and Kerberos. One of the main difference between these approaches is where you place your trust. I provide a simplified description only, you have to read about them to learn about the exact details (you wouldn't base your security on an extremely short summary, would you?).
In the web of trust there are some people who you trust, and you (ideally) verified their public keys personally. You can trust other public keys if they have been signed by several people bearing your initial trust. Using these trusted individuals you can check more and more keys.
In PKI (Personal Key Infrastructure) you trust several Certificate Authorities (CAs) and accept their public keys. You trust them that they thoroughly check the identity of key holders before signing their public keys. The combination of public key + signature from a CA (and some other data) forms a certificate. The PKI is used in SSL/TLS, it is the underlying infrastructure of the secure web. You use it when you read your mail on a web interface, when you do online banking, etc. If a CA is compromised, then every certificate signed by the CA will be come insecure.
In Kerberos is designed for computer networks and the key server is the single point of trust. It provides mutual authentication and a unique symmetric encryption key for clients and servers. The key server checks the identity of clients by a secret shared only between the key server and the client. Kereberos is used, for example, in Windows, AFS, Grid computing.
your answer gave me much of insight into the question. And also, I would like to put the wikipedia link http://en.wikipedia.org/wiki/X.509#Security here. Coz there is one stentence in the article solve my question "who certificate the Certificate Authority"
This is an example of a self-signed certificate, as the issuer and subject are the same. There's no way to verify this certificate except by checking it against itself; instead, these top-level certificates are manually stored by web browsers. Thawte is one of the root certificate authorities recognized by both Microsoft and Netscape. This certificate comes with the web browser and is trusted by default.
Just in case some one has the same question.
I've read about SSL protocol and now, I know how it encrypts data. But there is something I couldn't understand. With SSL , you're sure you're sending data to and getting data from correct server. But how?
I mean if I create a fake certificate and send it for requests of special website, how do browsers ( or other programs) detect the fake certificate?
Edit: I didn't mean to create a self-signed certificate. I meant how can someone validate my certificate if I create a certificate that its issuer and subject ,etc are something to real certificate! (the only things that are not real is Public key & signature)
TL;DR summary:
Validity of a server certificate is established by:
Host name verification
Verifying the signatures of the entire certificate chain
Performing additional checks on meta data for each certificate
Checking the revocation status of each of the certificates involved
Checking whether the self-signed root certificate of the chain is among the certificates that one trusts by default
Explanation
Let's assume you want to connect to https://mail.google.com (you can try this out in your browser!).
The (real) server will respond with a certificate that is issued to mail.google.com, i.e. in the 'Subject' field of the certificate you will find the Common Name (CN) 'mail.google.com' - cf. RFC 5280 for details on the fields of certificates. The fact that the subject is linked to the site URL is very important for the security of the whole model, and it is actively checked by your TLS implementation ("host name verification"), because otherwise there would be room for Man-In-The-Middle attacks. I.e. somebody could acquire an otherwise valid certificate and impersonate mail.google.com without you taking any notice of it.
In addition to the host name verification, your TLS implementation will also check the "validity" of the certificate. The whole procedure is rather complex and does include checking the trustworthiness of the certificate, but additionally a lot of other things will be checked, more on that in a minute.
If you view Google Mail's certificate in your browser, you will notice that there are actually three certificates shown:
mail.google.com
Thawte SGC CA
Class 3 Public Primary Certification Authority (VeriSign)
The model is that there are a few (well, unfortunately not so few anymore) trusted root certificate authorities ("root CAs") that either you could choose on your own or (more likely) that come preconfigued with your software (e.g. browser) that are blindly trusted. These trusted authorities form the anchors of the entire trust model of "PKI" (Public Key Infrastructure). The basic idea is that the trusted entities may issue certificates to other authorities and grant them permission to again issue certificates (these authorities are called intermediate certificate authorities). The intermediate CAs may again recursively apply this procedure up to a certain point, the number of intermediate CAs between an actual end entity certificate and a root CA certificate is generally limited.
At one point, an intermediate CA will issue certificates to an "end entity" ("mail.google.com" in our example). Now the process of issuing a certificate actually means that the party requesting a certificate will create a public/private key pair first, and use them to authenticate a certificate request that is sent to the certificate authority. The issuing authority creates a certificate for the subordinate entity (either intermediate CA or end entity) by "signing" that certificate using its own private key using an asymmetric algorithm such as RSA and by additionally including the public key of the requesting party within the newly generated certificate. The root CA possesses a so called self-signed certificate, i.e. the root CA is the only authority that may sign their own certificate and include their own public key. The private key remains hidden at all times, of course.
The recursive nature of the certificate issuing process implies that for each end entity certificate there is a unique way of establishing a "chain" of certificates that leads up to a root certificate authority. Now when you are presented with an end entity certificate while trying to connect to a TLS-secured site, the following procedure will be applied recursively until you end up with a root CA certificate:
Find the certificate of the authority that issued the certificate to be validated (see RFC 5280 for details). If none is found: exit with error.
Take the public key of the issuing certificate and verify the signature of the to-be-validated certificate using this public key.
Check a lot of additional things such as whether the certificate has neither expired nor is it not valid yet, "policy constraints", "key usages", "extended key usages"... (again, the gory details are in the RFC).
