access: SSH signatures

access::sshsig: OpenSSH's SSHSIG, verified and made natively (Ed25519,
sha512/sha256), checked against OpenSSH's own test vector.

Auth plan M7.

2 files changed, 260 insertions, 0 deletions

diff --git a/src/access/mod.rs b/src/access/mod.rs
index 4d96f9b..2ed19a5 100644
--- a/src/access/mod.rs
+++ b/src/access/mod.rs
@@ -18,6 +18,10 @@ pub(crate) mod policy;
 pub(crate) mod registry;
 pub(crate) mod rules;
 pub(crate) mod screen;
+// Users' keys and signatures: what signed values and logins by key use; until then,
+// the tests.
+#[cfg_attr(not(test), allow(dead_code))]
+pub(crate) mod sshsig;
 pub(crate) mod token;
 
 /// What a capability lets its holder do, at its path and below.
diff --git a/src/access/sshsig.rs b/src/access/sshsig.rs
new file mode 100644
index 0000000..1f01d21
--- /dev/null
+++ b/src/access/sshsig.rs
@@ -0,0 +1,256 @@
+//! Users' keys as SSH has them: Ed25519 public keys as an `authorized_keys` line has
+//! them (`PublicKey`), and signatures as `ssh-keygen -Y sign` makes them (OpenSSH's
+//! `PROTOCOL.sshsig`), checked (`verify`) and made (`sign`) here, so a user signs with
+//! the keys and the tools it has, and a client of ours can sign alike without them.
+//!
+//! A signature is armored (`-----BEGIN SSH SIGNATURE-----`) around `SSHSIG`, its
+//! version (1), the public key, a namespace (what it is for: a signature for one can't
+//! be taken for another), a reserved string, the hash algorithm (`sha512` or `sha256`)
+//! and the signature, over `SSHSIG`, the namespace, the reserved string, the algorithm
+//! and the message's hash. Strings are SSH's: a big-endian `u32` length, then the bytes.
+
+use base64::{Engine, engine::general_purpose::STANDARD as BASE64};
+use ed25519_dalek::{Signature, Signer, SigningKey, VerifyingKey};
+use sha2::{Digest, Sha256, Sha512};
+
+const MAGIC: &[u8] = b"SSHSIG";
+const KEY_TYPE: &str = "ssh-ed25519";
+const BEGIN: &str = "-----BEGIN SSH SIGNATURE-----";
+const END: &str = "-----END SSH SIGNATURE-----";
+
+/// An Ed25519 public key.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct PublicKey(pub(crate) [u8; 32]);
+
+impl PublicKey {
+    /// From an `authorized_keys` line: `ssh-ed25519 <base64> [comment]`.
+    pub(crate) fn parse(line: &str) -> Result<Self, String> {
+        let mut parts = line.split_whitespace();
+        let (Some(kind), Some(encoded)) = (parts.next(), parts.next()) else {
+            return Err("not `ssh-ed25519 <base64> [comment]`".to_owned());
+        };
+        if kind != KEY_TYPE {
+            return Err(format!("a {kind} key: only {KEY_TYPE} keys are taken"));
+        }
+        let blob = BASE64
+            .decode(encoded)
+            .map_err(|error| format!("its base64 doesn't decode: {error}"))?;
+        Self::from_blob(&blob)
+    }
+
+    /// From SSH's encoding: the key type, then the key, each a string.
+    fn from_blob(blob: &[u8]) -> Result<Self, String> {
+        let mut reader = Reader(blob);
+        let kind = reader.string()?;
+        if kind != KEY_TYPE.as_bytes() {
+            return Err(format!("not an {KEY_TYPE} key"));
+        }
+        let key: [u8; 32] = reader
+            .string()?
+            .try_into()
+            .map_err(|_| "a key that isn't 32 bytes".to_owned())?;
+        reader.end()?;
+        Ok(PublicKey(key))
+    }
+
+    fn blob(&self) -> Vec<u8> {
+        let mut blob = Vec::new();
+        string(&mut blob, KEY_TYPE.as_bytes());
+        string(&mut blob, &self.0);
+        blob
+    }
+
+    /// As an `authorized_keys` line has it, without a comment.
+    pub(crate) fn line(&self) -> String {
