feat(core): key-file armor (relicario-keyfile-v1) encode/decode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01McMF5pUJbCMricmFEnf2sG

crates/relicario-core/src/keyfile.rs

@@ -0,0 +1,73 @@
+//! Key-file armor for the pluggable second factor. The file holds the raw
+//! 32-byte secret (base64) behind a version header -- it is the "something
+//! you have", not an encrypted artifact (the passphrase is the other factor).
+//!
+//! The format is intentionally minimal and human-inspectable:
+//!
+//! ```text
+//! relicario-keyfile-v1
+//! <base64-standard of 32 bytes>
+//! ```
+//!
+//! Two lines, both terminated with `\n`. There is no encryption here: the file
+//! is as sensitive as the raw secret itself. Store it accordingly (encrypted
+//! drive, hardware token, offline backup). The passphrase remains the second
+//! factor; together they feed Argon2id to derive the master key.
+
+use base64::{engine::general_purpose::STANDARD, Engine};
+use zeroize::Zeroizing;
+
+use crate::error::{RelicarioError, Result};
+
+const HEADER: &str = "relicario-keyfile-v1";
+
+/// Encode a 32-byte secret into the `relicario-keyfile-v1` armor format.
+///
+/// Output is: `"relicario-keyfile-v1\n<base64>\n"` as UTF-8 bytes.
+/// The returned bytes are suitable for writing directly to a `.relkey` file.
+pub fn keyfile_encode(secret: &[u8; 32]) -> Vec<u8> {
+    format!("{HEADER}\n{}\n", STANDARD.encode(secret)).into_bytes()
+}
+
+/// Decode a `relicario-keyfile-v1` armored file back to the raw 32-byte secret.
+///
+/// Returns [`RelicarioError::Format`] if the header is missing or wrong, if the
+/// body is not valid base64, or if the decoded length is not exactly 32 bytes.
+pub fn keyfile_decode(bytes: &[u8]) -> Result<Zeroizing<[u8; 32]>> {
+    let text = std::str::from_utf8(bytes)
+        .map_err(|_| RelicarioError::Format("key file is not UTF-8".into()))?;
+    let mut lines = text.lines();
+    if lines.next() != Some(HEADER) {
+        return Err(RelicarioError::Format("bad key-file header".into()));
+    }
+    let b64 = lines.next().unwrap_or("").trim();
+    let decoded = STANDARD
+        .decode(b64)
+        .map_err(|_| RelicarioError::Format("key-file body not base64".into()))?;
+    let arr: [u8; 32] = decoded
+        .as_slice()
+        .try_into()
+        .map_err(|_| RelicarioError::Format("key-file secret must be 32 bytes".into()))?;
+    Ok(Zeroizing::new(arr))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn round_trip() {
+        let secret = [9u8; 32];
+        let armored = keyfile_encode(&secret);
+        assert!(std::str::from_utf8(&armored).unwrap().starts_with("relicario-keyfile-v1\n"));
+        let back = keyfile_decode(&armored).unwrap();
+        assert_eq!(*back, secret);
+    }
+    #[test]
+    fn rejects_bad_header() {
+        assert!(keyfile_decode(b"not-a-keyfile\nAAAA\n").is_err());
+    }
+    #[test]
+    fn rejects_wrong_length() {
+        assert!(keyfile_decode(b"relicario-keyfile-v1\nAAAA\n").is_err()); // decodes to <32 bytes
+    }
+}

crates/relicario-core/src/lib.rs

@@ -84,6 +84,8 @@ pub use vault::{
 
 pub mod imgsecret;
 
+pub mod keyfile;
+
 pub mod backup;
 pub use backup::{pack_backup, unpack_backup, BackupInput, BackupOutput, BackupItem, BackupAttachment};