Why the post?

Just wanted to write about the whle TLS process. So instead of writing the whole article in a formal tutorial kind of style, I thought to have a QnA format for better understanding.

For now I did this with Chatgpt. Will try to see later how does other models provide answers.

Question#1 - Explain diffie hellman with actual numbers

explain diffie hellman. And instead of variables like a, b, g, p etc. lets take some real numbers. The number need not be large as I just wanted to understand the math behind it.

Response:

Sure! Let’s break down the Diffie-Hellman key exchange with real small numbers so you can follow the math intuitively. This is a way for two parties to agree on a shared secret over an insecure channel without ever sending the secret directly.

🔐 Diffie-Hellman Key Exchange — Step by Step with Real Numbers

Let’s assume Alice and Bob want to communicate securely.

📌 Publicly Agreed Numbers

These two numbers are public and can be known by everyone (even an attacker):

• p = 23 (a prime number)

• g = 5 (a primitive root modulo 23)

So:

• p = 23

• g = 5

👩 Alice Picks Her Secret

• Alice picks a secret number: a = 6

• She computes:
  A = g^a mod p = 5^6 mod 23
5^6 = 15625
15625 mod 23 = 8

✅ Alice sends A = 8 to Bob.

👨 Bob Picks His Secret

• Bob picks a secret number: b = 15

• He computes:
  B = g^b mod p = 5^15 mod 23
5^15 = 30,517,578,125
5^15 mod 23 = 2

✅ Bob sends B = 2 to Alice.

🔒 Now Both Compute the Shared Secret

• Alice receives B = 2, and computes:
  s = B^a mod p = 2^6 mod 23 = 64 mod 23 = 18

• Bob receives A = 8, and computes:
  s = A^b mod p = 8^15 mod 23
8^15 = 35,184,372,088,832
8^15 mod 23 = 18

✅ Shared secret is 18

✅ Summary

Both now have the same shared secret (18) even though they never sent it directly!

🔐 Why It Works

The math relies on modular exponentiation and the difficulty of the discrete logarithm problem — it’s easy to compute g^a mod p, but hard to reverse it to find a.

Want to try with different values or want a visualization of how the numbers flow?

Question#2 : Attacker pretends to be Bob

The attacker can read the value sent by Alice. Now what if the attacker sends his own computed value and pretends to be Bob?

Response:

Excellent question — you’re now touching on a classic vulnerability in the basic Diffie-Hellman key exchange: it’s susceptible to a Man-in-the-Middle (MITM) attack if not authenticated.

Let’s walk through how that attack would work using our previous numbers.

🚨 Man-in-the-Middle Attack on Diffie-Hellman

Let’s say Mallory is an attacker who intercepts the messages between Alice and Bob.

Step-by-Step Breakdown:

1. Public Parameters:
   p = 23, g = 5 (everyone knows this, including Mallory)

👩 Alice Sends Her Public Value

• Alice’s secret: a = 6

• Computes: A = g^a mod p = 5^6 mod 23 = 8

• Sends A = 8 ➡️ Mallory intercepts this

🕵️ Mallory Fakes Being Bob

Mallory does not forward Alice’s A to Bob. Instead:

• Mallory picks her own secret: m1 = 13

• Computes: M1 = g^m1 mod p = 5^13 mod 23 = 21

• Sends M1 = 21 to Bob pretending it came from Alice

✅ Now Bob thinks he got Alice’s public key, but it’s actually Mallory’s.

👨 Bob Sends His Public Value

• Bob’s secret: b = 15

• Computes: B = g^b mod p = 5^15 mod 23 = 2

• Sends B = 2 ➡️ Mallory intercepts this too

🕵️ Mallory Fakes Being Alice

Mallory now sends her own value to Alice:

• Mallory picks a new secret: m2 = 7

• Computes: M2 = g^m2 mod p = 5^7 mod 23 = 17

• Sends M2 = 17 to Alice pretending it came from Bob

🔑 Final Shared Secrets

Now, two different secrets exist:

• Alice thinks she’s communicating with Bob:

  • Computes: s1 = 17^6 mod 23 = 18 (shared with Mallory)

• Bob thinks he’s communicating with Alice:

  • Computes: s2 = 21^15 mod 23 = 14 (shared with Mallory)

✅ Mallory now shares a different key with Alice and with Bob.

