diff --git a/crates/notsecrets/src/format.rs b/crates/notsecrets/src/format.rs
new file mode 100644
index 0000000..6a83aea
--- /dev/null
+++ b/crates/notsecrets/src/format.rs
@@ -0,0 +1,251 @@
+use crate::error::AgeError;
+use crate::identities::{Header, Stanza};
+use base64::{Engine, engine::general_purpose::STANDARD_NO_PAD};
+
+const VERSION_LINE: &[u8] = b"age-encryption.org/v1\n";
+const FOOTER_PREFIX: &[u8] = b"--- ";
+
+/// Parse an age binary file's header.
+///
+/// Returns `(Header, payload_offset)` where `payload_offset` is the byte
+/// index into `input` where the payload (nonce + ciphertext) begins.
+pub fn parse_header(input: &[u8]) -> Result<(Header, usize), AgeError> {
+    if !input.starts_with(VERSION_LINE) {
+        return Err(AgeError::ParseError(
+            "missing age version line".to_string(),
+        ));
+    }
+
+    let mut pos = VERSION_LINE.len();
+    let mut recipients: Vec<Stanza> = Vec::new();
+
+    loop {
+        if pos >= input.len() {
+            return Err(AgeError::ParseError("unexpected end of header".to_string()));
+        }
+
+        // Check for footer line: "--- <base64-mac>\n"
+        if input[pos..].starts_with(FOOTER_PREFIX) {
+            let line_end = input[pos..]
+                .iter()
+                .position(|&b| b == b'\n')
+                .ok_or_else(|| AgeError::ParseError("footer line not terminated".to_string()))?;
+            let mac_b64 = &input[pos + FOOTER_PREFIX.len()..pos + line_end];
+            let mac = STANDARD_NO_PAD
+                .decode(mac_b64)
+                .map_err(|e| AgeError::ParseError(format!("footer MAC base64: {e}")))?;
+            let payload_offset = pos + line_end + 1;
+            return Ok((Header { recipients, mac }, payload_offset));
+        }
+
+        // Must be a stanza line: "-> <tag> [args...]\n"
+        if !input[pos..].starts_with(b"-> ") {
+            return Err(AgeError::ParseError(format!(
+                "expected stanza or footer at byte {pos}"
+            )));
+        }
+
+        let line_end = input[pos..]
+            .iter()
+            .position(|&b| b == b'\n')
+            .ok_or_else(|| AgeError::ParseError("stanza header line not terminated".to_string()))?;
+
+        let header_line = std::str::from_utf8(&input[pos + 3..pos + line_end])
+            .map_err(|e| AgeError::ParseError(format!("stanza line UTF-8: {e}")))?;
+
+        let mut parts = header_line.split(' ');
+        let tag = parts
+            .next()
+            .ok_or_else(|| AgeError::ParseError("stanza missing tag".to_string()))?
+            .to_string();
+        let args: Vec<String> = parts.map(str::to_string).collect();
+
+        pos += line_end + 1;
+
+        // Read stanza body: base64 lines until an empty line
+        let mut body_b64 = String::new();
+        loop {
+            if pos >= input.len() {
+                return Err(AgeError::ParseError(
+                    "unexpected end during stanza body".to_string(),
+                ));
+            }
+            let line_end = input[pos..]
+                .iter()
+                .position(|&b| b == b'\n')
+                .ok_or_else(|| {
+                    AgeError::ParseError("stanza body line not terminated".to_string())
+                })?;
+            let line = &input[pos..pos + line_end];
+            pos += line_end + 1;
+            if line.is_empty() {
+                break;
+            }
+            body_b64.push_str(
+                std::str::from_utf8(line)
+                    .map_err(|e| AgeError::ParseError(format!("body UTF-8: {e}")))?,
+            );
+        }
+
+        let body = if body_b64.is_empty() {
+            Vec::new()
+        } else {
+            STANDARD_NO_PAD
+                .decode(&body_b64)
+                .map_err(|e| AgeError::ParseError(format!("stanza body base64: {e}")))?
+        };
+
+        recipients.push(Stanza { tag, args, body });
+    }
+}
+
+/// Serialize a `Header` to bytes.
+///
+/// Writes version line, all stanzas (with body split into 64-char base64 lines),
+/// and the `--- <base64-mac>` footer. Does NOT write the payload.
+pub fn serialize_header(header: &Header) -> Vec<u8> {
+    let mut out = Vec::new();
+    out.extend_from_slice(VERSION_LINE);
+
+    for stanza in &header.recipients {
+        out.extend_from_slice(b"-> ");
+        out.extend_from_slice(stanza.tag.as_bytes());
+        for arg in &stanza.args {
+            out.push(b' ');
+            out.extend_from_slice(arg.as_bytes());
+        }
+        out.push(b'\n');
+
+        if stanza.body.is_empty() {
+            out.push(b'\n');
+        } else {
+            let encoded = STANDARD_NO_PAD.encode(&stanza.body);
+            for chunk in encoded.as_bytes().chunks(64) {
+                out.extend_from_slice(chunk);
+                out.push(b'\n');
+            }
+            // Terminating empty line after body
+            out.push(b'\n');
+        }
+    }
+
+    // Footer
+    out.extend_from_slice(FOOTER_PREFIX);
+    out.extend_from_slice(STANDARD_NO_PAD.encode(&header.mac).as_bytes());
+    out.push(b'\n');
+
+    out
+}
+
+/// Return the header bytes up to and including `"--- "` (not including the MAC value).
