Merge #13756: wallet: "avoid_reuse" wallet flag for improved privacy

5ebc6b0eb267e0552c66fffc5e5afe7df8becf80 bitcoind: update -avoidpartialspends description to account for auto-enable for avoid_reuse wallets (Karl-Johan Alm)
ada258f8c8f92d44d893cf9f22d15acdeca40b1a doc: release notes for avoid_reuse (Karl-Johan Alm)
27669551da52099e4a6a401acd7aa32b32832423 wallet: enable avoid_partial_spends by default if avoid_reuse is set (Karl-Johan Alm)
8f2e208f7c0468f9ba92bc789a698281b1c81284 test: add test for avoidreuse feature (Karl-Johan Alm)
0bdfbd34cf4015de87741ff549db35e5064f4e16 wallet/rpc: add 'avoid_reuse' option to RPC commands (Karl-Johan Alm)
f904723e0d5883309cb0dd14b826bc45c5e776fb wallet/rpc: add setwalletflag RPC and MUTABLE_WALLET_FLAGS (Karl-Johan Alm)
8247a0da3a46d7c38943ee0304343ab7465305bd wallet: enable avoid_reuse feature (Karl-Johan Alm)
eec15662fad917b169f5e3b8baaf4301dcf00a7b wallet: avoid reuse flags (Karl-Johan Alm)
58928098c299efdc7c5ddf2dc20716ca5272f21b wallet: make IsWalletFlagSet() const (Karl-Johan Alm)
129a5bafd9a3efa2fa16d780885048a06566d262 wallet: rename g_known_wallet_flags constant to KNOWN_WALLET_FLAGS (Karl-Johan Alm)

Pull request description:

  Add a new wallet flag called `avoid_reuse` which, when enabled, will keep track of when a specific destination has been spent from, and will actively "blacklist" any new UTXOs which send to an already-spent-from destination.

  This improves privacy, as a payer could otherwise begin tracking a payee's wallet by regularly peppering a known UTXO with dust outputs, which would then be scooped up and used in payments by the payee, allowing the payer to map out (1) the inputs owned by the payee and (2) the destinations to which the payee is making payments.

  This replaces #10386 and together with the (now merged) #12257 it addresses #10065 in full. The concerns raised in https://github.com/bitcoin/bitcoin/pull/10386#issuecomment-302361381 are also addressed due to #12257.

  ~~Note: this builds on top of #15780.~~ (merged)

ACKs for commit 5ebc6b:
  jnewbery:
    ACK 5ebc6b0eb
  laanwj:
    Concept and code-review ACK 5ebc6b0eb267e0552c66fffc5e5afe7df8becf80
  meshcollider:
    Code review ACK https://github.com/bitcoin/bitcoin/pull/13756/commits/5ebc6b0eb267e0552c66fffc5e5afe7df8becf80
  achow101:
    ACK 5ebc6b0eb267e0552c66fffc5e5afe7df8becf80 modulo above nits

Tree-SHA512: fdef45826af544cbbb45634ac367852cc467ec87081d86d08b53ca849e588617e9a0a255b7e7bb28692d15332de58d6c3d274ac003355220e4213d7d9070742e

1 files changed, 39 insertions, 0 deletions

diff --git a/doc/release-notes-13756.md b/doc/release-notes-13756.md
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+Coin selection
+--------------
+
+### Reuse Avoidance
+
+A new wallet flag `avoid_reuse` has been added (default off). When enabled,
+a wallet will distinguish between used and unused addresses, and default to not
+use the former in coin selection.
+
+(Note: rescanning the blockchain is required, to correctly mark previously
+used destinations.)
+
+Together with "avoid partial spends" (present as of Bitcoin v0.17), this
+addresses a serious privacy issue where a malicious user can track spends by
+peppering a previously paid to address with near-dust outputs, which would then
+be inadvertently included in future payments.
+
+New RPCs
+--------
+
+- A new `setwalletflag` RPC sets/unsets flags for an existing wallet.
+
+
+Updated RPCs
+------------
+
+Several RPCs have been updated to include an "avoid_reuse" flag, used to control
+whether already used addresses should be left out or included in the operation.
+These include:
+
+- createwallet
+- getbalance
+- sendtoaddress
+
+In addition, `sendtoaddress` has been changed to enable `-avoidpartialspends` when
+`avoid_reuse` is enabled.
+
+The listunspent RPC has also been updated to now include a "reused" bool, for nodes
+with "avoid_reuse" enabled.