diff --git a/Cargo.lock b/Cargo.lock
index b19cef5..9ca30b3 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -290,6 +290,10 @@ version = "0.1.0"
 [[package]]
 name = "gatekeeper"
 version = "0.1.0"
+dependencies = [
+ "keystore",
+ "zeroize",
+]
 
 [[package]]
 name = "generic-array"
diff --git a/crates/servers/gatekeeper/Cargo.toml b/crates/servers/gatekeeper/Cargo.toml
index 5db048a..96e5c09 100644
--- a/crates/servers/gatekeeper/Cargo.toml
+++ b/crates/servers/gatekeeper/Cargo.toml
@@ -4,3 +4,5 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+keystore = { workspace = true }
+zeroize = { workspace = true }
diff --git a/crates/servers/gatekeeper/src/flow.rs b/crates/servers/gatekeeper/src/flow.rs
new file mode 100644
index 0000000..9a64454
--- /dev/null
+++ b/crates/servers/gatekeeper/src/flow.rs
@@ -0,0 +1,189 @@
+//! What the gatekeeper does, over its three ports: the keystore device,
+//! randomness, and the owner (`Prompt`). No kernel, no hardware: tested on
+//! the host.
+
+use keystore::{Block, Copies, KdfParams, Key, Outcome, RECORD_SIZE, Random, Record};
+use zeroize::Zeroize;
+
+/// The keystore device: the record's two copies.
+pub trait Keystore {
+    fn read(&mut self, copy: usize, out: &mut [u8; RECORD_SIZE]) -> bool;
+    fn write(&mut self, copy: usize, data: &[u8; RECORD_SIZE]) -> bool;
+}
+
+/// A passcode as typed, wiped from memory when dropped.
+pub struct Secret {
+    bytes: [u8; MAX_SECRET],
+    len: usize,
+}
+
+pub const MAX_SECRET: usize = 64;
+
+impl Secret {
+    pub const fn new() -> Self {
+        Self { bytes: [0; MAX_SECRET], len: 0 }
+    }
+
+    pub fn set(&mut self, typed: &[u8]) {
+        self.bytes.zeroize();
+        let n = typed.len().min(MAX_SECRET);
+        self.bytes[..n].copy_from_slice(&typed[..n]);
+        self.len = n;
+    }
+
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.bytes[..self.len]
+    }
+
+    /// The same bytes, compared without stopping at the first difference.
+    fn same(&self, other: &Secret) -> bool {
+        let diff = self.bytes.iter().zip(&other.bytes).fold(0u8, |d, (a, b)| d | (a ^ b));
+        (diff | (self.len ^ other.len) as u8) == 0 && self.len == other.len
+    }
+}
+
+impl Default for Secret {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Drop for Secret {
+    fn drop(&mut self) {
+        self.bytes.zeroize();
+    }
+}
+
+/// The owner, on the screen and keyboard.
+pub trait Prompt {
+    fn say(&mut self, text: &str);
+    /// Shows `question` and reads a passcode without showing it; false if
+    /// nothing could be read.
+    fn secret(&mut self, question: &str, out: &mut Secret) -> bool;
+}
+
+/// Why the gatekeeper could not hand over a key.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Failure {
+    /// The keystore could not be read or written.
+    Device,
+    /// No randomness for keys and salts.
+    Random,
+    /// The owner could not be asked.
+    Prompt,
+}
+
+/// Wrong passcodes in a row that wipe the keys.
+pub const THRESHOLD: u8 = 10;
+/// Shortest passcode accepted at setup.
+pub const MIN_PASSCODE: usize = 4;
+
+/// The keystore's newest record, and where the next write goes.
+fn load(ks: &mut impl Keystore) -> Result<(Option<Record>, usize), Failure> {
+    let (mut a, mut b) = ([0u8; RECORD_SIZE], [0u8; RECORD_SIZE]);
+    if !ks.read(0, &mut a) || !ks.read(1, &mut b) {
+        return Err(Failure::Device);
+    }
+    let copies = Copies { copies: [&a, &b] };
+    Ok((copies.newest().map(|(r, _)| r), copies.next_copy()))
+}
+
+fn store(ks: &mut impl Keystore, copy: usize, r: &Record) -> Result<(), Failure> {
+    if ks.write(copy, &r.encode()) { Ok(()) } else { Err(Failure::Device) }
+}
+
+/// Destroys the key slots: noise over both copies, twice, then blank (the
+/// device then looks never set up). Marked as wiping first, so a power cut
+/// part way is finished on the next boot.