She can:

• Decrypt messages from Alice using s1

• Read them

• Re-encrypt them using s2 and send to Bob

• And vice versa

Neither Alice nor Bob realizes they’re not talking to each other.

🔐 Solution: Authentication

Diffie-Hellman alone doesn’t provide authentication — it only sets up a shared secret.

To fix this:

• Use Digital Signatures (e.g., RSA, ECDSA) to sign the public values (A, B)

• Use authenticated key exchange protocols like:

  • Station-to-Station (STS) protocol

  • TLS (which uses DH with certificates)

  • ECDH with digital signature schemes (ECDSA)

✅ TL;DR

• Yes, an attacker can intercept and inject their own public keys.

• This leads to a Man-in-the-Middle attack.

• Diffie-Hellman without authentication is insecure.

• Always pair DH with authentication, e.g., digital signatures or certificates.

Would you like to see how this works with real-world protocols like TLS?

Question#3: Why not use public key for every communication?

So you are saying that using an authentication method like signatures or certificate, Alice can make sure that the message goes to Bob only. I believe it works by encrypting the message with Bob’s public key so that only Bob can decipher the message with its private key. In that case, can’t Alice just use that public key to encrypt everything thereby eliminating the use of the key exchange altogether?

Response:

You’re absolutely right to question the need for Diffie-Hellman if Alice already has Bob’s public key. Let’s break this down carefully.

✅ What You’re Asking

If Alice can authenticate Bob (via signature or certificate), and knows his public key, then:

Why not just encrypt messages with Bob’s public key and skip the whole Diffie-Hellman key exchange?

That’s a valid and insightful question. Here’s the full answer with trade-offs and real-world reasons.

🔐 Asymmetric Encryption vs. Diffie-Hellman

🔸 Asymmetric Encryption (e.g., RSA)

Alice uses Bob’s public key to encrypt a message.

• Only Bob can decrypt it (he has the private key).

• No key exchange is needed.

• This is called “public-key encryption.”

✅ Pro: Simple conceptually — no handshake needed.
❌ Con: Computationally expensive for large data.
❌ Con: Cannot easily establish forward secrecy.

🔸 Diffie-Hellman Key Exchange (DH or ECDH)

Used to negotiate a shared secret key, even over an insecure channel.

That shared secret is then used with:

• A fast symmetric cipher (like AES) to encrypt messages.

• Often used in conjunction with authentication (certificates or digital signatures).

✅ Pro: Much faster encryption/decryption (via symmetric ciphers).
✅ Pro: Enables Forward Secrecy (more below).
❌ Con: Needs extra steps (key exchange + authentication).

🔒 Why Use DH Instead of Just Public-Key Encryption?

1. Performance

• Public-key encryption (e.g., RSA) is very slow, especially for large data.

• In practice, it’s only used to encrypt:

  • Small secrets (like a session key).

  • Or to authenticate (via signatures).

DH + AES (symmetric crypto) is much faster and scales better.

2. Forward Secrecy

• With DH (especially ephemeral DH or ECDHE), the keys are generated per session.

• If long-term private keys are compromised in the future, past sessions remain secure.

But with public-key encryption:

• If Bob’s private key is compromised, all past encrypted messages can be decrypted.

🔐 Forward secrecy is a major reason why modern TLS (HTTPS) uses ECDHE, not just RSA encryption.

3. Key Agreement vs. Key Transport

With DH, neither party picks the final key — it’s mutually derived — this matters in some protocols for fairness and trust.

