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KMAC256 is a keyed hash function standardised by NIST in SP 800-185. It is built on the SHA-3 Keccak sponge and provides both message authentication and key derivation in a single, domain-separable construction. Umbra uses KMAC256 as the sole key derivation function across the entire key hierarchy. Every derived key in the system - master viewing key, X25519 keypairs, Poseidon private key, and all rotation offsets - is produced by the same primitive: 
output = KMAC256(
  key   = UTF8("Umbra Privacy - {domain_separator}"),
  msg   = master_seed,
  dkLen = 64,
  S     = personalization_string,
)

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Customisation Parameter

The S (customization) parameter encodes the protocol version, algorithm name, scheme name, and network: 
umbra/1.0.0|kmac256/1.0.0|kdf/1.0.0|mainnet
This ensures that:
  • Changing the network produces a completely different key space
  • Upgrading the protocol version rotates all keys without changing the wallet
  • Domain separation prevents key reuse across different key types within the same session

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References