+fn wipe(ks: &mut impl Keystore, record: &Record, next: usize, rng: &mut impl Random) -> Result<(), Failure> {
+    let first = record.wiping(rng).ok_or(Failure::Random)?;
+    let second = first.wiping(rng).ok_or(Failure::Random)?;
+    for r in [&first, &second] {
+        store(ks, next, r)?;
+        store(ks, 1 - next, r)?;
+    }
+    let blank = [0u8; RECORD_SIZE];
+    if ks.write(next, &blank) && ks.write(1 - next, &blank) { Ok(()) } else { Err(Failure::Device) }
+}
+
+/// Asks for a new passcode twice until both match; `optional` allows none.
+fn choose(prompt: &mut impl Prompt, question: &str, optional: bool, out: &mut Secret) -> Result<(), Failure> {
+    loop {
+        let mut again = Secret::new();
+        if !prompt.secret(question, out) || !prompt.secret("Again: ", &mut again) {
+            return Err(Failure::Prompt);
+        }
+        if !out.same(&again) {
+            prompt.say("They differ; once more.\n");
+        } else if out.len == 0 && optional {
+            return Ok(());
+        } else if out.len < MIN_PASSCODE {
+            prompt.say("At least 4 characters.\n");
+        } else {
+            return Ok(());
+        }
+    }
+}
+
+/// A device never set up (or wiped): the owner chooses a passcode, and
+/// may choose a duress one. Returns the new master key.
+fn setup(ks: &mut impl Keystore, rng: &mut impl Random, prompt: &mut impl Prompt, params: KdfParams, memory: &mut [Block]) -> Result<Key, Failure> {
+    prompt.say("This device has no passcode yet. Choose one; it protects everything on it.\n");
+    let mut real = Secret::new();
+    choose(prompt, "New passcode: ", false, &mut real)?;
+    prompt.say("A duress passcode wipes the device when it is entered instead. Leave it empty for none.\n");
+    let mut duress = Secret::new();
+    loop {
+        choose(prompt, "Duress passcode: ", true, &mut duress)?;
+        if duress.len == 0 || !duress.same(&real) {
+            break;
+        }
+        prompt.say("It must differ from the passcode.\n");
+    }
+    let duress = (duress.len > 0).then_some(duress.as_bytes());
+    let (record, key) = Record::setup(real.as_bytes(), duress, THRESHOLD, params, memory, rng).ok_or(Failure::Random)?;
+    store(ks, 0, &record)?;
+    store(ks, 1, &record)?;
+    Ok(key)
+}
+
+/// Authenticates the owner and returns the master key: setting one up on
+/// a fresh device, wiping on duress or too many failures (the device then
+/// looks fresh, and is set up anew).
+pub fn run(ks: &mut impl Keystore, rng: &mut impl Random, prompt: &mut impl Prompt, params: KdfParams, memory: &mut [Block]) -> Result<Key, Failure> {
+    loop {
+        let (record, next) = load(ks)?;
+        let Some(record) = record else {
+            return setup(ks, rng, prompt, params, memory);
+        };
+        if record.must_wipe() {
+            wipe(ks, &record, next, rng)?;
+            continue;
+        }
+        let mut typed = Secret::new();
+        if !prompt.secret("Passcode: ", &mut typed) {
+            return Err(Failure::Prompt);
+        }
+        // Counted before it is checked: pulling the power cannot save a guess.
+        let counted = record.next(|r| r.failures = r.failures.saturating_add(1));
+        store(ks, next, &counted)?;
+        match record.unlock(typed.as_bytes(), memory) {
+            Outcome::Real(key) => {
+                store(ks, 1 - next, &counted.next(|r| r.failures = 0))?;
+                return Ok(key);
+            }
+            Outcome::Duress => {
+                // Recorded as worn out first: a wipe cut short resumes.
+                store(ks, 1 - next, &counted.next(|r| r.failures = r.threshold))?;
+                let (worn, next) = load(ks)?;
+                wipe(ks, &worn.ok_or(Failure::Device)?, next, rng)?;
+            }
+            Outcome::Wrong => {
+                let left = counted.threshold.saturating_sub(counted.failures);
+                if left > 0 {
+                    prompt.say(if left == 1 { "Wrong passcode. One try left before the device is wiped.\n" } else { "Wrong passcode.\n" });
+                }
+            }
+        }
+    }
+}
diff --git a/crates/servers/gatekeeper/src/lib.rs b/crates/servers/gatekeeper/src/lib.rs
index 4d4946f..c3074a5 100644
--- a/crates/servers/gatekeeper/src/lib.rs
+++ b/crates/servers/gatekeeper/src/lib.rs
@@ -1,16 +1,14 @@
+//! The gatekeeper (see the vision's Gatekeeper note): authenticates the
+//! owner before anything that touches user data starts, and hands the
+//! master key over. `flow` is the logic; `run` wires the devices.