4. Real-World Example: TLS (HTTPS)

TLS (used in HTTPS) used to support RSA key exchange (Alice encrypts pre-master secret with Bob’s public key). But this:

• Does not provide forward secrecy.

• Is less flexible.

Modern TLS (1.2+ and especially 1.3) prefers:

• ECDHE (Ephemeral Diffie-Hellman over Elliptic Curves)

• Authenticated with digital certificates (X.509)

So even though Bob has a certificate, the actual session key is negotiated via DH.

✅ TL;DR

You’re right — if Alice trusts Bob’s public key, she can encrypt a message directly.

But in practice, Diffie-Hellman is preferred for:

So: authentication (e.g., signatures, certs) doesn’t replace key exchange — it secures it.

Would you like a quick visual flowchart of how TLS 1.3 does this, or a code example (Node.js or Python)?

Question#4: How certificates work?

How do these certificates work? Alice will need the public key of Bob right? To get that Alice will need to connect with the outer world over internet. Mallory can intercept that request and return a key to Alice. Now Alice will think she is talking to Bob, but actually its Mallory.

Response:

Yes — you’ve hit another crucial security challenge. This is exactly the kind of attack that digital certificates and Certificate Authorities (CAs) are designed to prevent.

Let’s walk through it step by step.

🧠 The Problem You’re Highlighting

If Alice tries to get Bob’s public key (e.g., by visiting https://bob.com), Mallory can intercept the connection and pretend to be Bob, giving Alice a fake public key.

That’s a Man-in-the-Middle (MITM) attack.

So:
How does Alice know that the public key she receives truly belongs to Bob?

✅ The Solution: Digital Certificates & Certificate Authorities (CAs)

🔐 What is a Digital Certificate?

A digital certificate is a file that contains:

• Bob’s public key

• Identity information (e.g., CN=bob.com)

• The digital signature of a trusted Certificate Authority (CA)

The signature is the key part: it’s what prevents Mallory from impersonating Bob.

🏛️ What is a Certificate Authority (CA)?

A CA is a trusted third party that:

• Verifies Bob’s identity (e.g., domain ownership of bob.com)

• Issues a digitally signed certificate binding Bob’s identity to his public key

Examples of CAs:

• Let’s Encrypt

• DigiCert

• Sectigo

• GlobalSign

Your browser and operating system come preloaded with public keys of trusted CAs.

🔐 How Certificate Validation Works

Let’s say Alice visits https://bob.com:

Step 1: Bob’s server responds with:

• Its certificate, which includes:

  • Bob’s public key

  • A CA signature (made using CA’s private key)

Step 2: Alice’s browser:

• Uses the CA’s public key (already stored locally)

• Verifies the signature on Bob’s certificate

• If the signature is valid, it trusts that:

  • The public key belongs to bob.com

  • The identity was verified by the CA

✅ Mallory cannot fake this, because she doesn’t have the CA’s private key and therefore cannot forge the signature.

🔄 Why This Prevents MITM

If Mallory tries to:

• Intercept the connection

• Present her own public key

She would need to present a certificate for bob.com signed by a trusted CA — which she can’t get, because the CA will only sign certificates for entities that prove control of bob.com.

🔒 Bonus: Certificate Chains

In reality, there might be intermediate CAs involved:

• Root CA (trusted by browsers)
  → Intermediate CA
  → Issues Bob’s certificate

Alice’s browser can validate this chain of trust, all the way up to a trusted root certificate.

✅ TL;DR

Would you like to see an example certificate and how the signature is validated using OpenSSL or Node.js?

Question#5: Where is the key stored?

Where does Bob store the key that was made using CA’s private key?

Response:

Great question — and it reveals a common misconception about who holds which key when it comes to certificates.

Let’s clarify the roles and answer your question directly:

🔐 Short Answer

Bob does not store a key made with the CA’s private key.

