Tag Archives: public key

Digital Certificates Explained

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Digital Certificates Explained KB ID 0001846

Problem

From setting up PKI environments, to migrating them, and a myriad of errors and problems in between, we have a lot of content on PNL referring to digital certificates and PKI. I’ve dealt with these technologies a lot over the past 20+ years and I’m always surprised how frightened of digital certificates people are. So in the space of a few minutes I’ll try and give you a basic understanding of what PKI is, how that relates to Digital certificates, then finish off with a few certificate ‘Golden rules‘.

Solution : Digital Certificates Explained

You will see I use the terms PKI environment, and Certificate Services  and interchange those two things a lot, Certificate Services is the name of the Microsoft ‘Role” that provides Certificate Services. If you simply want to setup certificate services, then use the following link.

Microsoft PKI Planning and Deploying Certificate Services

Before you understand certificates, you need to have a basic understanding of Public Keys, and Private Keys, how this key pair is used, and what Asymmetric Cryptography is (bear with me the maths might be mind boggling, but the concept is simple).

Public Key

The public key is shared openly and is used to encrypt messages or data. It is associated with a user (or entity) and can be freely distributed to anyone. Its primary function is to ensure confidentiality by allowing others to encrypt messages that only the owner of the corresponding private key can decrypt.

Private Key

The private key is kept secret and is known only to the owner. It is used for decrypting messages or data that have been encrypted with the corresponding public key. The private key is crucial for ensuring the authenticity and integrity of the data because only the person with the private key can decrypt and access the information.

Digital Certificates Explained: How These Keys are Used for Encryption

Encryption

1. Sender obtains the recipient’s public key and uses the recipient’s public key to encrypt the message.
2. Encrypted message is sent to the recipient.

Decryption

3. The recipient, (who possesses the corresponding private key), can then decrypt the message.

The strength of the security lies in the complexity of the key pair. Asymmetric cryptography is based on mathematical principles, making it computationally infeasible for someone to calculate the private key even if they have the corresponding public key. This makes it a secure method for tasks such as secure communication, digital signatures, and online transactions.

Digital Certificates Explained: Use of Keys and Certificates

Public Key Infrastructure (PKI) is a framework that manages digital keys and certificates. it requires the following:

Key Pair Generation: The process begins with the generation of a key pair (the public and private keys) for a user or entity. As shown above the public key is shared openly, and the private key is kept secret.

Certificate Authority (CA):A Certificate Authority is a trusted third party (public) or you own (private) system for authenticating digital certificates. in a PKI system.
The user’s public key is combined with other information (such as identity details and time stamps,) and then signed by the CA to create a digital certificate.

Digital Certificates: A digital certificate is a data structure that binds a public key to an individual, device, or service. It includes the public key, and information about the entity, the digital signature of the CA, and other metadata. So the digital certificate serves as a way for others to verify the authenticity of the associated public key and ultimately the identity of the entity.

Certificate Lifecycle: Certificates have a defined lifecycle, including issuance, and expiration. Note: They can also be revoked before expiration for example if compromised, or if the associated private key is lost.

Usage in Secure Communication

When parties want to communicate securely, they exchange their digital certificates. So just like the example above, the sender encrypts data with the recipient’s public key, which they get from the certificate that the receiver has published publicly, ensuring only the recipient, with the corresponding private key, can decrypt it.
Note: Digital signatures, created using the sender’s private key, can also be attached to messages to ensure authenticity and integrity. The recipient can verify the signature using the sender’s public key.

Digital Certificates Explained: Golden Rules Of Certificates

You must trust the CA that issued (authorised) the Certificate.

Every certificate has a CA that issued/approved it you NEED TO TRUST that CA, there are several trusted Certification Authorities that are built into your computer, and these are updated regularly with normal updates. this is why if you self-sign a certificate or build your own CA – then you may need to manually get people to trust your CA before they will be trusted. Also Certificate Authorities are hierarchical (they can have parent CAs) these form a CHAIN, and you need to be able to see that chain and trust ALL the CAs in that chain, (all the way back to the Root CA).

