Support loading Ed448 public keys in OpenSSH format

The 'ssh-ed448' key type is documented along with 'ssh-ed25519' in [1], but
has never been supported by any as-yet-released version of OpenSSH.

However, LANcom router devices (which appear to be primarily used in
Germany, see [2] for examples on the public Internet) appear to support
these keys, so this library can and should support loading them.

See also jtesta/ssh-audit#277 in which I extended
`ssh-audit` to allow it to scan and discover host keys of type 'ssh-ed488'.

[1] https://datatracker.ietf.org/doc/html/rfc8709#name-public-key-format
[2] https://www.shodan.io/search?query=ssh+%22ed448%22

src/cryptography/hazmat/primitives/serialization/ssh.py

@@ -20,6 +20,7 @@
     dsa,
     ec,
     ed25519,
+    ed448,
     padding,
     rsa,
 )
@@ -57,6 +58,7 @@ def _bcrypt_kdf(
 
 
 _SSH_ED25519 = b"ssh-ed25519"
+_SSH_ED448 = b"ssh-ed448"
 _SSH_RSA = b"ssh-rsa"
 _SSH_DSA = b"ssh-dss"
 _ECDSA_NISTP256 = b"ecdsa-sha2-nistp256"
@@ -152,6 +154,10 @@ def _get_ssh_key_type(key: SSHPrivateKeyTypes | SSHPublicKeyTypes) -> bytes:
         key, (ed25519.Ed25519PrivateKey, ed25519.Ed25519PublicKey)
     ):
         key_type = _SSH_ED25519
+    elif isinstance(
+        key, (ed448.Ed448PublicKey,)  # private keys are not yet supported
+    ):
+        key_type = _SSH_ED448
     else:
         raise ValueError("Unsupported key type")
 
@@ -582,6 +588,59 @@ def encode_private(
         f_priv.put_sshstr(f_keypair)
 
 
+class _SSHFormatEd448:
+    """Format for Ed448 keys.
+
+    Public:
+        bytes point
+    Private:
+        bytes point
+        bytes secret_and_point
+    """
+
+    def get_public(
+        self, data: memoryview
+    ) -> tuple[tuple[memoryview], memoryview]:
+        """Ed448 public fields"""
+        point, data = _get_sshstr(data)
+        return (point,), data
+
+    def load_public(
+        self, data: memoryview
+    ) -> tuple[ed448.Ed448PublicKey, memoryview]:
+        """Make Ed448 public key from data."""
+        (point,), data = self.get_public(data)
+        public_key = ed448.Ed448PublicKey.from_public_bytes(
+            point.tobytes()
+        )
+        return public_key, data
+
+    def load_private(
+        self, data: memoryview, pubfields
+    ) -> tuple[ed448.Ed448PrivateKey, memoryview]:
+        """Make Ed448 private key from data."""
+        raise UnsupportedAlgorithm(
+            "Loading Ed448 SSH private keys is unsupported"
+        )
+
+    def encode_public(
+        self, public_key: ed448.Ed448PublicKey, f_pub: _FragList
+    ) -> None:
+        """Write Ed448 public key"""
+        raw_public_key = public_key.public_bytes(
+            Encoding.Raw, PublicFormat.Raw
+        )
+        f_pub.put_sshstr(raw_public_key)
+
+    def encode_private(
+        self, private_key: ed448.Ed448PrivateKey, f_priv: _FragList
+    ) -> None:
+        """Write Ed448 private key"""
+        raise UnsupportedAlgorithm(
+            "Serializing Ed448 SSH private keys is unsupported"
+        )
+
+
 def load_application(data) -> tuple[memoryview, memoryview]:
     """
     U2F application strings
@@ -636,6 +695,7 @@ def load_public(
     _SSH_RSA: _SSHFormatRSA(),
     _SSH_DSA: _SSHFormatDSA(),
     _SSH_ED25519: _SSHFormatEd25519(),
+    _SSH_ED448: _SSHFormatEd448(),
     _ECDSA_NISTP256: _SSHFormatECDSA(b"nistp256", ec.SECP256R1()),
     _ECDSA_NISTP384: _SSHFormatECDSA(b"nistp384", ec.SECP384R1()),
     _ECDSA_NISTP521: _SSHFormatECDSA(b"nistp521", ec.SECP521R1()),

