feat: enable random anti-fee sniping #5

aagbotemi · 2025-05-06T11:35:29Z

This PR implements Anti-Fee-Sniping with randomization as discussed in issue #4

Notes to the reviewers

The implementation adds randomization to anti-fee-sniping behavior:

Uses a 50/50 chance to choose between nLockTime and nSequence (when possible)
Adds a 10% chance to set either value further back in time (by a random value between 0-99)
Detects taproot inputs and their confirmation status

I've signed all my commits
I ran cargo fmt and cargo clippy before committing
I've added docs for the new feature

Closes #4

ValuedMammal · 2025-05-19T13:19:46Z

Thanks @aagbotemi I'm still in the process of reviewing. I agree it would be nice to resolve the clippy error - my other suggestion would be to globally allow the lints in lib.rs for now and open an issue to fix them in a future PR, but in general I agree with your approach to simply Box the large enum variants.

aagbotemi · 2025-05-19T13:25:58Z

Thanks @aagbotemi I'm still in the process of reviewing.

Thank you @ValuedMammal. Clippy error and large enum variant error in the CI build has been fixed. For subsequent ones(if there is), I'll open another for it.

ValuedMammal

I have some review notes

For imports, prefer alloc over std. This way we can actually support no-std.
I want to see a test of some kind that creates a PSBT with enable_anti_fee_sniping and checks the expected values of the locktime and/or sequence.

Cargo.toml

src/selection.rs

src/utils.rs

aagbotemi · 2025-05-28T20:12:39Z

I have some review notes

Noted.
alloc is now preferred over std.
adding test to check enable_anti_fee_sniping

Cargo.toml

src/selection.rs

src/utils.rs

ValuedMammal · 2025-06-01T16:31:43Z

We should also be able to remove the clippy allow attributes in lib.rs.

aagbotemi · 2025-06-01T23:35:10Z

We should also be able to remove the clippy allow attributes in lib.rs.

Alright, I will remove the clippy allow attributes in lib.rs

ValuedMammal · 2025-06-09T13:15:39Z

I'm still in favor of using Box on PlanOrPsbtInput and ScriptSource::Descriptor.

aagbotemi · 2025-06-09T21:51:45Z

I'm still in favor of using Box on PlanOrPsbtInput and ScriptSource::Descriptor.

Fixed! I've updated the code to use Box.

src/selection.rs

src/utils.rs

src/lib.rs

ValuedMammal · 2025-10-26T16:07:51Z

In general I think the git history would be cleaner if we didn't merge with merge commits but instead rebase the commits in this PR.

aagbotemi · 2025-10-26T23:30:50Z

In general I think the git history would be cleaner if we didn't merge with merge commits but instead rebase the commits in this PR.

Alright, I've done the cleanup

ValuedMammal · 2025-10-28T20:09:28Z

Approach ACK.

nymius · 2025-11-06T13:40:30Z

src/utils.rs

+    Ok(())
+}
+
+/// Returns true with probability 1/n.


I would add a comment explaining why we are using these functions and not rand::random_bool.

The anti fee sniping implementation started from here. So, I have been using rand_core for the implementation since then. I can change to rand if we agree to use that.

cc: @ValuedMammal, we have access to using Rng trait using rand

nymius · 2025-11-06T13:42:39Z

src/utils.rs

+}
+
+/// Returns a random value in the range [0, end).
+fn random_range(rng: &mut OsRng, end: u32) -> u32 {


Same here for rand::random_range. Also, I would mention here we are using unbiased methods to pick from the range.

nymius · 2025-11-11T13:53:05Z

src/utils.rs

+            // if random_probability(TEN_PERCENT_PROBABILITY_RANGE) {
+            // let random_offset = random_range(MAX_RANDOM_OFFSET);


nit: remove these comments.

nymius · 2025-11-11T13:53:25Z

src/utils.rs

+        || must_use_locktime
+        || taproot_inputs.is_empty()
+        || random_bool(FIFTY_PERCENT_PROBABILITY_RANGE);
+    // || random_probability(FIFTY_PERCENT_PROBABILITY_RANGE);


nit: remove this comment

nymius · 2025-11-11T14:03:30Z

examples/anti_fee_sniping.rs

+    // - RBF is disabled, OR
+    // - Any input requires locktime (non-taproot, unconfirmed, or >65535 confirmations), OR
+    // - There are no taproot inputs, OR
+    // - Random 50/50 coin flip chose locktime


To make the example more dynamic I would set up all the conditions to make the example dependent of this 50/50 coin flip. Because I'm never hitting the sequence anti fee sniping branch below.

