(14) Simple Hash Functions

• Hashes (or message digests) are well-known in computer science
• Hash values are of fixed and short length and make it easier to compare data
• Collisions are the most problematic case in hash algorithms
  • length in bytes is even/uneven: risk of collision is 50%
  • length in bytes: collisions happen when data is simply replaced
• Hashing is often done on an ad-hoc basis
  • lengths are a form of hashes
  • time stamps are a form of hashes
• Hashing is also used for computing error correction codes
  • many technologies (hard drives, networks, …) use Cyclic Redundancy Code (CRC) [http://en.wikipedia.org/wiki/Cyclic_redundancy_check] hashes
  • error correction codes computation has to be done very fast

(15) One-Way Function

• One-way functions are cryptographically safe hashes [Simple Hash Functions (1)] (a.k.a. cryptographic hash)
  • very hard to find an input producing a given output
  • very hard to find two inputs producing the same output (collision)
  • small changes in input should cause entirely different output
• MD5 [http://en.wikipedia.org/wiki/MD5] has been a very popular cryptographic hash
  • MD5 turns data into a 128bit hash value (often encoded as 32 hex characters)
  • various security flaws have been discovered over the years
  • MD5 Hash Function → e367cdcfd2e16f28e81bbc58c9d3339c
• SHA [http://en.wikipedia.org/wiki/SHA] is the most popular cryptographic hash in use today
  • SHA-1 turns data into a 160bit hash value (often encoded as 40 hex characters)
  • SHA-1 Hash Function → afd38b77186afba44123093827c2e0f3732726c4

(17) Plausible Encryption

• Secret-Key is was most people think of when thinking of encryption
  • symmetric cryptography is another popular term
• One key for encryption and decryption
• Revealing the key makes encrypted data openly readable
  • there must be a secure channel to transport keys, such as diplomatic pouches [http://en.wikipedia.org/wiki/Diplomatic_bag]
• Good for long-term relationships with few partners
  • exchange secret keys as part of the initial setup of a relationships
  • adding partners requires a secure channel for key exchange
  • changing keys requires a secure channel for key exchange
• Almost impractical in an environment with many ad-hoc partners

(18) Notice the Arrow

(20) Implausible Encryption

• Public-Key intuitively is hard to accept as a concept
  • asymmetric cryptography is another popular term
• Key pairs of one public and one secret key
  • key generation is the process of generating these key pairs
• The public key can be made available to the public
  • only the secret key can do the inverse operation of the public key
• Good for short-term relationships with many partners
  • publish your public key so that it can be used worldwide
  • everybody can encrypt data using the public key
  • only the owner of the secret can can decrypt the message and read it
• Computationally expensive and not good for a large amounts of data

(21) No Arrow Here …

(23) Building Secure Applications

• Cryptographic primitives in most cases are not sufficient
  • they provide basic functionality for fundamental tasks
  • they must by combined to provide solutions for real-world problems
• Typical problem #1: How to ensure key authenticity
  • with insecure keys, the majority of cryptographic methods is worthless
• Typical problem #2: How to communicate securely without shared keys
  • many interesting scenarios are based on ad-hoc interactions
  • secret-key does not work, public-key needs to verify the peer
• Typical problem #3: How to check authenticity and integrity of data
  • integrity can be done with checksums, but these could be forged
  • authenticity needs a cryptographically secure way of combining identity and data

(24) Certificate

• Certificates are digital signatures issued by a trusted party
  • most digital signatures are created with certified public keys
  • this means the digital signature is created based on a digitally signed key
• Who can you trust on the Web?
  • trust can only start to grow based on initial trust in something
  • many systems come with pre-installed trust (root certificates)
  • certificates from other issuers will cause browsers to complain [https://katapultmedia.com/]
• Certificates (like domain names) are a very easy way to make money
  • in theory there are different levels of certificates with different levels of identity checking
  • in practice most sites choose the cheapest one that does not give an error message

(25) Creating a Digital Signature

(26) Verifying a Digital Signature