< All Cryptography Notes

redvers | Cryptography | Section 1

By: redvers | Related Course: Cryptography | Published: February 23, 2017 | Modified: March 7, 2017
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NotepadIntro to cryptography, part 2

Privacy/confidentiality, integrity, authenticity, non-repudiation. (Rearrange them a bit, and the acronym is PAIN ;-| )

Plaintext + initialization vector (to add randomness) + algorithm + key = ciphertext

All symmetric (= private, secret, shared, or session) algorithms/ciphers are either block or stream.

Asymmetric: discrete logarithms or factorisation.

AES = advanced encryption standard. The default and most common one.

3DES — powerful but extremely processor-intensive.


RC4 — the most common stream cipher. Not considered nearly as secure as a block cipher such as AES. Fast to encrypt, but also fast to decrypt if you have the key. WEP and WPA use RC4; WPA2 uses AES.

A pseudo-random key is matched up with plain text bit by bit. If the values are the same (both 0s or both 1s), the bit gets coded as 0; if they’re different, it’s 1.


Symmetric cryptography


  • Out-of-band key distribution (need to figure out a way to provide the key to the other parties)
  • Not scalable
  • No authenticity, integrity, or non-repudiation


  • Very fast


Asymmetric cryptography

Privacy: encrypt with the receiver’s public key.

Integrity: hash.

Authenticity: encrypt the hash with the sender’s private key (so it can be decrypted with the sender’s public key).

Non-repudiation: integrity combined with authenticity (= digital signature; can’t deny either the source or the content)


Asymmetric keys are used to establish symmetric data exchange sessions.

  • Https –> A client requests the server’s public key
  • Receives the key along with the certificate digitally signed by a trusted authority, e.g. Verisign (proof that they key comes from the server the client is trying to reach and not an impersonator)
  • Client generates a symmetric key, encrypts it with the asymmetric key, sends back to server
  • Establishes a symmetric session to speed things up


PKI — public key infrastructure

CRL — certificate revocation list

OCSP — online certificate status protocol

TPM — trusted platform module (a chip on the motherboard for full disk encryption)

NotepadKey concepts of cryptography

Symmetric algorithms mnemonic:

”AES, DES, 3 guys had an Idea to Cast out a rod 2Fish for Blowfish but instead they received Serpents in the Rain.”

(AES, DES, 3DES, Idea, Cast, 2Fish, Blowfish, Serpents, Rijndael, PGP, OTP (one-time password))

Asymmetric algorithms:

DEREKS: DH (Diffie-Hellman), El Gamal, Eliptical Curve, Knapsack, DSA (digital signature algorithm)


< All Cryptography Notes
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