fix(credentials): mark the encrypted store so a random nonce cannot fake plaintext

The on-disk format was detected by sniffing the first byte for `[` or `{`.
Encrypted blobs begin with a random 12-byte nonce, so roughly 1 in 128
saves produced a valid encrypted file whose first byte was 0x5B or 0x7B;
those were misread as plaintext JSON, failed `String::from_utf8`, and the
store became permanently unreadable. This was surfacing as a flaky
`test_list_credentials_no_filter`, but it is a real data-loss bug: a node
whose ciphertext happened to start with one of those bytes could not load
its credentials.

Writes now carry a fixed `ARCHYCRED1` marker, which cannot collide with a
random nonce, so detection of the current format is exact.

Legacy unmarked files are detected by SUCCESSFUL AEAD DECRYPTION rather
than by another byte sniff. A verifying Poly1305 tag under the node key is
a cryptographic discriminator (~2^-128 false-positive rate), strictly
stronger than any structural guess — which is why the deferred item's
suggested "keep the first-byte sniff as the legacy fallback" was not the
shape adopted. Plaintext JSON remains the last resort, and is still
reachable on a node that has no node key at all.

An undecodable file now errors instead of returning an empty store, so a
transiently unreadable file is never silently replaced by an empty one
that the next save would commit to disk (CLAUDE.md: migrations never
destroy data). Legacy files upgrade on write, never on read.

Tests drive the collision deterministically via an explicit nonce rather
than waiting on the 1-in-128 draw, and cover all three on-disk
populations, the read-path-does-not-rewrite guarantee, and tamper
rejection. 28 passed, 0 failed.

Closes the 10-01 deferred item.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/archipelago/src/credentials/store.rs

@@ -18,6 +18,17 @@ fn store_path(data_dir: &Path) -> std::path::PathBuf {
     data_dir.join(CREDENTIALS_DIR).join("credentials.json")
 }
 
+/// Magic prefix marking an encrypted credential store written by this version
+/// onward.
+///
+/// Historically the on-disk format was detected by sniffing the first byte for
+/// `[`/`{`. Encrypted blobs begin with a *random* 12-byte nonce, so roughly 2 in
+/// 256 (~1 in 128) saves produced a valid encrypted file whose first byte was
+/// `[` (0x5B) or `{` (0x7B); those files were misread as plaintext JSON and
+/// failed to load forever after. A fixed multi-byte marker cannot collide with a
+/// random nonce, so detection is now exact rather than probabilistic.
+const ENCRYPTED_MAGIC: &[u8] = b"ARCHYCRED1";
+
 pub async fn load_credentials(data_dir: &Path) -> Result<CredentialStore> {
     ensure_dir(data_dir).await?;
     let path = store_path(data_dir);
@@ -25,27 +36,68 @@ pub async fn load_credentials(data_dir: &Path) -> Result<CredentialStore> {
         return Ok(CredentialStore::default());
     }
     let raw = fs::read(&path).await.context("Reading credentials")?;
-    // Detect plaintext JSON (migration path) vs encrypted binary
-    if raw.first().is_some_and(|b| *b == b'[' || *b == b'{') {
-        let data = String::from_utf8(raw).context("UTF-8 credentials")?;
-        return serde_json::from_str(&data).context("Parsing credentials");
+    decode_credentials(data_dir, &raw).await
+}
+
+/// Decode any of the three on-disk credential formats that exist in the fleet.
+///
+/// Detection order, and why each step is unambiguous:
+///
+/// 1. **Current format** — `MAGIC ‖ nonce ‖ ciphertext`. The magic is a fixed
+///    10-byte literal, so this test has no false positives and no false
+///    negatives.
+/// 2. **Legacy encrypted** (no magic) — `nonce ‖ ciphertext`. Detected by
+///    *successful AEAD decryption*, not by byte shape. ChaCha20-Poly1305 is
+///    authenticated: a successful `decrypt` means the 16-byte Poly1305 tag
+///    verified under the node key, which a non-ciphertext file passes only with
+///    probability ~2^-128. This is a cryptographic discriminator, strictly
+///    stronger than any structural sniff.
+/// 3. **Legacy plaintext JSON** (original migration path) — reached only when
+///    the bytes did not authenticate, then parsed strictly as a whole document.
+///
+/// If none match we return an error rather than a default store, so a
+/// transiently unreadable file is never silently replaced with empty
+/// credentials on the next save (CLAUDE.md: migrations never destroy data).
+async fn decode_credentials(data_dir: &Path, raw: &[u8]) -> Result<CredentialStore> {
+    // 1. Current format: explicit marker.
+    if let Some(body) = raw.strip_prefix(ENCRYPTED_MAGIC) {
+        let key = load_encryption_key(data_dir).await?;
+        let plaintext = decrypt_credentials(body, &key)?;
+        return serde_json::from_slice(&plaintext).context("Parsing decrypted credentials");
     }
-    // Encrypted: decrypt using node key
-    let key = load_encryption_key(data_dir).await?;
-    let plaintext = decrypt_credentials(&raw, &key)?;
-    serde_json::from_slice(&plaintext).context("Parsing decrypted credentials")
+
+    // 2. Legacy encrypted, unmarked. The node key may legitimately be absent on
+    //    a node that only ever wrote plaintext, so a key-load failure falls
+    //    through to the plaintext path instead of aborting.
+    if let Ok(key) = load_encryption_key(data_dir).await {
+        if let Ok(plaintext) = decrypt_credentials(raw, &key) {
+            return serde_json::from_slice(&plaintext)
+                .context("Parsing decrypted credentials (legacy unmarked)");
+        }
+    }
+
+    // 3. Legacy plaintext JSON migration path.
+    serde_json::from_slice(raw).context(
+        "Credentials file is not magic-prefixed encrypted data, does not authenticate \
+         as a legacy encrypted blob, and is not valid plaintext JSON — refusing to \
+         treat it as empty",
+    )
 }
 