tests/hazmat/primitives/test_ssh.py

@@ -15,6 +15,7 @@
     dsa,
     ec,
     ed25519,
+    ed448,
     rsa,
 )
 from cryptography.hazmat.primitives.serialization import (
@@ -55,6 +56,7 @@ class TestOpenSSHSerialization:
             ("ecdsa-nopsw.key.pub", "ecdsa-nopsw.key-cert.pub"),
             ("ed25519-psw.key.pub", None),
             ("ed25519-nopsw.key.pub", "ed25519-nopsw.key-cert.pub"),
+            ("ed448-nopsw.key.pub", None),
             ("sk-ecdsa-psw.key.pub", None),
             ("sk-ecdsa-nopsw.key.pub", None),
             ("sk-ed25519-psw.key.pub", None),
@@ -64,6 +66,8 @@ class TestOpenSSHSerialization:
     def test_load_ssh_public_key(self, key_file, cert_file, backend):
         if "ed25519" in key_file and not backend.ed25519_supported():
             pytest.skip("Requires OpenSSL with Ed25519 support")
+        if "ed448" in key_file and not backend.ed448_supported():
+            pytest.skip("Requires OpenSSL with Ed448 support")
 
         # normal public key
         pub_data = load_vectors_from_file(
@@ -170,6 +174,8 @@ def run_partial_pubkey(self, pubdata, backend):
     def test_load_ssh_private_key(self, key_file, backend):
         if "ed25519" in key_file and not backend.ed25519_supported():
             pytest.skip("Requires OpenSSL with Ed25519 support")
+        if "ed448" in key_file and not backend.ed448_supported():
+            pytest.skip("Requires OpenSSL with Ed448 support")
         if "-psw" in key_file and not ssh._bcrypt_supported:
             pytest.skip("Requires bcrypt module")
 
@@ -1130,6 +1136,40 @@ def test_load_ssh_public_key_trailing_data(self, backend):
             load_ssh_public_key(ssh_key, backend)
 
 
+@pytest.mark.supported(
+    only_if=lambda backend: backend.ed448_supported(),
+    skip_message="Requires OpenSSL with Ed448 support",
+)
+class TestEd448SSHSerialization:
+    def test_load_ssh_public_key(self, backend):
+        ssh_key = (
+            b"ssh-ed448 AAAACXNzaC1lZDQ0OAAAADnVY+2PC4Oj9MSsYZORD7xivKK3zy"
+            b"yHFKYj3eMCMPsAwNVk6fqGHeSIRDN39ld5Jto8S5Y1lemtJHA= user@chir"
+            b"on.local"
+        )
+        key = load_ssh_public_key(ssh_key, backend)
+        assert isinstance(key, ed448.Ed448PublicKey)
+        assert key.public_bytes(Encoding.Raw, PublicFormat.Raw) == (
+            bytes.fromhex(
+                "d5 63 ed 8f 0b 83 a3 f4  c4 ac 61 93 91 0f bc 62"
+                "bc a2 b7 cf 2c 87 14 a6  23 dd e3 02 30 fb 00 c0"
+                "d5 64 e9 fa 86 1d e4 88  44 33 77 f6 57 79 26 da"
+                "3c 4b 96 35 95 e9 ad 24  70"
+            )
+        )
+
+    def test_public_bytes_openssh(self, backend):
+        ssh_key = (
+            b"ssh-ed448 AAAACXNzaC1lZDQ0OAAAADnVY+2PC4Oj9MSsYZORD7xivKK3zy"
+            b"yHFKYj3eMCMPsAwNVk6fqGHeSIRDN39ld5Jto8S5Y1lemtJHA="
+        )
+        key = load_ssh_public_key(ssh_key, backend)
+        assert isinstance(key, ed448.Ed448PublicKey)
+        assert (
+            key.public_bytes(Encoding.OpenSSH, PublicFormat.OpenSSH) == ssh_key
+        )
+
+
 class TestSSHCertificate:
     @pytest.mark.supported(
         only_if=lambda backend: backend.ed25519_supported(),

vectors/cryptography_vectors/asymmetric/OpenSSH/ed448-nopsw.key.pub

@@ -0,0 +1 @@
+ssh-ed448 AAAACXNzaC1lZDQ0OAAAADnVY+2PC4Oj9MSsYZORD7xivKK3zyyHFKYj3eMCMPsAwNVk6fqGHeSIRDN39ld5Jto8S5Y1lemtJHA=