Essence of Data

• Hashes (or message digests) are a well-known principle in computer science
  • fast to compute (the goal is to make data handling more efficient)
  • few collisions (there are always collisions because of the smaller size)
  • checksums and Cyclic Redundancy Check (CRC) are popular hashes
• One-way functions are cryptographically safe hashes
  • not just for detecting errors, but also for preventing tampering
  • often referred to as cryptographic hash or digital fingerprint
• One-way functions must satisfy additional criteria
  • it must be very hard to find an input producing a given output
  • it must be very hard to find two inputs producing the same output (collision)

Reducing Data

Plausible Encryption

• Secret-Key is was most people think of when thing of encryption
  • symmetric cryptography is another popular term
• One key for encryption and decryption
• Losing the key makes encrypted data openly readable
  • there must be a secure channel to transport keys
• 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

Notice the Arrow

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

No Arrow Here …

And No Arrow Here …

Encrypted Fingerprints

• Hashes are used to check data integrity
• One-Way Functions are used to check data integrity securely
  • it is not possible to reverse engineer data for a given hash
• Signed hashes can be used to ensure data authenticity
  • if the hash sum is signed, it cannot be changed
  • if the data is changed, its hash will not match the signed hash
• Digital signatures work as long as the hash can be securely signed
  • there must be a trusted public key for checking the hash signature

Creating a Digital Signature

Verifying a Digital Signature

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
• 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

Encrypted Keys

• Public-Key cryptography is computationally expensive
  • it is possible to encrypt all traffic using asymmetric key pairs
  • this generates considerably more load on the server side
• Combining public- and secret-key cryptography
  1. check the public key for authenticity (using a Certificate)
  2. generate a key for a secret-key encryption scheme
  3. use the public key to securely transmit the secret key
  4. use the secret key for securely transmitting the payload
• Combines the advantages of both methods
  • the lower complexity of secret-key algorithms
  • the ability of public-key algorithms to work without a secure channel