 pub async fn save_credentials(data_dir: &Path, store: &CredentialStore) -> Result<()> {
     ensure_dir(data_dir).await?;
     let path = store_path(data_dir);
     let data = serde_json::to_vec(store)?;
-    // Encrypt using node key
+    // Encrypt using node key. Always written in the current, magic-prefixed
+    // format — this is how legacy files are opportunistically upgraded: they are
+    // read in whatever format they are on disk, and the next save re-emits them
+    // marked. Nothing is ever rewritten from a read path.
     let key = load_encryption_key(data_dir).await?;
     let encrypted = encrypt_credentials(&data, &key)?;
-    fs::write(&path, encrypted)
-        .await
-        .context("Writing credentials")
+    let mut output = Vec::with_capacity(ENCRYPTED_MAGIC.len() + encrypted.len());
+    output.extend_from_slice(ENCRYPTED_MAGIC);
+    output.extend_from_slice(&encrypted);
+    fs::write(&path, output).await.context("Writing credentials")
 }
 
 /// Derive a 32-byte encryption key from the node's identity key via SHA-256.
@@ -65,8 +117,22 @@ async fn load_encryption_key(data_dir: &Path) -> Result<[u8; 32]> {
 }
 
 fn encrypt_credentials(data: &[u8], key: &[u8; 32]) -> Result<Vec<u8>> {
-    use chacha20poly1305::aead::{Aead, KeyInit};
     let nonce_bytes: [u8; 12] = rand::random();
+    encrypt_credentials_with_nonce(data, key, nonce_bytes)
+}
+
+/// Encrypt with a caller-supplied nonce, returning `nonce ‖ ciphertext` (no
+/// magic prefix — `save_credentials` adds that).
+///
+/// Split out from [`encrypt_credentials`] so tests can construct a blob whose
+/// first byte is a specific value and exercise format detection deterministically
+/// instead of waiting on a 1-in-128 random draw.
+fn encrypt_credentials_with_nonce(
+    data: &[u8],
+    key: &[u8; 32],
+    nonce_bytes: [u8; 12],
+) -> Result<Vec<u8>> {
+    use chacha20poly1305::aead::{Aead, KeyInit};
     let cipher = chacha20poly1305::ChaCha20Poly1305::new_from_slice(key)
         .map_err(|e| anyhow::anyhow!("Cipher init: {}", e))?;
     let ciphertext = cipher
@@ -101,6 +167,46 @@ fn decrypt_credentials(data: &[u8], key: &[u8; 32]) -> Result<Vec<u8>> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use super::super::types::{CredentialProof, CredentialSubject, VerifiableCredential};
+
+    /// Tempdir with a deterministic `identity/node_key` so the encryption key
+    /// can be derived. Never a real key.
+    fn test_dir_with_node_key() -> tempfile::TempDir {
+        let dir = tempfile::tempdir().unwrap();
+        let identity_dir = dir.path().join("identity");
+        std::fs::create_dir_all(&identity_dir).unwrap();
+        std::fs::write(identity_dir.join("node_key"), [0xAB; 32]).unwrap();
+        std::fs::create_dir_all(dir.path().join(CREDENTIALS_DIR)).unwrap();
+        dir
+    }
+
+    fn sample_store(marker: &str) -> CredentialStore {
+        CredentialStore {
+            credentials: vec![VerifiableCredential {
+                context: vec!["https://www.w3.org/ns/credentials/v2".to_string()],
+                id: format!("urn:uuid:{marker}"),
+                credential_type: vec![
+                    "VerifiableCredential".to_string(),
+                    "NodeOperator".to_string(),
+                ],
+                issuer: "did:key:issuer".to_string(),
+                credential_subject: CredentialSubject {
+                    id: "did:key:subject".to_string(),
+                    claims: serde_json::json!({"role": "admin"}),
+                },
+                issuance_date: "2026-01-01T00:00:00Z".to_string(),
+                expiration_date: None,
+                proof: CredentialProof {
+                    proof_type: "Ed25519Signature2020".to_string(),
+                    created: "2026-01-01T00:00:00Z".to_string(),
+                    verification_method: "did:key:issuer#key-1".to_string(),
+                    proof_purpose: "assertionMethod".to_string(),
+                    proof_value: "sig".to_string(),
+                },
+                credential_status: None,
+            }],
+        }
+    }
 
