From 28298d3ce69a86b81a0b3eeae02559b4347183fc Mon Sep 17 00:00:00 2001
From: Franziska Kunsmann <hi@kunsmann.eu>
Date: Fri, 31 Mar 2023 21:41:12 +0200
Subject: [PATCH] replace predefined ssh keys with generated ones

---
 bundles/backup-client/items.py |  7 ++-
 bundles/backup-server/items.py |  8 +--
 libs/ssh.py                    | 96 ++++++++++++++++++++++++++++++++++
 3 files changed, 105 insertions(+), 6 deletions(-)
 create mode 100644 libs/ssh.py

diff --git a/bundles/backup-client/items.py b/bundles/backup-client/items.py
index 6538803..a4c9a11 100644
--- a/bundles/backup-client/items.py
+++ b/bundles/backup-client/items.py
@@ -33,14 +33,17 @@ else:
     backup_target = repo.get_node(node.metadata.get('backup-client/target'))
 
     files['/etc/backup.priv'] = {
-        'content': repo.vault.decrypt_file(join('backup', 'keys', f'{node.name}.key.vault')),
+        'content': repo.libs.ssh.generate_ed25519_private_key(
+            node.metadata.get('backup-client/user-name'),
+            backup_target,
+        ),
         'mode': '0400',
     }
 
     files['/usr/local/bin/generate-backup'] = {
         'content_type': 'mako',
         'context': {
-            'username': node.metadata['backup-client']['user-name'],
+            'username': node.metadata.get('backup-client/user-name'),
             'server': backup_target.metadata.get('backup-server/my_hostname'),
             'port': backup_target.metadata.get('backup-server/my_ssh_port'),
             'paths': backup_paths,
diff --git a/bundles/backup-server/items.py b/bundles/backup-server/items.py
index 11d0624..bd4d12f 100644
--- a/bundles/backup-server/items.py
+++ b/bundles/backup-server/items.py
@@ -27,9 +27,6 @@ directories['/etc/backup-server/clients'] = {
 sudoers = {}
 
 for nodename, config in node.metadata.get('backup-server/clients', {}).items():
-    with open(join(repo.path, 'data', 'backup', 'keys', f'{nodename}.pub'), 'r') as f:
-        pubkey = f.read().strip()
-
     sudoers[config['user']] = nodename
 
     users[config['user']] = {
@@ -41,7 +38,10 @@ for nodename, config in node.metadata.get('backup-server/clients', {}).items():
     }
 
     files[f'/srv/backups/{nodename}/.ssh/authorized_keys'] = {
-        'content': pubkey,
+        'content': repo.libs.ssh.generate_ed25519_public_key(
+            config['user'],
+            node,
+        ),
         'owner': config['user'],
         'mode': '0400',
         'needs': {
diff --git a/libs/ssh.py b/libs/ssh.py
new file mode 100644
index 0000000..90fb674
--- /dev/null
+++ b/libs/ssh.py
@@ -0,0 +1,96 @@
+from base64 import b64decode, b64encode
+from functools import lru_cache
+from hashlib import sha3_224
+
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+from cryptography.hazmat.primitives.serialization import (
+    Encoding,
+    NoEncryption,
+    PrivateFormat,
+    PublicFormat,
+)
+
+from bundlewrap.utils import Fault
+
+
+@lru_cache(maxsize=None)
+def generate_ed25519_private_key(username, node):
+    return Fault(
+        f'private key {username}@{node.name}',
+        lambda username, node: _generate_ed25519_private_key(username, node),
+        username=username,
+        node=node,
+    )
+
+
+@lru_cache(maxsize=None)
+def generate_ed25519_public_key(username, node):
+    return Fault(
+        f'public key {username}@{node.name}',
+        lambda username, node: _generate_ed25519_public_key(username, node),
+        username=username,
+        node=node,
+    )
+
+
+def _generate_ed25519_private_key(username, node):
+    privkey_bytes = Ed25519PrivateKey.from_private_bytes(_secret(username, node))
+
+    nondeterministic_privatekey = privkey_bytes.private_bytes(
+        encoding=Encoding.PEM,
+        format=PrivateFormat.OpenSSH,
+        encryption_algorithm=NoEncryption(),
+    ).decode()
+
+    # get relevant lines from string
+    nondeterministic_bytes = b64decode(
+        ''.join(nondeterministic_privatekey.split('\n')[1:-2])
+    )
+
+    # sanity check
+    if nondeterministic_bytes[98:102] != nondeterministic_bytes[102:106]:
+        raise Exception("checksums should be the same: whats going on here?")
+
+    # replace random bytes with deterministic values
+    random_bytes = sha3_224(_secret(username, node)).digest()[0:4]
+    deterministic_bytes = (
+        nondeterministic_bytes[:98]
+        + random_bytes
+        + random_bytes
+        + nondeterministic_bytes[106:]
+    )
+
+    # reassemble file
+    deterministic_privatekey = '\n'.join(
+        [
+            '-----BEGIN OPENSSH PRIVATE KEY-----',
+            b64encode(deterministic_bytes).decode(),
+            '-----END OPENSSH PRIVATE KEY-----',
+        ]
+    ) + '\n'
+
+    return deterministic_privatekey
+
+
+def _generate_ed25519_public_key(username, node):
+    return (
+        Ed25519PrivateKey.from_private_bytes(_secret(username, node))
+        .public_key()
+        .public_bytes(
+            encoding=Encoding.OpenSSH,
+            format=PublicFormat.OpenSSH,
+        )
+        .decode()
+        + f' {username}@{node.name}'
+    )
+
+
+@lru_cache(maxsize=None)
+def _secret(username, node):
+    return b64decode(
+        str(
+            node.repo.vault.random_bytes_as_base64_for(
+                f"{username}@{node.name}", length=32
+            )
+        )
+    )