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CryptoAlgorithms.md

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Crypto Algorithms

Cryptographic primitives

Password-based key derivation function (PKDF)

Let MasterKey be the Argon2id result of the password with

  • output length: 32 bytes
  • iterations: 20
  • memory usage: 64 MiB
  • salt: 00000000000000000000000000000000 (16 bytes)

and derive the subkeys with

  • algorithm: Blake2b
  • output length: 32 bytes
  • message: empty string
  • key: the MasterKey
  • personalization: ascii("CHATv001") plus 8 bytes of zero-padding to the right (16 bytes)
  • salt: ??000000000000000000000000000000 (16 bytes), where ?? is one subkey-dependent byte

Subkey salt values

  • LoginKey: 0x01
  • PasswordVerificationKey: 0x02
  • EncryptionPrivkeyEncryptingKey: 0x03

Cryptographic hash

  • BLAKE2b

Known use cases: symmetric key to ID.

Fingerprints

  • A BLAKE2b-128 hash, hex encoded

Symmetric authenticated encryption

  • IETF ChaCha20-Poly1305

The algorithm identifier "chacha20poly1305-ietf-nonce12prefixed" means ChaCha20-Poly1305 (IETF variant) with a 12 bytes nonce that is prefixed to the ciphertext.

Signatures

  • Ed25519

Public-key encryption

  • libsodium's sealed boxes

Signatures

Signatures are generated using Devices' Keypairs. They are stored as a string in the form <deviceId>,base64(<rawSignature>).

Device idOwnerIdSignature

rawSignature = ed25519(utf8(
    id + "|" + ownerId
))

ConversationPermission signature

rawSignature = ed25519(utf8(
    conversationId + "|" +
    conversationKeyId + "|" +
    base64(conversationKey) + "|" +
    ownerId + "|" +
    creatorId + "|" +
    validFrom
))

Authentication LoginKey signature

In this case, the message to be signed is not constructed from a string but from binary data directly.

rawSignature = ed25519(loginKey)