diff --git a/Cargo.lock b/Cargo.lock
index 0f28161..8dd71c3 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1177,6 +1177,7 @@ dependencies = [
  "rpassword",
  "serde",
  "serde_json",
+ "sha2 0.10.9",
  "tokio",
  "uuid",
  "zeroize",
@@ -1841,7 +1842,7 @@ dependencies = [
  "num",
  "once_cell",
  "serde",
- "sha2",
+ "sha2 0.11.0",
  "zbus",
 ]
 
@@ -1933,6 +1934,17 @@ dependencies = [
  "digest 0.10.7",
 ]
 
+[[package]]
+name = "sha2"
+version = "0.10.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
+dependencies = [
+ "cfg-if",
+ "cpufeatures 0.2.17",
+ "digest 0.10.7",
+]
+
 [[package]]
 name = "sha2"
 version = "0.11.0"
diff --git a/Cargo.toml b/Cargo.toml
index 888ddfd..6212a43 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -17,5 +17,6 @@ base64 = "0.22"
 getrandom = "0.2"
 zeroize = "1"
 rpassword = "7"
+sha2 = "0.10"
 async-trait = "0.1"
 keyring = "4.2"
diff --git a/src/audit.rs b/src/audit.rs
new file mode 100644
index 0000000..1e7512f
--- /dev/null
+++ b/src/audit.rs
@@ -0,0 +1,476 @@
+use std::fs;
+use std::fs::OpenOptions;
+use std::io::Write;
+use std::os::unix::fs::OpenOptionsExt;
+use std::path::PathBuf;
+use std::sync::Mutex;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use base64::engine::general_purpose::STANDARD as B64;
+use base64::Engine;
+use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
+
+use crate::errors::AppError;
+
+/// File name for the append-only audit log.
+const AUDIT_LOG_FILE: &str = "audit.log";
+
+/// Algorithm used for hash-chaining.
+/// Hash algorithm used for chain entries (informational only).
+#[allow(dead_code)]
+const HASH_ALGORITHM: &str = "sha256";
+
+/// Canonical audit log entry.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct AuditEntry {
+    /// Unix timestamp in seconds.
+    pub timestamp: u64,
+    /// The profile npub this action relates to.
+    pub profile_npub: String,
+    /// Action type.
+    pub action: AuditAction,
+    /// Human-readable reason for the action (required for exports).
+    pub reason: String,
+    /// Whether the action succeeded.
+    pub success: bool,
+    /// Error message if failed.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub error: Option<String>,
+    /// Hash of the previous entry for chain integrity.
+    pub prev_hash: String,
+    /// Hash of this entry (timestamp|profile|action|reason|success|error|prev_hash).
+    pub this_hash: String,
+}
+
+/// Actions that are audited.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum AuditAction {
+    /// Private key export requested.
+    KeyExport,
+    /// NIP-46 connection created.
+    ConnectionCreated,
+    /// NIP-46 connection revoked.
+    ConnectionRevoked,
+    /// Signing request approved/rejected.
+    SignRequest,
+    /// Encrypt/decrypt request.
+    Nip44Request,
+    /// Vault unlocked.
+    VaultUnlocked,
+    /// Vault locked.
+    VaultLocked,
+    /// NIP-46 operation denied by permission check.
+    ConnectionPermissionDenied,
+}
+
+/// The audit log writer.
+pub struct AuditLog {
+    path: PathBuf,
+    last_hash: Mutex<String>,
+}
+
+impl AuditLog {
+    /// Open or create the audit log, returning the last hash for chaining.
+    pub fn open() -> Result<Self, AppError> {
+        let path = crate::vault::data_dir().join(AUDIT_LOG_FILE);
+        let last_hash = Self::compute_last_hash(&path)?;
+        Ok(Self {
+            path,
+            last_hash: Mutex::new(last_hash),
+        })
+    }
+
+    /// Compute the hash of the last entry in the log, or genesis hash if empty.
+    fn compute_last_hash(path: &PathBuf) -> Result<String, AppError> {
+        if !path.exists() {
+            return Ok(Self::genesis_hash());
+        }
+        let content =
+            fs::read_to_string(path).map_err(|e| AppError::io("Could not read audit log", e))?;
+        let lines: Vec<&str> = content.lines().collect();
+        if lines.is_empty() {
+            return Ok(Self::genesis_hash());
+        }
+        // Parse the last line as JSON and extract its this_hash
+        let last_line = lines.last().unwrap();
+        let entry: AuditEntry = serde_json::from_str(last_line)
+            .map_err(|e| AppError::vault_malformed(format!("Audit log corrupted: {e}")))?;
+        Ok(entry.this_hash)
+    }
+
+    /// Genesis hash for empty log.