     #[tokio::test]
     async fn test_load_credentials_returns_empty_when_no_file() {
@@ -109,4 +215,153 @@ mod tests {
         assert!(store.credentials.is_empty());
         assert!(dir.path().join(CREDENTIALS_DIR).exists());
     }
+
+    /// Regression: an encrypted blob whose random nonce happens to start with
+    /// `[` (0x5B) or `{` (0x7B) used to be misdetected as plaintext JSON, so
+    /// `String::from_utf8` failed and the store became permanently unreadable.
+    /// ~1 in 128 saves hit this. Both colliding bytes are exercised here
+    /// deterministically via an explicit nonce.
+    #[tokio::test]
+    async fn test_legacy_encrypted_blob_with_json_first_byte_still_loads() {
+        for (first_byte, label) in [(0x5Bu8, "open-bracket"), (0x7Bu8, "open-brace")] {
+            let dir = test_dir_with_node_key();
+            let key = load_encryption_key(dir.path()).await.unwrap();
+
+            // Force the collision: nonce[0] is exactly the byte the old sniffer
+            // treated as "this file is plaintext JSON".
+            let mut nonce = [0u8; 12];
+            nonce[0] = first_byte;
+            let plaintext = serde_json::to_vec(&sample_store(label)).unwrap();
+            let blob = encrypt_credentials_with_nonce(&plaintext, &key, nonce).unwrap();
+            assert_eq!(blob[0], first_byte, "test must actually trigger the collision");
+
+            // Written WITHOUT magic: this is the legacy on-disk population.
+            std::fs::write(store_path(dir.path()), &blob).unwrap();
+
+            let loaded = load_credentials(dir.path())
+                .await
+                .unwrap_or_else(|e| panic!("{label}: colliding blob failed to load: {e:#}"));
+            assert_eq!(loaded.credentials.len(), 1, "{label}");
+            assert_eq!(loaded.credentials[0].id, format!("urn:uuid:{label}"));
+        }
+    }
+
+    /// Same collision, but in the current magic-prefixed format.
+    #[tokio::test]
+    async fn test_new_format_with_colliding_nonce_loads() {
+        for first_byte in [0x5Bu8, 0x7Bu8] {
+            let dir = test_dir_with_node_key();
+            let key = load_encryption_key(dir.path()).await.unwrap();
+            let mut nonce = [0u8; 12];
+            nonce[0] = first_byte;
+            let plaintext = serde_json::to_vec(&sample_store("magic")).unwrap();
+            let body = encrypt_credentials_with_nonce(&plaintext, &key, nonce).unwrap();
+            let mut blob = ENCRYPTED_MAGIC.to_vec();
+            blob.extend_from_slice(&body);
+            std::fs::write(store_path(dir.path()), &blob).unwrap();
+
+            let loaded = load_credentials(dir.path()).await.unwrap();
+            assert_eq!(loaded.credentials[0].id, "urn:uuid:magic");
+        }
+    }
+
+    /// Population 1: legacy plaintext JSON from the original migration path.
+    #[tokio::test]
+    async fn test_legacy_plaintext_json_still_loads() {
+        let dir = test_dir_with_node_key();
+        let json = serde_json::to_vec(&sample_store("plaintext")).unwrap();
+        assert_eq!(json[0], b'{');
+        std::fs::write(store_path(dir.path()), &json).unwrap();
+
+        let loaded = load_credentials(dir.path()).await.unwrap();
+        assert_eq!(loaded.credentials[0].id, "urn:uuid:plaintext");
+    }
+
+    /// Legacy plaintext must still load on a node that has no node key at all
+    /// (pre-onboarding), where the encrypted path cannot even derive a key.
+    #[tokio::test]
+    async fn test_legacy_plaintext_json_loads_without_node_key() {
+        let dir = tempfile::tempdir().unwrap();
+        std::fs::create_dir_all(dir.path().join(CREDENTIALS_DIR)).unwrap();
+        let json = serde_json::to_vec(&sample_store("nokey")).unwrap();
