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kmd - Key Management Daemon

Overview

kmd is the Key Management Daemon, the process responsible for securely managing spending keys. It is the implementation of the design specified here.

Useful facts

  • kmd has a data directory separate from algod's data directory. By default, however, the kmd data directory is in the kmd subdirectory of algod's data directory.
  • kmd starts an HTTP API server on localhost:7833 by default.
  • You talk to the HTTP API by sending json-serialized request structs from the kmdapi package.

Preventing memory from swapping to disk

kmd tries to ensure that secret keys never touch the disk unencrypted. At startup, kmd tries to call mlockall in order to prevent the kernel from swapping memory to disk. You can check kmd.log after starting kmd to see if the call succeeded.

In order for the mlockall call to succeed, your kernel must support mlockall, and the user running kmd must be able to lock the necessary amount of memory. On many linux distributions, you can achieve this by calling sudo setcap cap_ipc_lock+ep /path/to/kmd. We also provide a make target for this: run make capabilities from the go-algorand project root.

Project structure

  • ./
    • api/v1/
      • This folder contains all of the HTTP handlers for the kmd API V1. In general, these handlers each parse a kmdapi.APIV1Request, and use it to run commands against a wallet.
      • Initializing these handlers requires passing a session.Manager to handle wallet auth and persistent state between requests.
    • client/
      • The client package provides client.KMDClient. client.KMDClient.DoV1Request infers the HTTP endpoint and method from the request type, serializes the request with msgpack, makes the request over the unix socket, and deserializes a kmdapi.APIV1Response.
      • The client package also provides wrappers for these API calls in wrappers.go
    • config/
      • This folder contains code that parses kmd_config.json and merges values from that file with any default values.
    • lib/
      • This folder contains the kmdapi package, which provides the canonical structs used for requests and responses.
    • server/
      • The server package is in charge of starting and stopping the kmd API server.
    • session/
      • The session package provides session.Manager, which allows users to interact with wallets without having to enter a password repeatedly. It achieves this by temporarily storing wallet keys in memory once they have been decrypted.
    • wallet/
      • driver
        • This folder contains the definitions of a "Wallet Driver", as well as the "SQLite Wallet Driver", kmd's default wallet backend.
        • Wallet Drivers are responsible for creating and retrieving Wallets, which store, retrieve, generate, and perform cryptographic operations on spending keys.