+    fn genesis_hash() -> String {
+        "0".repeat(64)
+    }
+
+    /// Write an audit entry atomically. Fails closed if write fails.
+    ///
+    /// The mutex is held across the entire check-write-update cycle to prevent
+    /// concurrent threads from reading the same `prev_hash`, which would cause
+    /// one entry to silently overwrite another on rename.
+    pub fn write_entry(&self, entry: &AuditEntry) -> Result<(), AppError> {
+        let mut last = self.last_hash.lock().expect("audit mutex poisoned");
+
+        // Verify chain integrity before appending
+        if entry.prev_hash != *last {
+            return Err(AppError::storage(
+                "Audit chain integrity check failed: prev_hash mismatch",
+            ));
+        }
+
+        // Serialize canonically: sorted keys, no whitespace, deterministic
+        let json = serde_json::to_string(entry)
+            .map_err(|e| AppError::json("Could not serialize audit entry", e))?;
+
+        // Atomic append: read existing, write all to temp, sync, rename
+        let tmp_path = self.path.with_extension("log.tmp");
+        {
+            // Read existing content (empty file is fine)
+            let existing = fs::read_to_string(&self.path).unwrap_or_default();
+
+            let mut file = OpenOptions::new()
+                .write(true)
+                .create(true)
+                .truncate(true)
+                .mode(0o600)
+                .open(&tmp_path)
+                .map_err(|e| AppError::io("Could not open audit log temp file", e))?;
+            file.write_all(existing.as_bytes())
+                .map_err(|e| AppError::io("Could not write existing audit log content", e))?;
+            file.write_all(json.as_bytes())
+                .map_err(|e| AppError::io("Could not write audit log temp file", e))?;
+            file.write_all(b"\n")
+                .map_err(|e| AppError::io("Could not write audit log newline", e))?;
+            file.sync_all()
+                .map_err(|e| AppError::io("Could not sync audit log temp file", e))?;
+        }
+
+        // Rename temp to actual (atomic on POSIX)
+        fs::rename(&tmp_path, &self.path)
+            .map_err(|e| AppError::io("Could not finalize audit log", e))?;
+
+        // Update last hash — still under the same lock
+        *last = entry.this_hash.clone();
+
+        Ok(())
+    }
+
+    /// Build and write a new entry, returning the entry for the caller.
+    ///
+    /// The entire read-compute-write-update cycle is under a single mutex
+    /// acquisition to prevent concurrent writers from interleaving.
+    pub fn record(
+        &self,
+        profile_npub: &str,
+        action: AuditAction,
+        reason: &str,
+        success: bool,
+        error: Option<String>,
+    ) -> Result<AuditEntry, AppError> {
+        let timestamp = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .map_err(|e| AppError::internal(format!("System clock error: {e}")))?
+            .as_secs();
+
+        let mut last = self.last_hash.lock().expect("audit mutex poisoned");
+        let prev_hash = last.clone();
+
+        // Compute this hash from canonical fields
+        let this_hash = Self::compute_hash(&AuditEntry {
+            timestamp,
+            profile_npub: profile_npub.to_string(),
+            action,
+            reason: reason.to_string(),
+            success,
+            error: error.clone(),
+            prev_hash: prev_hash.clone(),
+            this_hash: String::new(), // placeholder
+        });
+
+        let entry = AuditEntry {
+            timestamp,
+            profile_npub: profile_npub.to_string(),
+            action,
+            reason: reason.to_string(),
+            success,
+            error,
+            prev_hash,
+            this_hash,
+        };
+
+        // Serialize canonically
+        let json = serde_json::to_string(&entry)
+            .map_err(|e| AppError::json("Could not serialize audit entry", e))?;
+
+        // Atomic append: read existing, write all to temp, sync, rename
+        let tmp_path = self.path.with_extension("log.tmp");
+        {
+            let existing = fs::read_to_string(&self.path).unwrap_or_default();
+            let mut file = OpenOptions::new()
+                .write(true)
+                .create(true)
+                .truncate(true)
+                .mode(0o600)
+                .open(&tmp_path)
+                .map_err(|e| AppError::io("Could not open audit log temp file", e))?;
+            file.write_all(existing.as_bytes())
+                .map_err(|e| AppError::io("Could not write existing audit log content", e))?;
+            file.write_all(json.as_bytes())
+                .map_err(|e| AppError::io("Could not write audit log temp file", e))?;
+            file.write_all(b"\n")
+                .map_err(|e| AppError::io("Could not write audit log newline", e))?;
+            file.sync_all()
+                .map_err(|e| AppError::io("Could not sync audit log temp file", e))?;
+        }
+
+        // Rename temp to actual (atomic on POSIX)
+        fs::rename(&tmp_path, &self.path)
+            .map_err(|e| AppError::io("Could not finalize audit log", e))?;
+
+        // Update last hash — still under the same lock
+        *last = entry.this_hash.clone();
+
+        Ok(entry)
+    }
+
+    /// Canonical hash: timestamp|profile_npub|action|reason|success|error|prev_hash
+    /// All fields are JSON-encoded to avoid delimiter ambiguity.