+        format!("{KEY_TYPE} {}", BASE64.encode(self.blob()))
+    }
+
+    /// Its fingerprint, as `ssh-keygen -l` shows it: `SHA256:` and the base64 of its
+    /// encoding's SHA-256, unpadded.
+    pub(crate) fn fingerprint(&self) -> String {
+        let digest = Sha256::digest(self.blob());
+        format!(
+            "SHA256:{}",
+            base64::engine::general_purpose::STANDARD_NO_PAD.encode(digest)
+        )
+    }
+}
+
+/// The key that made `armored`, a signature of `message` for `namespace`, if it is
+/// one: valid, of an Ed25519 key, for that namespace.
+pub(crate) fn verify(armored: &str, namespace: &str, message: &[u8]) -> Result<PublicKey, String> {
+    let body: String = armored
+        .trim()
+        .strip_prefix(BEGIN)
+        .and_then(|rest| rest.trim_end().strip_suffix(END))
+        .ok_or("not an armored SSH signature")?
+        .split_whitespace()
+        .collect();
+    let blob = BASE64
+        .decode(body)
+        .map_err(|error| format!("its base64 doesn't decode: {error}"))?;
+    let mut reader = Reader(
+        blob.strip_prefix(MAGIC)
+            .ok_or("not an SSH signature (no SSHSIG)")?,
+    );
+    if reader.u32()? != 1 {
+        return Err("an SSH signature of another version".to_owned());
+    }
+    let key = PublicKey::from_blob(reader.string()?)?;
+    let signed_for = reader.string()?;
+    if signed_for != namespace.as_bytes() {
+        return Err(format!(
+            "signed for {:?}, not {namespace:?}",
+            String::from_utf8_lossy(signed_for)
+        ));
+    }
+    let reserved = reader.string()?;
+    let algorithm = reader.string()?;
+    let hash = match algorithm {
+        b"sha512" => Sha512::digest(message).to_vec(),
+        b"sha256" => Sha256::digest(message).to_vec(),
+        other => {
+            return Err(format!("hashed with {}", String::from_utf8_lossy(other)));
+        }
+    };
+    let mut signature = Reader(reader.string()?);
+    reader.end()?;
+    if signature.string()? != KEY_TYPE.as_bytes() {
+        return Err(format!("not an {KEY_TYPE} signature"));
+    }
+    let bytes: [u8; 64] = signature
+        .string()?
+        .try_into()
+        .map_err(|_| "a signature that isn't 64 bytes".to_owned())?;
+    signature.end()?;
+    let signed = signed_data(signed_for, reserved, algorithm, &hash);
+    VerifyingKey::from_bytes(&key.0)
+        .map_err(|_| "not a valid Ed25519 key".to_owned())?
+        .verify_strict(&signed, &Signature::from_bytes(&bytes))
+        .map_err(|_| "the signature doesn't match".to_owned())?;
+    Ok(key)
+}
+
+/// `message` signed with `key` for `namespace`, armored, as `ssh-keygen -Y sign` makes
+/// it (SHA-512).
+pub(crate) fn sign(key: &SigningKey, namespace: &str, message: &[u8]) -> String {
+    let public = PublicKey(key.verifying_key().to_bytes());
+    let hash = Sha512::digest(message);
+    let signed = signed_data(namespace.as_bytes(), b"", b"sha512", &hash);
+    let mut signature = Vec::new();
+    string(&mut signature, KEY_TYPE.as_bytes());
+    string(&mut signature, &key.sign(&signed).to_bytes());
+    let mut blob = MAGIC.to_vec();
+    blob.extend_from_slice(&1u32.to_be_bytes());
+    string(&mut blob, &public.blob());
+    string(&mut blob, namespace.as_bytes());
+    string(&mut blob, b"");
+    string(&mut blob, b"sha512");
+    string(&mut blob, &signature);
+    let encoded = BASE64.encode(blob);
+    let mut armored = format!("{BEGIN}\n");
+    for line in encoded.as_bytes().chunks(70) {
+        armored.push_str(std::str::from_utf8(line).expect("base64 is ASCII"));
