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PKI & SSL/TLS

Overview

The fundamental Diffie-Hellman key exchange is susceptible to a Man-in-the-Middle (MITM) attack because it lacks a mechanism for participants to authenticate each other's identity, allowing an attacker to establish two separate secret keys and decrypt all communication. This critical security gap is filled by Public Key Infrastructure (PKI), which introduces trust by using digital certificates signed by a universally trusted third party called a Certificate Authority (CA). Consequently, modern protocols like HTTPS rely on the client's ability to verify the server's certificate signature, ensuring that the initial key exchange and subsequent encrypted communication are indeed with the legitimate intended party.

Key Information

  • Diffie-Hellman (DH) Flaw: The standard DH key exchange lacks authentication, leaving it vulnerable to Man-in-the-Middle (MITM) attacks.
  • MITM Attack: An attacker can intercept public values and establish two separate secret keys (one with Alice, one with Bob), allowing them to read and modify all communication.
  • PKI Solution: Public Key Infrastructure (PKI) resolves this by providing an identity verification mechanism.
  • Digital Certificates: PKI uses digital certificates which cryptographically bind a public key to an identity.
  • Trusted CAs: These certificates are signed by a Certificate Authority (CA) (a trusted third party), enabling protocols like HTTPS to ensure clients are communicating with the genuine server.

Notes

Creating a certificate with openssl

openssl req -new -nodes -newkey rsa:4096 -keyout key.pem -out cert.csr

  • req -new create a new certificate signing request
  • -nodes save private key without a passphrase
  • -newkey generate a new private key
  • rsa:4096 generate an RSA key of size 4096 bits
  • -keyout specify where to save the key
  • -out save the certificate signing request

Task

  1. What is the size of the public key in bits?
  2. Till which year is this certificate valid?

Conclusion

The inherent lack of identity verification in the basic Diffie-Hellman key exchange leaves it vulnerable to a crippling MITM attack where all communication is compromised. This fundamental flaw is securely mitigated by PKI, which leverages CA-signed digital certificates to authenticate the server's identity, thereby guaranteeing the integrity and confidentiality of modern communication protocols like HTTPS.

Resources

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