PKI

tags : SSH, GPG, Security, Networking, Cryptography

What §

Basics §

• X.509 ( See X-Standards) PKI enables each certificate to be signed by a single party: a certificate authority (CA).
• It allows severs to be authenticated without requiring clients to manually install root certificates.
• Bestows trust throughout the hierarchy of certificates which lead back to them.
• One alternative to PKI is Web of Trust which GNUPG uses

Main Components §

• Certificate Authorities (CAs) (See TLS)
• Registration Authorities
• Digital certificates
• Public and private keys
• Certificate revocation lists

Key protocols §

PKI does not necessarily mean asymmetric key encryption. For example, DH is used for PKI but it aims to create a symmetric key in the end. See this and this answer for more details.

Diffie hellman §

• Relies on the hardness of taking logarithms (discrete logs)
• DH uses a multiplicative group of integers modulo a prime p

RSA §

• Relies on the hardness of factoring.

DSA (Digital Signature Algorithm) §

• It weak, can be broken?

Elliptic curve (ECC) §

• See How does SSH use both RSA and Diffie-Hellman?

Key exchange method §

• The key exchange yields the secret key which will be used to encrypt data for that session.
• SSH uses DH to establish an ephemeral (ie: one time) key to establish forward-security.

• ECDH

  • Variant of DH that uses a multiplicative group of points on an elliptic curve
  • Uses a curve
    • NIST curve P-256
    • Curve25519/cv25519/X25519

Signature schemes §

• SSH uses RSA/ECC variants to establish an initial secure, authenticated connection
• The signature is so that the client can make sure that it talks to the right server

• ECDSA

  • NIST produced, political concerns
  • Can be susceptible to attacks. Underlying DSA.
  • Just use Ed25519 instead

• Ed25519

  • Strongest so far

Resources §

• Ed25519 Keys | Brian Warner
• cv25519 vs ed25519
• Introducing Elliptic Curves – Math ∩ Programming
• Comparing SSH Keys - RSA, DSA, ECDSA, or EdDSA?
• A (relatively easy to understand) primer on elliptic curve cryptography
• EdDSA, Ed25519, Ed25519-IETF, Ed25519ph, Ed25519ctx, HashEdDSA, PureEdDSA, WTF?

Keys §

Public key §

• Public key can be derived from the private key
• They are meant to be public, fr. No point trying to keep it secret.
• The design does not guarantee that the public key cannot be derived from the encrypted material.

Private key §

• The design guarantee that the private key cannot be derived from the encrypted material.

Encrypting §

See Encryption

Signing §

• ORDERING MATTERS. Do due diligence. See this
  • It’s suggested that we do sign-then-encrypt
  • Because with encrypt-then-sign the sender might be unaware of the actual content of the message.
• When X wants to send you an encrypted message, they’ll use your public key.
• So now you got a package that you think came from X.
  • You open it, it’s encrypted.
  • You decrypt it, it gets decrypted. Nice.
  • But now you have no idea, if that package was tampered or MITM’ed or something, because anyone with access to the public key can easily create a file that will pass any file modification tests
  • This is why we need signing
• When signing, X can sign the message with their private key.
  • Now if you want to verify if it was X that sent the message
  • You use X’s public key to verify that the message certainly came from X
  • And ofc then use your own private key to decrypt rest of the message for yourself.