Instead, Bob stores:

• His own private key (which only he knows)

• A certificate issued by the CA (which contains:

  • Bob’s public key

  • CA’s digital signature over that public key and Bob’s identity)

🧠 Let’s Break It Down Clearly

📄 What Bob Receives from the CA

When Bob gets a certificate from a Certificate Authority (CA), here’s what happens:

1. Bob first generates his own key pair:

   • A private key (kept secret)

   • A public key

2. Bob creates a Certificate Signing Request (CSR):

   • Contains his public key and identity (e.g., bob.com)

   • Signs the CSR using his private key (optional for integrity)

3. Bob sends the CSR to the CA.

4. The CA:

   • Verifies Bob’s identity

   • Creates a certificate by:

     • Taking Bob’s public key + identity info

     • Signing that bundle with the CA’s private key

5. The CA gives Bob a certificate file (e.g., cert.pem), which Bob stores on his server.

🗂️ So What Does Bob Store?

🔒 When a Client Connects (e.g., Alice’s Browser):

1. Bob sends:

   • His certificate (with public key + CA signature)

2. Alice’s browser:

   • Looks up the CA’s public key from its local trust store

   • Verifies the CA’s signature on Bob’s certificate

   • If valid, trusts Bob’s identity and public key

🔐 Important Summary

The CA’s private key is never given to Bob.
Instead, the CA uses its private key to sign Bob’s public key, creating a certificate.
Bob stores:

• His private key securely.

• His certificate (signed by CA) publicly.

That’s it — no mystery CA key hidden on Bob’s side.

Would you like to see an example of this with OpenSSL commands? I can show how a CSR, certificate, and key are generated and how they relate.

Question#6: What if attacker intercepts the first message itself?

but when Alice connects with Bob for the first time. It may happen that Mallory can intercept that message and start pretending Bob by giving its own ceritificate which would be a valid one because Mallory also has taken it from the same CA?

Response:

Excellent — you’re now asking the core question of TLS trust:

What if Mallory has her own valid certificate from the same CA — how does Alice know she’s talking to Bob, not Mallory?

Let’s answer this clearly and directly.

🧠 The Key Insight

A certificate is valid only for the identity it was issued to.

So yes, Mallory can get her own certificate from the same CA — but it will be valid for:

mallory.com

not:

bob.com

So if Mallory tries to use her own certificate while pretending to be bob.com, Alice will detect the mismatch.

✅ How Alice Detects the Attack

When Alice visits https://bob.com, her browser checks:

1. Is the certificate’s Common Name (CN) or Subject Alternative Name (SAN) equal to bob.com?

2. Is the certificate valid (not expired/revoked)?

3. Is the certificate signed by a trusted CA?

If Mallory presents:

• A valid cert for mallory.com

• While Alice is visiting bob.com

Then ❌ certificate validation fails:

ERROR: Certificate does not match host 'bob.com'

This is called a hostname mismatch error, and browsers will block the connection.

🧪 But What If Mallory Gets a Certificate for bob.com?

Then we’re in real danger. This would mean:

• The CA was tricked or hacked.

• Mallory managed to fraudulently validate ownership of bob.com.

This has actually happened in the past — here are real-world examples:

• 2011: DigiNotar breach – attackers issued fraudulent certs for google.com.

• 2015: CNNIC issued intermediate CA to an unauthorized company.

🔐 That’s why:

1. CAs use domain ownership checks (like DNS challenges, HTTP challenges).

2. Certificate Transparency (CT) logs now exist — public logs where all certificates must be recorded.

   • Browsers can verify that a cert for bob.com was legitimately issued and logged.

3. Certificate pinning (deprecated in browsers but used elsewhere) can detect unexpected certs.

🔒 Summary

✅ TL;DR

Mallory having a certificate doesn’t help unless it’s specifically for bob.com — and CAs prevent that via domain validation and transparency mechanisms.

Would you like a full example showing how CN/SAN and hostname verification works in an actual certificate?

Question#7: How subdomains are handled?

How does CN or SAN work for subdomains. Will bob need separate certificates for blog.bob.com and app.bob.com?