Okay. I have fixed it.

nymius · 2025-11-11T14:03:46Z

examples/anti_fee_sniping.rs

+        if height_val >= min_expected && height_val <= max_expected {
+            println!("✓ Locktime is within expected range");
+        } else {
+            println!("⚠ Locktime is outside expected range");


Is this line reachable? I guess you want to show the feature is working, but if anti fee sniping works as intended, this is not going to ever happen. This isn't a test. Maybe showing the nLockTime or nSequence in a meaningful way is a better approach for the example.

Thank you for the review. I have fixed it.

nymius · 2025-11-11T23:10:32Z

examples/anti_fee_sniping.rs

+
+    println!("nLockTime approach used: {} times", locktime_count);
+    println!("nSequence approach used: {} times", sequence_count);
+    println!("Both approaches provide anti-fee-sniping protection:");


nit: this isn't providing new information, I recommend to remove it

@aagbotemi sorry for the confusion here, but I was just referring to line 149 here, 147 and 148 were good

nymius · 2025-11-12T13:07:01Z

Cargo.toml


 [dependencies]
 miniscript = { version = "12", default-features = false }
 bdk_coin_select = "0.4.0"


@ValuedMammal what do you think about rand as additional dependency? I don't want to fall in the slippery slope of adding new dependencies with each feature, but I wouldn't implement anti-fee sniping without rand, so, is anti-fee sniping something bdk-tx should ship by default? Should we consider adding it as a feature dependency?

Ideally we want to support no-std, so my question is how are we providing a source of randomness for no-std environments.

The approach we took for bdk_wallet is to depend on rand_core in order to have RngCore trait and essentially split the interface based on whether std feature is enabled. If std, then we have access to rand via bitcoin::secp256k1::rand. If no-std then the user can pass in something that implements RngCore.

/// Create PSBT. #[cfg(feature = "std")] pub fn create_psbt(&self, params: PsbtParams) -> Result<bitcoin::Psbt, CreatePsbtError> { self.create_psbt_with_rng(params, &mut rand::thread_rng()) } /// Create PSBT with `rng`. pub fn create_psbt_with_rng(&self, params: PsbtParams, rng: &mut impl RngCore) -> Result<bitcoin::Psbt, CreatePsbtError> { ... }

The other option is to feature-gate the enable_anti_fee_sniping option entirely but that seems a bit less useful.

@aagbotemi could you adapt your code here and the current state of the PR to the above solution? Using feature gates

Alright, will do that.

I think you are missing the std implementation using rand full features.

Looks good overall, not sure if any review comments are still unresolved.

@ValuedMammal I'm not sure we have fulfilled this requirement yet. Do you think we should roll a custom random range even if the user has std available?

ValuedMammal · 2025-11-14T18:35:17Z

Sorry about the merge conflict.

aagbotemi · 2025-11-14T21:20:25Z

Sorry about the merge conflict.

Alright. I will rebase to resolve the conflict.

aagbotemi · 2025-11-19T20:48:55Z

Currently running cargo build is throwing error

error[E0433]: failed to resolve: could not find `rand` in `key`
   --> src/selection.rs:201:74
    |
201 | ...::bitcoin::key::rand::thread_rng())
    |                    ^^^^ could not find `rand` in `key`
    |
note: found an item that was configured out
   --> /home/abiodun/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/bitcoin-0.32.7/src/crypto/key.rs:29:20
    |
29  | pub use secp256k1::rand;
    |                    ^^^^
note: the item is gated behind the `rand-std` feature
   --> /home/abiodun/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/bitcoin-0.32.7/src/crypto/key.rs:28:7
    |
28  | #[cfg(feature = "rand-std")]
    |       ^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0433`.
error: could not compile `bdk_tx` (lib) due to 1 previous error

The solution I could think of is moving bitcoin from dev-dependencies to dependencies. But it's a very big dependency.

EDIT: another solution is to add the rand crate, which is better (I would like to know if it's okay we want to add a new dependency)

src/utils.rs

src/selection.rs

ValuedMammal · 2025-12-03T02:19:04Z

Looks good overall, not sure if any review comments are still unresolved.

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Update Cargo.toml for initial release #6

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[test] Add test_enable_anti_fee_sniping #10

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refactor: remove unnecessary clippy allow attributes

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Release tracking 0.2.0 #30

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feat: implement anti-fee-sniping protection

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		// if random_probability(TEN_PERCENT_PROBABILITY_RANGE) {
		// let random_offset = random_range(MAX_RANDOM_OFFSET);

feat: enable random anti-fee sniping #5

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feat: enable random anti-fee sniping #5

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