+///
+/// Used for MAC computation.
+pub fn header_bytes_up_to_footer(input: &[u8]) -> Result<Vec<u8>, AgeError> {
+    let footer_pos = input
+        .windows(FOOTER_PREFIX.len())
+        .position(|w| w == FOOTER_PREFIX)
+        .ok_or_else(|| AgeError::ParseError("footer not found".to_string()))?;
+    Ok(input[..footer_pos + FOOTER_PREFIX.len()].to_vec())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::identities::{Header, Stanza};
+
+    fn minimal_age_file() -> Vec<u8> {
+        let mut out = Vec::new();
+        out.extend_from_slice(b"age-encryption.org/v1\n");
+        out.extend_from_slice(b"-> X25519 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=\n");
+        // empty stanza body
+        out.extend_from_slice(b"\n");
+        // MAC line: "--- " + base64(32 zero bytes) -- note: STANDARD_NO_PAD encodes 32 zeros as 43 chars
+        out.extend_from_slice(b"--- AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\n");
+        // payload: 16-byte nonce + 16-byte ciphertext placeholder
+        out.extend_from_slice(&[0x01u8; 16]);
+        out.extend_from_slice(&[0x02u8; 16]);
+        out
+    }
+
+    #[test]
+    fn parse_minimal_header_roundtrip() {
+        let input = minimal_age_file();
+        let (header, payload_offset) = parse_header(&input).expect("parse should succeed");
+        assert_eq!(header.recipients.len(), 1);
+        assert_eq!(header.recipients[0].tag, "X25519");
+        assert_eq!(header.recipients[0].args.len(), 1);
+        assert!(header.recipients[0].body.is_empty());
+        // payload offset should point to the 32 bytes at the end
+        assert_eq!(&input[payload_offset..], {
+            let mut v = vec![0x01u8; 16];
+            v.extend_from_slice(&[0x02u8; 16]);
+            v
+        }.as_slice());
+    }
+
+    #[test]
+    fn serialize_header_roundtrip() {
+        let stanza = Stanza {
+            tag: "X25519".to_string(),
+            args: vec!["dGVzdA".to_string()],
+            body: vec![0xde, 0xad, 0xbe, 0xef],
+        };
+        let header = Header {
+            recipients: vec![stanza],
+            mac: vec![0u8; 32],
+        };
+        let serialized = serialize_header(&header);
+        // Append dummy payload so parse_header can find the end
+        let mut with_payload = serialized.clone();
+        with_payload.extend_from_slice(&[0u8; 32]);
+        let (parsed, _) = parse_header(&with_payload).expect("round-trip parse should succeed");
+        assert_eq!(parsed.recipients.len(), 1);
+        assert_eq!(parsed.recipients[0].tag, "X25519");
+        assert_eq!(parsed.recipients[0].body, vec![0xde, 0xad, 0xbe, 0xef]);
+    }
+
+    #[test]
+    fn parse_missing_version_line_returns_error() {
+        let input = b"not-age-encryption\n-> X25519 arg\n\n--- AAAA\n";
+        let result = parse_header(input);
+        assert!(result.is_err(), "expected ParseError for missing version line");
+    }
+
+    #[test]
+    fn parse_truncated_before_footer_returns_error() {
+        let input = b"age-encryption.org/v1\n-> X25519 arg\n";
+        let result = parse_header(input);
+        assert!(result.is_err(), "expected ParseError for truncated header");
+    }
+
+    #[test]
+    fn parse_empty_input_returns_error() {
+        let result = parse_header(b"");
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn stanza_body_split_across_multiple_64char_lines() {
+        // 49 bytes -> base64 no-pad = 66 chars -> must split: 64 + 2
+        let body = vec![0xabu8; 49];
+        let stanza = Stanza {
+            tag: "scrypt".to_string(),
+            args: vec!["salt".to_string(), "18".to_string()],
+            body: body.clone(),
+        };
+        let header = Header { recipients: vec![stanza], mac: vec![0u8; 32] };
+        let serialized = serialize_header(&header);
+        let mut with_payload = serialized;
+        with_payload.extend_from_slice(&[0u8; 32]);
+        let (parsed, _) = parse_header(&with_payload).unwrap();
+        assert_eq!(parsed.recipients[0].body, body);
+    }
+
+    #[test]
+    fn header_bytes_up_to_footer_excludes_mac() {
+        let input = minimal_age_file();
+        let bytes = header_bytes_up_to_footer(&input).expect("should find footer");
+        // Must end with "--- " (not include the MAC value)
+        assert!(bytes.ends_with(b"--- "), "header_bytes must end at '--- '");
+    }
+}
diff --git a/crates/notsecrets/src/lib.rs b/crates/notsecrets/src/lib.rs
index 79fd423..aa46b6f 100644
--- a/crates/notsecrets/src/lib.rs
+++ b/crates/notsecrets/src/lib.rs
@@ -1,4 +1,5 @@
 pub mod error;
+pub mod format;
 pub mod identities;
 pub mod recipients;
 pub mod sources;