+
+#![no_std]
 #![forbid(unsafe_code)]
 
-pub fn add(left: u64, right: u64) -> u64 {
-    left + right
-}
+extern crate alloc;
+
+pub mod flow;
 
 #[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
-    }
-}
+#[path = "tests/mod.rs"]
+mod tests;
diff --git a/crates/servers/gatekeeper/src/tests/mod.rs b/crates/servers/gatekeeper/src/tests/mod.rs
new file mode 100644
index 0000000..91579aa
--- /dev/null
+++ b/crates/servers/gatekeeper/src/tests/mod.rs
@@ -0,0 +1,212 @@
+extern crate std;
+
+use crate::flow::*;
+use keystore::{Block, KdfParams, RECORD_SIZE, Random, Record};
+use std::collections::VecDeque;
+use std::string::String;
+use std::vec;
+use std::vec::Vec;
+
+const FAST: KdfParams = KdfParams { memory_kib: 8, passes: 1 };
+
+/// The keystore in memory; `cut_after` writes simulate the power going.
+#[derive(Default, Clone)]
+struct Device {
+    copies: [Vec<u8>; 2],
+    writes: usize,
+    cut_after: Option<usize>,
+    /// The power goes in the middle of the first write that starts a
+    /// wipe: only its start lands.
+    tear_wipe: bool,
+}
+
+impl Device {
+    fn new() -> Self {
+        Self { copies: [vec![0; RECORD_SIZE], vec![0; RECORD_SIZE]], ..Self::default() }
+    }
+
+    fn record(&self) -> Option<Record> {
+        keystore::Copies { copies: [&self.copies[0], &self.copies[1]] }.newest().map(|(r, _)| r)
+    }
+}
+
+impl Keystore for Device {
+    fn read(&mut self, copy: usize, out: &mut [u8; RECORD_SIZE]) -> bool {
+        out.copy_from_slice(&self.copies[copy]);
+        true
+    }
+    fn write(&mut self, copy: usize, data: &[u8; RECORD_SIZE]) -> bool {
+        if self.cut_after.is_some_and(|n| self.writes >= n) {
+            return false;
+        }
+        let starts_wipe = Record::decode(data).is_some_and(|r| r.state == keystore::State::Wiping);
+        if self.tear_wipe && starts_wipe {
+            // Into the slots: the record (and its checksum) is not whole.
+            self.copies[copy][..100].copy_from_slice(&data[..100]);
+            self.cut_after = Some(self.writes);
+            return false;
+        }
+        self.writes += 1;
+        self.copies[copy] = data.to_vec();
+        true
+    }
+}
+
+struct Counter(u8);
+
+impl Random for Counter {
+    fn fill(&mut self, buf: &mut [u8]) -> bool {
+        for b in buf {
+            self.0 = self.0.wrapping_mul(37).wrapping_add(11);
+            *b = self.0;
+        }
+        true
+    }
+}
+
+/// An owner typing from a script; what was shown is kept.
+struct Owner {
+    typing: VecDeque<&'static str>,
+    shown: String,
+}
+
+impl Owner {
+    fn types(lines: &[&'static str]) -> Self {
+        Self { typing: lines.iter().copied().collect(), shown: String::new() }
+    }
+}
+
+impl Prompt for Owner {
+    fn say(&mut self, text: &str) {
+        self.shown.push_str(text);
+    }
+    fn secret(&mut self, question: &str, out: &mut Secret) -> bool {
+        self.shown.push_str(question);
+        let Some(line) = self.typing.pop_front() else { return false };
+        out.set(line.as_bytes());
+        true
+    }
+}
+
+fn memory() -> Vec<Block> {
+    vec![Block::default(); FAST.memory_kib as usize]
+}
+
+fn run_with(dev: &mut Device, owner: &mut Owner) -> Result<keystore::Key, Failure> {
+    run(dev, &mut Counter(5), owner, FAST, &mut memory())
+}
+
+/// A device set up with passcode 1234 and duress 0000; its master key.
+fn set_up() -> (Device, [u8; 32]) {
+    let mut dev = Device::new();
+    let key = run_with(&mut dev, &mut Owner::types(&["1234", "1234", "0000", "0000"])).unwrap();
+    (dev, key.0)
+}
+
+#[test]
+fn a_fresh_device_is_set_up_and_then_unlocks_with_its_passcode() {
+    let (mut dev, key) = set_up();
+    let mut owner = Owner::types(&["1234"]);
+    assert_eq!(run_with(&mut dev, &mut owner).unwrap().0, key, "the same master key every boot");
+    assert_eq!(dev.record().unwrap().failures, 0);
+}
+
+#[test]
+fn setup_insists_on_a_confirmed_passcode_of_four_or_more() {
+    let mut dev = Device::new();
+    let mut owner = Owner::types(&["1234", "1243", "12", "12", "123456", "123456", "", ""]);
+    run_with(&mut dev, &mut owner).unwrap();
+    assert!(owner.shown.contains("They differ"));
+    assert!(owner.shown.contains("At least 4"));
+    let mut unlock = Owner::types(&["123456"]);
+    assert!(run_with(&mut dev, &mut unlock).is_ok(), "no duress chosen: the passcode is 123456");
+}
+
+#[test]
+fn the_duress_passcode_must_differ() {
+    let mut dev = Device::new();
+    let mut owner = Owner::types(&["1234", "1234", "1234", "1234", "", ""]);
+    run_with(&mut dev, &mut owner).unwrap();
+    assert!(owner.shown.contains("must differ"));
+}
+
+#[test]
+fn every_guess_is_counted_before_it_is_checked() {
+    let (mut dev, _) = set_up();
+    // The power goes after the guess was counted, before the check could
+    // clear it: even the right passcode stays counted.
