Cryptography for [web]app developers

Cryptography for [web]app developers

Sean Comeau

Who am I

• Security and crypto enthusiast

• Not a professional cryptographer

• Can probably judge whether or not your project requires professional advice

Why should you care?

What crypto can and can't do

Encryption /Confidentiality

Authentication /Integrity

Identification /Authenticity

Talk Outline

• One-way hash functions

• Private key cryptography

• Public key cryptography

• Digital signatures

• Secure pseudorandom number generation

Hash functions

• md5("hello") = 5d41402abc4b2a76b9719d911017c592

• sha1("hello") = aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d

• sha256("hello") = 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824

Ideal hash functions

• easy to compute the hash value for any given message

• infeasible to generate a message that has a given hash

• infeasible to modify a message without changing the hash

• infeasible to find two different messages with the same hash

Leakage of our secrets (monthly)

Private Key Crypto

• Cleartext = the unencrypted, readable data we care about.

• Ciphertext = the message after encryption, the data the adversary gets to see.

• Key = the secret required to encrypt and decrypt the message

• Encryption: ciphertext = f(key, cleartext)

• Decryption: cleartext = f(key, ciphertext)

Block vs. Stream

o Avoid stream ciphers.

Use AES with 256 bit keys. It’s a block cipher.

Modes of Operation

XTS for file systems

CTR mode for anything else

Public Key Crypto

• Use RSA

• Use 4096 bit keys

• Great for securely logging things

• Just use GnuPG

• If you can’t just use GnuPG, get help

• Just need key agreement? Curve25519

Asymmetric Digital Signatures

Symmetric Digital Signatures

Random

Questions?