Response:

Excellent follow-up — you’re now getting into the practicalities of certificate scoping and subdomain handling.

Let’s break it down clearly:

🔑 TL;DR

Bob can either get separate certificates for each subdomain
OR use a wildcard certificate to cover all subdomains under a domain like *.bob.com.

🧾 How CN and SAN Work

📌 CN (Common Name)

• Traditionally specified the main domain for the certificate.

• Nowadays, it’s largely deprecated in favor of SAN (Subject Alternative Names).

• Modern browsers mostly ignore CN if SAN is present.

📌 SAN (Subject Alternative Name)

• A list of all domain names the certificate is valid for.

• Supports:

  • Exact domains (blog.bob.com)

  • Multiple domains (blog.bob.com, app.bob.com, bob.net)

  • Wildcards (*.bob.com)

✅ Certificate Options for Subdomains

Option 1: Separate Certificates

• One certificate for blog.bob.com

• One for app.bob.com

• Useful if different teams/services manage each subdomain

🔻 Downsides:

• Harder to manage

• More certificates to renew

Option 2: Wildcard Certificate

• A single cert for *.bob.com covers:

  • blog.bob.com

  • app.bob.com

  • anything.bob.com

⛔ Does not cover:

• bob.com (must be listed separately)

• sub.blog.bob.com (wildcards only go one level deep)

✅ Example:

CN: *.bob.com
SAN: *.bob.com, bob.com

Option 3: Multi-SAN Certificate

• A single certificate listing all domains/subdomains explicitly.

✅ Example:

SAN:
  - bob.com
  - blog.bob.com
  - app.bob.com
  - api.bob.net

This is what’s commonly used for multi-site applications or Content Delivery Networks (CDNs).

🔐 Which is Better?

💡 Bonus: Let’s Encrypt & Wildcards

Let’s Encrypt supports wildcard certificates — but:

• Requires DNS-01 challenge (not HTTP-01)

• You must prove domain control via DNS TXT record

✅ Summary

Would you like to see how a wildcard or multi-SAN certificate looks in OpenSSL format or a real PEM file?

Question#8: SAN vs CN

so for www.bob.com and bob.com, what approach would be better?

Response:

Great question — and very practical.

✅ TL;DR:

The best approach is to use a SAN certificate that includes both bob.com and www.bob.com.

🔍 Why This Happens

• bob.com and www.bob.com are technically different hostnames.

• Many users type bob.com, while others click links to www.bob.com.

• You want your certificate to cover both, or browsers will show “not secure” warnings on the one that’s not covered.

🔒 Recommended: SAN Certificate with Both Domains

This is the most common and recommended approach for most websites.

Example SAN:

Subject Alternative Name:
  DNS: bob.com
  DNS: www.bob.com

Bonus:

Most certificate providers (including Let’s Encrypt) do this automatically if you request a cert for both.

⚠️ Why Not Use a Wildcard (*.bob.com)?

You could, but there are trade-offs:

🟩 Pros:

• Covers www.bob.com, blog.bob.com, store.bob.com, etc.

• Good for flexible subdomain use

🟥 Cons:

• Does not cover the root domain bob.com (wildcards only match one level)

• Must explicitly add bob.com to the SAN list anyway

• Overkill for just one subdomain like www

So a wildcard is useful only if you’re managing many subdomains — otherwise, it’s unnecessarily broad.

✅ Best Practice Summary

Would you like to see how to generate such a SAN certificate manually using OpenSSL or using Let’s Encrypt tools?

Question#9: Where does hosting services store the keys?

I am using Netlify to host my website. Netlify manages ssl certifactes using Letsencrypt. Where does netlify stores them? On my space or does it store in its database?

And if netlify stores it, then it can use that it to pretend me and communicate to others on my behalf?

Response:

Great set of questions — you’re now digging into trust boundaries in cloud hosting, especially regarding SSL certificates and impersonation risk.