+    fn compute_hash(entry: &AuditEntry) -> String {
+        let mut hasher = Sha256::new();
+        // Use JSON values for canonical representation
+        let timestamp_json = serde_json::to_string(&entry.timestamp).unwrap();
+        let profile_json = serde_json::to_string(&entry.profile_npub).unwrap();
+        let action_json = serde_json::to_string(&entry.action).unwrap();
+        let reason_json = serde_json::to_string(&entry.reason).unwrap();
+        let success_json = serde_json::to_string(&entry.success).unwrap();
+        let error_json = serde_json::to_string(&entry.error).unwrap();
+        let prev_hash_json = serde_json::to_string(&entry.prev_hash).unwrap();
+
+        hasher.update(timestamp_json.as_bytes());
+        hasher.update(b"|");
+        hasher.update(profile_json.as_bytes());
+        hasher.update(b"|");
+        hasher.update(action_json.as_bytes());
+        hasher.update(b"|");
+        hasher.update(reason_json.as_bytes());
+        hasher.update(b"|");
+        hasher.update(success_json.as_bytes());
+        hasher.update(b"|");
+        hasher.update(error_json.as_bytes());
+        hasher.update(b"|");
+        hasher.update(prev_hash_json.as_bytes());
+
+        B64.encode(hasher.finalize())
+    }
+
+    /// Verify the entire chain from genesis to end.
+    pub fn verify_chain(&self) -> Result<bool, AppError> {
+        if !self.path.exists() {
+            return Ok(true);
+        }
+        let content = fs::read_to_string(&self.path)
+            .map_err(|e| AppError::io("Could not read audit log for verification", e))?;
+        let mut expected_prev = Self::genesis_hash();
+        for line in content.lines() {
+            let entry: AuditEntry = match serde_json::from_str(line) {
+                Ok(e) => e,
+                Err(_) => return Ok(false), // corrupted line = invalid chain
+            };
+            if entry.prev_hash != expected_prev {
+                return Ok(false);
+            }
+            let computed = Self::compute_hash(&entry);
+            if computed != entry.this_hash {
+                return Ok(false);
+            }
+            expected_prev = entry.this_hash;
+        }
+        Ok(true)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::env;
+    use std::sync::atomic::{AtomicU32, Ordering};
+
+    static COUNTER: AtomicU32 = AtomicU32::new(0);
+
+    fn temp_audit_dir() -> PathBuf {
+        let dir = env::temp_dir().join(format!(
+            "keynectr-audit-test-{}-{}",
+            std::process::id(),
+            COUNTER.fetch_add(1, Ordering::SeqCst)
+        ));
+        fs::create_dir_all(&dir).unwrap();
+        dir
+    }
+
+    #[test]
+    fn audit_log_chain_works() {
+        let dir = temp_audit_dir();
+        let log_path = dir.join(AUDIT_LOG_FILE);
+        // Manually create an AuditLog pointing to our temp dir
+        let audit = AuditLog {
+            path: log_path.clone(),
+            last_hash: Mutex::new(AuditLog::genesis_hash()),
+        };
+
+        // Write first entry
+        let e1 = audit
+            .record(
+                "npub1alice",
+                AuditAction::KeyExport,
+                "migration backup",
+                true,
+                None,
+            )
+            .unwrap();
+        assert_eq!(e1.prev_hash, AuditLog::genesis_hash());
+        assert!(AuditLog::compute_hash(&e1) == e1.this_hash);
+
+        // Write second entry
+        let e2 = audit
+            .record(
+                "npub1bob",
+                AuditAction::KeyExport,
+                "key rotation",
+                true,
+                None,
+            )
+            .unwrap();
+        assert_eq!(e2.prev_hash, e1.this_hash);
+        assert!(AuditLog::compute_hash(&e2) == e2.this_hash);
+
+        // Verify chain
+        assert!(audit.verify_chain().unwrap());
+
+        // Read back and verify
+        let content = fs::read_to_string(&log_path).unwrap();
+        let lines: Vec<&str> = content.lines().collect();
+        assert_eq!(lines.len(), 2);
+        let parsed1: AuditEntry = serde_json::from_str(lines[0]).unwrap();
+        let parsed2: AuditEntry = serde_json::from_str(lines[1]).unwrap();
+        assert_eq!(parsed1.this_hash, e1.this_hash);