 

Above on my (current) site certificate you can see it was issued by a CA called RapidSSL, and that CA was authorised by a Root CA called Digicert Global Root G2. This means my certificate was signed by an intermediate CA (sometimes called and Sub CA).

The Certificate needs to be In Date

Certificates a have a date they were issued and a date when they will expire, If you are not at a point in time between these two dates, then you will not trust the certificate. (Note: this can happen if the internal clock/date on YOUR machine is incorrect).

 

 

Note: A Certificate that is in date can also be revoked. This is done by the CA, which publishes a list of certificates that have been revoked, (that your computer can check.)

The Common Name (CN) and/or Subject Alternative Name (SAN)  needs to be correct.

In times gone by, the certificate CN had to be correct, but more modern browsers will require BOTH Common Name and Subject Alternative Name to be correct. these are fields in the certificate that hold the name of the entity for example mine is www.petenetlive.com (that is the Common Name) This is the address (or FQDN) that is typed into the browser window to get to the site that presents this certificate.

 

Note: That means if you are putting a certificate on ‘something‘ be that a website, or a device, the NAME needs to be both correct, and resolvable by the person (or entity) that you want to trust it, e.g. if the common name was www.petes-site.com and you needed to type in https://www.petenetlive.com to get there, it would not be trusted (even if both names resolved to the correct IP address!)

You need to trust the ‘Algorithm‘ that signed the certificate.

This is a more recent development, I mentioned above that there’s some very complicated maths going on, as time moves on the mathematics needs to get more complicated, as computers get more powerful to maintain security. So older methods of signing have been depreciated, if a certificate is using technology that’s been depreciated it will also not be trusted.

 

Here’s an example of what happens when the signature algorithm is not trusted.

ERR_CERT_WEAK_SIGNATURE_ALGORITHM

Here’s an example of what happens if the CN or SAN values are wrong.

ERR_CERT_COMMON_NAME_INVALID

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Cisco ASA Site to Site VPN’sSite to Site ISAKMP VPN (Main Mode)

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KB ID 0000213

Problem

As with most things, before you have a hope of fixing something, you will stand a better chance if you know how it works in the first place. Below is a quick run though of what’s happening with your site to site VPN‘s and how they work.

For the entire process we will have two Cisco ASA 5500 firewalls and a site to site VPN.

Solution

What’s an Initiator and a Responder?

1. Our Laptop 192.168.1.50 wants to talk to a server on the other site at 172.16.1.50

2. To get out of the local network the Laptop goes through the ASA at its local site, The ASA knows that traffic destined for 172.16.1.50 needs to be sent down the VPN tunnel, so it needs to bring up the tunnel. IT BECOMES THE INITIATOR, contacts the ASA on the other site THAT BECOMES THE RESPONDER.

3 Once that’s complete the tunnel is up and traffic can pass.

So how does it bring up the Tunnel?

To establish an ISAKMP VPN tunnel 3 things have to happen.

1. Phase 1 has to complete.

2. Phase 2 has to complete.

3. The Traffic has to be allowed to pass.

VPN Phase 1 (ISAKMP)

This stage brings up the first secure tunnel (eventually there will be three tunnels) and for it to establish the firewalls need to agree what they are going to do to bring up the tunnel, then Secure the tunnel. This process uses SIX MESSAGES (Note: We are dealing to Main Mode here not Aggressive mode). Both firewalls need a matching Phase 1 Policy to continue. And the Policy is proposed in MESSAGE1 and accepted in MESSAGE2.

A Phase 1 policy consists of,

1. The Authentication method (either a pre shared key or an RSA signature is usual).

2. The Encryption method (DES, 3DES, AES, AES-192, or AES-256).

3. The Hashing Method (MD5 or SHA).

4. The Diffie Helman Group (1, 2 or 5 usually).

5. Lifetime (In seconds before phase 1 should be re-established – usually 86400 seconds [1 day]).

MESSAGE 1

The Initiator sends policies that it proposes to use, for phase 1 to the other ASA.

MESSAGE 2

Providing the responder has a matching policy it will accept one of those proposed by the initiator and send it back in message 2.

