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Cryptography

Jun 30, 2024, 5 min read

tags : Security, PKI, TLS, Encryption, Hashing

FAQ

What is a construction?

  • A way of combining basic building blocks, such as encryption algorithms, hash functions, and protocols, to create a higher-level cryptographic scheme or system.
  • Examples
    • Block Ciphers : Modes of operations
    • Hashing: Eg. HMAC use a combination of a hash function and a secret key to provide integrity and authenticity.
    • PKI: RSA, ECC(Elliptic Curve Cryptography) etc.
    • Key Exchange: Diffie-Hellman key exchange etc.

PRF?

pseudorandom function

AEAD?

TODO Modes??? (ChatGPT generated)

block

  1. Electronic Codebook (ECB):

    • Each block of plaintext is independently encrypted with the same key.
    • Identical plaintext blocks result in identical ciphertext blocks.

  2. Cipher Block Chaining (CBC):

    • Each plaintext block is XORed with the previous ciphertext block before encryption.
    • Initialization Vector (IV) is used for the first block to break patterns.

  3. Cipher Feedback (CFB):

    • Turns a block cipher into a stream cipher.
    • Previous ciphertext block is fed back into the block cipher to generate a keystream.

  4. Output Feedback (OFB):

    • Similar to CFB but the block cipher encrypts a constant value (IV) to produce a keystream.
    • The keystream is then XORed with the plaintext.

  5. Counter (CTR):

    • Turns a block cipher into a stream cipher.
    • Uses a counter as the input to the block cipher to generate a keystream.
    • The counter is usually combined with a nonce.

  6. Galois/Counter Mode (GCM):

    • Combines the counter mode of operation with Galois field multiplication.
    • Provides both confidentiality and authenticity.
    • Widely used for authenticated encryption.

Stream Cipher Modes:

  1. Synchronous Stream Ciphers:
    • Keyed bits are generated to form a pseudorandom keystream.
    • The keystream is then XORed with the plaintext to produce the ciphertext.
  2. Self-Synchronizing Stream Ciphers:
    • Previous ciphertext bits are used to generate the next key bits.
    • Allows synchronization even if some ciphertext bits are lost or corrupted.
  3. Stream Cipher with Feedback (SCFB):
    • Similar to CFB mode for block ciphers.
    • Uses the previous ciphertext bits to generate the next keystream.
  4. Combining Generators:
    • Multiple keystream generators are combined to improve security.
  5. Filter Generators:
    • Combines a linear feedback shift register (LFSR) with a nonlinear filter.

Passwords

  • PAKE

Encrypt passwords?

  • Storing passwords HMAC encrypted is technique outdated by at least 10 years

SCRAM

The beauty of SCRAM is that both authenticating parties (in this case, your client/application and PostgreSQL) can both verify that each party knows a secret without ever exchanging the secret. In this case, the secret is a PostgreSQL password!

Basics

  • The basis of SCRAM is that both a client and a server
    • will send each other a series of cryptographic proofs demonstrating that they know the secret (i.e. the password).
    • Needed keys
      • “Client Key”
      • “Server Key” (Server needs to know)
      • “Stored Key” (Server needs to know)

  • Client key

  • Stored key (Derived from Client key)

  • Server key

Flow

You connect to you PG server using normal username and pass only, all this mumbo/jumbo is what your postgrest client library will handle. SCRAM is well supported.

  • Client

    Sends the server a "client proof"

    • client proof = client key XOR client signature
      • client signature = HMAC(one-time info of signing key) + stored key
        • stored key = sha256(client key)

  • Server

    Derives the client key from client proof and verifies if it OK by taking a SHA-256(client key) and comparing it against stored key

    i.e Server uses the stored key for two different usecases

    • Since server has stored key, and it only sends the one time info
      • It can generate the client signature
    • So once it gets the client proof from a client,
      • client key = client_signature XOR client_proof

  • Flow when we’re using PGBouncer

    • How to SCRAM in Postgres with pgBouncer | Crunchy Data Blog
    • The above nicely describes how 2 parties are talking, when we have a 3rd party(pgbouncer) things get a little bit different
    • client key = salt(password) , where salt is sent by the server at initial request
    • stored key = sha256(client key)
    • Flow
      • In this flow, we’re trusting the pgbouncer instance.
      • client sends client proof to pgbouncer
      • pgbouncer extracts the client key just liek pg would
      • pgbouncer now generates a new client proof where pgbouncer is now the actual client and sends the new client proof to pg

Comparison with md5(See Hashing) for PostgreSQL

'md5' || md5(password + username)
  • If you have access to someone’s
    • username / password combination
    • or their PostgreSQL MD5-styled hash
  • You could log into any PostgreSQL cluster where the user has the same username / password.
  • The md5 has a challenge/response thing but the password itself is not salted!

  • Attacks

    • Replay attacks: These are prevented by md5 because there is a challenge/response component, so the md5 hash which is stored is not what is sent across the wire.
    • Gaining dump of the at-rest-hash: If an attacker somehow gains access to the at-rest-hash, then they can figure out the actual password.
      • Using something like SCRAM prevents this.

Cipher

See Cipher

Other topics

Forward secrecy

  • If you lose your key to an attacker today
    • They still can’t go back and read yesterday’s messages
    • They had to be there with the key yesterday to read them
  • Layman implementation
    • 2 secret keys: a short term session key and a longer-term trusted key.
    • The session key is ephemeral (usually the product of a DH exchange)
    • The trusted key signs the session key.
  • See Forward Secrecy {The Call of the Open Sidewalk}

Things I keep hearing about

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