+        assert_eq!(parsed2.this_hash, e2.this_hash);
+    }
+
+    #[test]
+    fn audit_log_rejects_tampered_chain() {
+        let dir = temp_audit_dir();
+        let log_path = dir.join(AUDIT_LOG_FILE);
+        let audit = AuditLog {
+            path: log_path.clone(),
+            last_hash: Mutex::new(AuditLog::genesis_hash()),
+        };
+
+        let e1 = audit
+            .record("npub1alice", AuditAction::KeyExport, "reason", true, None)
+            .unwrap();
+        // Tamper: modify the file directly
+        let mut content = fs::read_to_string(&log_path).unwrap();
+        content = content.replace(&e1.reason, "tampered");
+        fs::write(&log_path, content).unwrap();
+
+        // New AuditLog should detect mismatch
+        let audit2 = AuditLog {
+            path: log_path.clone(),
+            last_hash: Mutex::new(AuditLog::genesis_hash()),
+        };
+        assert!(!audit2.verify_chain().unwrap());
+    }
+
+    #[test]
+    fn audit_entry_serialization_deterministic() {
+        let entry = AuditEntry {
+            timestamp: 1_700_000_000,
+            profile_npub: "npub1test".to_string(),
+            action: AuditAction::KeyExport,
+            reason: "test reason".to_string(),
+            success: true,
+            error: None,
+            prev_hash: "0".repeat(64),
+            this_hash: "1".repeat(64),
+        };
+        let json1 = serde_json::to_string(&entry).unwrap();
+        let json2 = serde_json::to_string(&entry).unwrap();
+        assert_eq!(json1, json2);
+    }
+
+    #[test]
+    fn audit_log_fails_on_write_error() {
+        // Use a path we can't write to
+        let audit = AuditLog {
+            path: PathBuf::from("/root/cannot_write.log"),
+            last_hash: Mutex::new(AuditLog::genesis_hash()),
+        };
+        let entry = AuditEntry {
+            timestamp: 1,
+            profile_npub: "npub1test".to_string(),
+            action: AuditAction::KeyExport,
+            reason: "test".to_string(),
+            success: true,
+            error: None,
+            prev_hash: AuditLog::genesis_hash(),
+            this_hash: "x".repeat(64),
+        };
+        assert!(audit.write_entry(&entry).is_err());
+    }
+
+    #[test]
+    fn concurrent_audit_writes_are_serialized() {
+        use std::sync::Arc;
+        use std::thread;
+
+        let dir = temp_audit_dir();
+        let log_path = dir.join(AUDIT_LOG_FILE);
+        let audit = Arc::new(AuditLog {
+            path: log_path.clone(),
+            last_hash: Mutex::new(AuditLog::genesis_hash()),
+        });
+
+        let num_writers = 8;
+        let mut handles = vec![];
+
+        for i in 0..num_writers {
+            let audit_clone = Arc::clone(&audit);
+            handles.push(thread::spawn(move || {
+                audit_clone
+                    .record(
+                        &format!("npub1writer{i}"),
+                        AuditAction::KeyExport,
+                        &format!("concurrent write {i}"),
+                        true,
+                        None,
+                    )
+                    .unwrap();
+            }));
+        }
+
+        for h in handles {
+            h.join().unwrap();
+        }
+
+        // Verify chain: all entries present and chain valid
+        let content = fs::read_to_string(&log_path).unwrap();
+        let lines: Vec<&str> = content.lines().collect();
+        assert_eq!(lines.len(), num_writers);
+
+        // Verify chain integrity
+        assert!(audit.verify_chain().unwrap());
+
+        // Verify no duplicate npubs (each writer wrote a unique entry)
+        let npubs: Vec<String> = lines
+            .iter()
+            .filter_map(|line| {
+                let entry: AuditEntry = serde_json::from_str(line).ok()?;
+                Some(entry.profile_npub)
+            })
+            .collect();
+        let unique: std::collections::HashSet<_> = npubs.iter().collect();
+        assert_eq!(unique.len(), num_writers);
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index f28ccf5..45c78f4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,4 +1,5 @@
 pub mod app;
+pub mod audit;
 pub mod bunker;
 pub mod crypto;
 pub mod errors;