Let’s break it down clearly in the context of Netlify + Let’s Encrypt:

🔐 Where does Netlify store the certificate?

When you use Netlify and enable HTTPS (which is automatic), here’s what happens:

1. Netlify uses Let’s Encrypt to issue an SSL certificate for your domain (e.g., yourdomain.com, www.yourdomain.com).

2. That certificate (public + private key pair) is:

   • Generated and stored by Netlify.

   • Not stored in “your” project folder or accessible in your Netlify account’s file system.

   • Stored on Netlify’s internal infrastructure, not visible to you.

This is by design, because Netlify acts as the reverse proxy / TLS terminator for your domain.

🔑 Who holds the private key?

Netlify does.

Since Netlify is terminating TLS connections on your behalf, it must have the private key associated with your certificate.

So yes:

• Netlify stores the private key securely (ideally using encrypted hardware or similar protection).

• Your application doesn’t handle raw TLS — Netlify does that at the CDN edge layer.

😨 Can Netlify impersonate you?

Technically: Yes.

• Since they hold the private key for your domain’s certificate, Netlify can:

  • Initiate TLS connections as yourdomain.com.

  • Decrypt TLS traffic destined for your domain.

  • Pretend to be you when communicating with others (e.g., APIs or clients) if they misuse their access.

But Practically: They shouldn’t and won’t.

Why?

• Trust & contracts: You’re trusting Netlify just like you’d trust any cloud host (AWS, Cloudflare, etc.).

• Abuse = reputational/legal risk: If they impersonate you, that’s a breach of trust and terms of service.

• No incentive: You’re the customer — they have no reason to impersonate you without cause.

• Security controls: Internally, access to private keys is likely restricted to systems, not humans.

🔄 Can I bring my own certificate?

Yes — Netlify does support custom certificates:

• You can upload your own TLS cert + private key.

• This gives you full control over the key material.

• Downside: You need to handle renewal and secure key storage yourself.

Reference: Netlify docs – Custom certificates

🔐 Summary

Would you like to see a diagram of how Netlify handles TLS termination and passes traffic to your site?

Question#10: TLS termination

What do you mean by TLS termination with respect to Netlify?

Response:

Great question. TLS termination is a key concept in understanding how platforms like Netlify securely handle HTTPS traffic.

🔐 What Is TLS Termination?

TLS termination means decrypting incoming HTTPS traffic at a specific point (usually a server or proxy), so that the encrypted data becomes readable to the backend.

In simpler terms:

• When a browser (client) sends an HTTPS request, the data is encrypted using TLS.

• The TLS terminator is the server that decrypts this data so it can be processed normally.

📍 TLS Termination in Netlify

Here’s how it works:

1. 🧑‍💻 A user visits your site:

They go to https://yourdomain.com.

2. 🌍 The request hits Netlify’s CDN edge node (e.g., in Cloudflare-like fashion):

• This edge server holds your TLS certificate and private key.

• It performs TLS termination:

  • Decrypts the HTTPS request.

  • Extracts the plaintext HTTP data (headers, body, etc.).

3. 🧠 Netlify routes that plain HTTP request to your app:

• This could be a static site or a serverless function.

• Your app does not see the HTTPS part — it just gets normal HTTP.

4. 🌐 If the app needs to make outgoing HTTPS requests:

• Those are separate TLS sessions initiated by your backend or browser.

📉 Why Is This Done?

TLS termination at the edge (like Netlify does):

🧱 TLS Termination vs TLS Passthrough

Netlify uses TLS termination — it decrypts the traffic, then sends plain HTTP internally.

🔐 Visual Summary

[Client Browser] --(HTTPS)--> [Netlify CDN (TLS termination)] --(HTTP)--> [Your App or Static Site]

Netlify handles the secure handshake, then passes your app just the clean, readable HTTP request.

Let me know if you’d like to go one level deeper into:

• How TLS handshakes work

• What info Netlify sees after termination

• How mutual TLS (mTLS) differs