+        std::fs::write(store_path(dir.path()), &json).unwrap();
+
+        let loaded = load_credentials(dir.path()).await.unwrap();
+        assert_eq!(loaded.credentials[0].id, "urn:uuid:nokey");
+    }
+
+    /// Population 2: legacy encrypted with an ordinary (non-colliding) nonce.
+    #[tokio::test]
+    async fn test_legacy_encrypted_without_magic_still_loads() {
+        let dir = test_dir_with_node_key();
+        let key = load_encryption_key(dir.path()).await.unwrap();
+        let plaintext = serde_json::to_vec(&sample_store("legacy-enc")).unwrap();
+        let blob = encrypt_credentials_with_nonce(&plaintext, &key, [0x01; 12]).unwrap();
+        std::fs::write(store_path(dir.path()), &blob).unwrap();
+
+        let loaded = load_credentials(dir.path()).await.unwrap();
+        assert_eq!(loaded.credentials[0].id, "urn:uuid:legacy-enc");
+    }
+
+    /// Population 3: current format round-trips and is actually marked on disk.
+    #[tokio::test]
+    async fn test_new_format_roundtrip_and_is_magic_prefixed() {
+        let dir = test_dir_with_node_key();
+        save_credentials(dir.path(), &sample_store("current"))
+            .await
+            .unwrap();
+
+        let on_disk = std::fs::read(store_path(dir.path())).unwrap();
+        assert!(on_disk.starts_with(ENCRYPTED_MAGIC), "save must mark the file");
+        // Still genuinely encrypted, not plaintext.
+        assert!(!on_disk.windows(4).any(|w| w == b"did:"));
+
+        let loaded = load_credentials(dir.path()).await.unwrap();
+        assert_eq!(loaded.credentials[0].id, "urn:uuid:current");
+    }
+
+    /// Opportunistic upgrade happens on write, never on read: reading a legacy
+    /// file must leave it byte-identical; the next save re-emits it marked.
+    #[tokio::test]
+    async fn test_legacy_file_upgraded_on_write_not_on_read() {
+        let dir = test_dir_with_node_key();
+        let json = serde_json::to_vec(&sample_store("upgrade")).unwrap();
+        std::fs::write(store_path(dir.path()), &json).unwrap();
+
+        let loaded = load_credentials(dir.path()).await.unwrap();
+        // Read path must not have rewritten anything.
+        let after_read = std::fs::read(store_path(dir.path())).unwrap();
+        assert_eq!(after_read, json, "read path must not rewrite the file");
+
+        save_credentials(dir.path(), &loaded).await.unwrap();
+        let after_write = std::fs::read(store_path(dir.path())).unwrap();
+        assert!(after_write.starts_with(ENCRYPTED_MAGIC));
+
+        let reloaded = load_credentials(dir.path()).await.unwrap();
+        assert_eq!(reloaded.credentials[0].id, "urn:uuid:upgrade");
+    }
+
+    /// An unreadable file must surface an error, never a silent empty store —
+    /// otherwise the next save would overwrite recoverable user data.
+    #[tokio::test]
+    async fn test_undecodable_file_errors_instead_of_returning_empty() {
+        let dir = test_dir_with_node_key();
+        std::fs::write(store_path(dir.path()), b"\x00\x01\x02 not json, not ours").unwrap();
+        assert!(load_credentials(dir.path()).await.is_err());
+    }
+
+    /// A magic-prefixed file that fails authentication (tampered / wrong key)
+    /// must error rather than fall through to another format.
+    #[tokio::test]
+    async fn test_tampered_magic_file_errors() {
+        let dir = test_dir_with_node_key();
+        save_credentials(dir.path(), &sample_store("tamper"))
+            .await
+            .unwrap();
+        let mut blob = std::fs::read(store_path(dir.path())).unwrap();
+        *blob.last_mut().unwrap() ^= 0x01;
+        std::fs::write(store_path(dir.path()), &blob).unwrap();
+
+        assert!(load_credentials(dir.path()).await.is_err());
+    }
 }