Certificate revocation status (more on that later)
If all checks were positive, you will ultimately end up with a certificate being self-signed, i.e. where the subject is also the issuer (such as the VeriSign certificate in our example). Now the last thing you have to verify is whether this certificate is among those that you blindly trust: if it is, all is well and the connection will succeed, if it is not, the connection attempt will be rejected.
As if this were not complicated enough already, the checks described so far do not handle cases where once valid certificates suddenly become rogue, examples being cases where a certificate is stolen or private keys are compromised (think of Comodo and DigiNotar). In these cases, the normal procedure is to "revoke" those certificates gone bad, that is you want to mark them as being invalid starting from a distinct point in time (they will expire at some point anyway, but for the remainder of that period they shall already be marked as invalid). For these cases, CAs have the possibility to issue CRLs (a catalog of certificates declared as invalid) or OCSP responses (information for one or in rare cases a set of certificates) that provides clients with information whether a given certificate has been marked as invalid or not. The revocation status needs to be checked for all certificates in a chain, should one of them be marked as invalid then the end entity certificate cannot be trusted and the connection must be rejected as well.
SSL certificates are signed by a certificate authority (CA), which is someone the user already trusts (or more likely, the people who designed their operating system trusts).
The CA digitally signs the certificate using public key encryption. The basic explanation is that the CA has a "private key", and a "public key" that everyone knows. Via some math I don't understand, the CA can create a signature using its private key which can easily be verified with its public key (but the public key can't be used to create a new signature).
When you get an SSL certificate from a server, you get the server's public key, and a signature from a CA saying that it's valid (along with some other info). If you know and trust that CA, you can check the signature and determine if it's valid. You can also use a certificate revocation list to make sure it wasn't revoked.
So basically, you can recognize a bad SSL certificate because it isn't signed by a certificate authority that you trust.
Any fake certificate you create will be a self-signed certificate.
The browser will display big scary warnings when connecting to a site with a self-signed certificate which the user will promptly ignore.
In order to avoid warnings, you need a certificate signed by a certificate authority that the browser trusts, such as VeriSign.
These companies will hopefully make sure that you actually own the domain for the certificate they're signing.
Re: Edit: You can only create a non-self-signed certificate if you get it signed from a trusted CA.
They will refuse to sign a certificate for a different subject.
Process from my understanding:
server sends servers public key
server sends certificate (all information encrypted by trusted CA with their private key)
Your PC decrypts certificate with public key (built into OS from trusted CA)
Your PC hashes (with sha1 and sha256) the servers public key
Your PC compares the hashes of servers public key with certificate stored hash, if not same browser will block site
Your PC compares allowed domains from certificate and the domain, if not allowed, if not same browser will block site
Your PC compares valid date from certificate and your date, if not valid browser will block site.
To fake this you would either need to:
obtain a CA private key (extremely hard to get),
be a CA,
be part of the 5 eyes (Government intelligence agency alliance) and ask a CA for their private key
So if you see a padlock in the address bar you are almost always safe.
Certificates work because they follow a chain of trust. Certificates have a chain of one or more issuers that are trusted; this chain is the backbone of why it works at all. Browsers and nearly all SSL certificate libraries do this chain check, or at least provide the option to.
Self-signed certificates (or those issued by chains that end in a self-signed certificate) would fail this check.
My question is about certificates specifically in ssl but I think the questions should apply to all certificates. I have included the SSL procedure for the sake of clarity.
In SSL this is what I understand the procedure is:
1)Client
sends supported crypto algorithms
sends client nonce
Server
chooses (and sends) a
symmetric algorithm
a public key algorithm
a MAC algorithm
sends it's certificate
sends server nonce
Client
verifies certificate
Extracts public key
Generates a pre-master secret key (pms)
encrypts with servers public key and sends
Client and Server
compute master secrete (MS) from PMS and nonces
PMS sliced to generate two encryption & two mac keys
Client
sends a mac of all handshakes (to ensure they were not previously modifide)
Server
sends a mac of all handshakes
Question
What stops a man in the middle attack from happening at step two? Why can't a man in the middle, say trudy, capture the certificate sent by the server and change the public key in it (to something it has the private key to).
I assume that the certificate is encrypted somehow.
However the server cannot encrypt the certificate because the client does not have the public key yet. When the server gets the key from an authority (like veri-sign) would the key be pre-encrypted using verisign's public key? I think this should work because all web browsers should have the public keys of most authorities.
No, the certificate is not encrypted. But it is signed by a certification authority (CA). Since those check the information included in the certificate (especially the URL to which the cert belongs), there shouldn't be a second valid certificate for a given URL.
The cert of the CA is checked against a trust store (e.g. in your browser). If this truststore is compromised, or if you trust not valid certificates, there is no protection against man in the middle attacks
Certificates are signed by some trusted authority, such as Verisign.
The certificates for these root authorities are built right into the browsers when you download them. You can view the root certificates in Firefox, for example, by going to tools-->options-->advanced-->encryption-->view certificates-->authorities.
If any one of these root-certificate authorities is compromised, however, you are right that a certificate could be forged, making a man-in-the-middle attack possible.
You actually pointed out a weak spot of PKI.
Say Trudy is in the middle of you and yourbank (bank.com). Trudy can change the public key at will at step 2 but the certificate's signature will be invalid. So Trudy has to find a way to generate the signature again. It's safe to say that the trusted CAs will not do this for him. So he has to sign with a fake CA, which is not trusted by your browser. This is still safe theoretically.
However, most browsers (especially IE 6) display a vague security warning and most people don't understand and just ignore, according to some tests.