+    dev.cut_after = Some(dev.writes + 1);
+    let mut owner = Owner::types(&["1234"]);
+    assert_eq!(run_with(&mut dev, &mut owner).err(), Some(Failure::Device));
+    assert_eq!(dev.record().unwrap().failures, 1, "the guess stays counted");
+}
+
+#[test]
+fn wrong_passcodes_are_counted_and_a_right_one_resets_the_count() {
+    let (mut dev, key) = set_up();
+    let mut owner = Owner::types(&["1111", "2222", "1234"]);
+    assert_eq!(run_with(&mut dev, &mut owner).unwrap().0, key);
+    assert_eq!(owner.shown.matches("Wrong passcode").count(), 2);
+    assert_eq!(dev.record().unwrap().failures, 0);
+}
+
+#[test]
+fn too_many_wrong_passcodes_wipe_the_keys_and_the_device_starts_fresh() {
+    let (mut dev, key) = set_up();
+    let mut guesses = vec!["9999"; THRESHOLD as usize];
+    // Then it looks fresh: the owner sets it up again.
+    guesses.extend(["5678", "5678", "", ""]);
+    let mut owner = Owner::types(&guesses);
+    let new = run_with(&mut dev, &mut owner).unwrap();
+    assert!(owner.shown.contains("One try left"));
+    assert!(owner.shown.ends_with("This device has no passcode yet. Choose one; it protects everything on it.\nNew passcode: Again: A duress passcode wipes the device when it is entered instead. Leave it empty for none.\nDuress passcode: Again: "));
+    assert_ne!(new.0, key, "the old master key is gone");
+    let mut old = Owner::types(&["1234"]);
+    let mut dev2 = dev.clone();
+    dev2.cut_after = None;
+    assert!(run_with(&mut dev2, &mut old).is_err(), "the old passcode opens nothing");
+}
+
+#[test]
+fn the_duress_passcode_wipes_silently_and_the_device_looks_fresh() {
+    let (mut dev, key) = set_up();
+    let mut owner = Owner::types(&["0000", "4321", "4321", "", ""]);
+    let new = run_with(&mut dev, &mut owner).unwrap();
+    assert!(!owner.shown.contains("Wrong"), "nothing tells it was a duress passcode");
+    assert!(owner.shown.contains("no passcode yet"));
+    assert_ne!(new.0, key);
+}
+
+#[test]
+fn a_wipe_cut_short_is_finished_on_the_next_boot() {
+    let (mut dev, key) = set_up();
+    // Duress, and the power goes during the wipe (after its first write).
+    let marked = dev.writes + 2;
+    dev.cut_after = Some(marked + 1);
+    assert_eq!(run_with(&mut dev, &mut Owner::types(&["0000"])).err(), Some(Failure::Device));
+    dev.cut_after = None;
+    // Next boot: the wipe completes before anything is asked; then setup.
+    let mut owner = Owner::types(&["4321", "4321", "", ""]);
+    let new = run_with(&mut dev, &mut owner).unwrap();
+    assert!(owner.shown.starts_with("This device has no passcode yet"), "no passcode is asked of a half-wiped device");
+    assert_ne!(new.0, key);
+}
+
+#[test]
+fn a_duress_cut_short_before_the_wipe_still_wipes_next_boot() {
+    let (mut dev, _) = set_up();
+    // The power goes right after the duress was recorded (before any wipe write).
+    dev.cut_after = Some(dev.writes + 2);
+    assert_eq!(run_with(&mut dev, &mut Owner::types(&["0000"])).err(), Some(Failure::Device));
+    assert!(dev.record().unwrap().must_wipe());
+}
+
+#[test]
+fn a_duress_whose_wipe_is_torn_still_wipes_next_boot() {
+    let (mut dev, _) = set_up();
+    // Counted, recorded, then the wipe's first write is torn.
+    dev.tear_wipe = true;
+    assert_eq!(run_with(&mut dev, &mut Owner::types(&["0000"])).err(), Some(Failure::Device));
+    dev.cut_after = None;
+    assert!(dev.record().unwrap().must_wipe(), "the duress was recorded before the wipe began");
+}