+        armored.push('\n');
+    }
+    armored.push_str(END);
+    armored.push('\n');
+    armored
+}
+
+/// What a signature is made over.
+fn signed_data(namespace: &[u8], reserved: &[u8], algorithm: &[u8], hash: &[u8]) -> Vec<u8> {
+    let mut signed = MAGIC.to_vec();
+    string(&mut signed, namespace);
+    string(&mut signed, reserved);
+    string(&mut signed, algorithm);
+    string(&mut signed, hash);
+    signed
+}
+
+fn string(out: &mut Vec<u8>, bytes: &[u8]) {
+    out.extend_from_slice(&(bytes.len() as u32).to_be_bytes());
+    out.extend_from_slice(bytes);
+}
+
+/// Reads SSH's encoding.
+struct Reader<'a>(&'a [u8]);
+
+impl<'a> Reader<'a> {
+    fn u32(&mut self) -> Result<u32, String> {
+        let (bytes, rest) = self.0.split_first_chunk::<4>().ok_or("cut short")?;
+        self.0 = rest;
+        Ok(u32::from_be_bytes(*bytes))
+    }
+
+    fn string(&mut self) -> Result<&'a [u8], String> {
+        let length = self.u32()? as usize;
+        if self.0.len() < length {
+            return Err("cut short".to_owned());
+        }
+        let (bytes, rest) = self.0.split_at(length);
+        self.0 = rest;
+        Ok(bytes)
+    }
+
+    fn end(&self) -> Result<(), String> {
+        if self.0.is_empty() {
+            Ok(())
+        } else {
+            Err("more than it should hold".to_owned())
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // OpenSSH's test data (regress/unittests/sshsig/testdata: ed25519.pub, ed25519.sig,
+    // signed-data, namespace), made by `ssh-keygen -Y sign`.
+    const OPENSSH_KEY: &str = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIlizSIN3DQV78VPNjVvvwjgf+P5HqbPYCYu3BOMdjAE ED25519 test key";
+    const OPENSSH_SIGNATURE: &str = "-----BEGIN SSH SIGNATURE-----
+U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgiWLNIg3cNBXvxU82NW+/COB/4/
+keps9gJi7cE4x2MAQAAAAIdW5pdHRlc3QAAAAAAAAABnNoYTUxMgAAAFMAAAALc3NoLWVk
+MjU1MTkAAABAihQsbUzuNEFflk5Tw1+H9aLS7tZQk0RG8KW1DtOmDYYnWe3D3UKiG3fcJa
+DNg4vBWp1j1gLRiBMOF+gwYNegDg==
+-----END SSH SIGNATURE-----
+";
+    const OPENSSH_MESSAGE: &[u8] = b"This is a test, this is only a test";
+
+    /// A signature `ssh-keygen -Y sign` made checks, by its key, for its namespace and
+    /// message only.
+    #[test]
+    fn what_ssh_keygen_signs_checks() {
+        let key = PublicKey::parse(OPENSSH_KEY).unwrap();
+        assert_eq!(
+            verify(OPENSSH_SIGNATURE, "unittest", OPENSSH_MESSAGE),
+            Ok(key)
+        );
+        assert!(verify(OPENSSH_SIGNATURE, "continuum-value", OPENSSH_MESSAGE).is_err());
+        assert!(verify(OPENSSH_SIGNATURE, "unittest", b"This is a test").is_err());
+        assert_eq!(PublicKey::parse(&key.line()), Ok(key));
+        assert!(key.fingerprint().starts_with("SHA256:"));
+    }
+
+    /// What `sign` makes checks, and reads as `ssh-keygen` writes it.
+    #[test]
+    fn signed_here_checks() {
+        let key = SigningKey::from_bytes(&[7; 32]);
+        let armored = sign(&key, "continuum-value", b"a value");
+        assert!(armored.starts_with(BEGIN));
+        assert_eq!(
+            verify(&armored, "continuum-value", b"a value"),
+            Ok(PublicKey(key.verifying_key().to_bytes()))
+        );
+        assert!(verify(&armored, "continuum-value", b"another").is_err());
+    }
+
+    #[test]
+    fn other_keys_are_refused() {
+        assert!(PublicKey::parse("ssh-rsa AAAAB3NzaC1yc2E= someone").is_err());
+        assert!(PublicKey::parse("ssh-ed25519 !!!").is_err());
+        assert!(PublicKey::parse("").is_err());
+        assert!(verify("not a signature", "unittest", b"").is_err());
+    }
+}