 

Now the two ends have agreed HOW they will establish phase 1, they then need to agree on a “Shared Key” both ends must use the same shared key, but the shared key cant be sent between them because the network link is not secure. To do this they use a Diffie Hellman key exchange, this uses a mathematical process called modular exponentiation, a simple example of how that works (The math’s involved in a real key exchange are much more complicated!).

How Diffie Hellman works (simply)

Problem Site A and Site B need to use the same secret key (which will be a big long number). they cant send that number to each other because if they do it will be seen.

Solution:

Both sites pick a random number, and they have a common number, this common number can be passed between sites, In our example Site A chooses 4 and Site B chooses 5

Both sites use the common number and raise it by the power of the random number they are using so Site A arrives at 16, and Site B at 32.

The sites then send the number they have arrived at, to the other site.

Each site uses the other sites total and raises it to the power of their original random number, this results in them both having the same key, with only the numbers 2, 16 and 32 being passed between them.

Back to our VPN Tunnel

The next two messages are the initiator and responder swapping their Diffie Hellman information, Each side produces a DH Public Key, and mathematically computes a long number called a “Nonce”

MESSAGE 3

The initiator generates a “Public Key” also called the DH Public Value or Xa It also generates a Nonce or Ni and sends both of them to the responder.

MESSAGE 4

The responder generates a “Public Key” also called the DH Public Value or Xb It also generates a Nonce or Nr and sends both of them to the initiator.

At this point both the initiator and the responder can calculate the DH Shared secret key, they then use the DH Secret Key, the “Shared Secret” that is manually entered onto both peers, and the Nonce from the other peer to create 3 DIGITAL KEYS, because of the nature of Diffie Hellman each end will produce the same keys.

Key 1 = SKEYID_d Used to work out any future IPsec keying Key 2 = SKEYID_a Used for data integrity and authentication (IKE) Key 3 = SKEYID_e Used to encrypt all further IKE traffic.

MESSAGE 5

The initiator now sends its ID to the responder (this is either its IP address or a hostname). It also sends a “Hash” this authenticates the initiator to the responder as its made from the SKEYID, the pre-shared key and other information only known to the two peers.

MESSAGE 6

Message 6 is basically the mirror of Message 5, the responder sends its ID (IP or Hostname) Back the the initiator with its “Hash” and authenticates itself back to the initiator.

At this point both peers recalculate the hash they have received from the other peer, and they should both come out the same, if this happens then the IKE SA’s are established and phase 1 is complete.

So what’s PFS?

Perfect Forward Secrecy is a method by which new keys are generated, each new key is mathematically linked to the key that came before it, the prior key being a “Grandfather” key. With PFS enabled this link is broken so a key can not be forward/reverse engineered to guess a previous/new key value). Every new negotiation produces a new fresh key.

VPN Phase 2

Once Phase 1 has completed the second stage of the VPN can start. Like phase 1 this state also requires messages to be sent between the peers, IPsec usually executes in “Quick mode” this means that there are only 3 MESSAGES.

Note: If PFS is configured only on one end then it will fail at this point with an “Attribute not supported” error.

MESSAGE 1

The Initiator sends another Hash to the responder, this is similar to the one used in phase 1 but also includes info within this message to guarantee integrity.

The Phase 2 proposal includes

1. Encapsulation method either ESP or AH.

2. Hashing method (Integrity checking) either SHA-HMAC or MD5-HMAC.

3. Diffie Hellman Group (1, 2, or 5).

4. The SPI – This number is the LABEL for the end of the tunnel the initiator will use for outbound traffic.

Tunnel mode (Tunnel or Transport). A timeout in seconds is specified, as is the ID (usually the subnet of both ends of the tunnel).

MESSAGE 2

The Responder replies with its own “Hash” with the accepted proposal and its own SPI for outgoing encrypted traffic from the responder, and finally its own Key Exchange Payload.

Once this is complete both peers generate new DH secret keys and combine them with the SKEYID_d key from phase 1 to create keys for IPsec encryption.

MESSAGE 3

The final Message is sent from imitator to responder, and serves to inform the responder that its previous message was received.

Once phase 2 is complete IPsec SA’s have been established and the tunnel is up.

 

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