Initial implementation

It should work, nobody tested it, there are TODO's which will likely
never be implemented :-)

authorizedkeys/parser.py

@@ -0,0 +1,63 @@
+from typing import IO
+
+# Copied from sshd(8)
+key_types = [
+	"sk-ecdsa-sha2-nistp256@openssh.com",
+	"ecdsa-sha2-nistp256",
+	"ecdsa-sha2-nistp384",
+	"ecdsa-sha2-nistp521",
+	"sk-ssh-ed25519@openssh.com",
+	"ssh-ed25519",
+	"ssh-dss",
+	"ssh-rsa",
+	]
+
+
+class AuthorizedKey:
+	def __init__(self, line):
+		self.original = line
+		line = line.strip()
+		if line.startswith('#') or line == '':
+			raise ValueError('This is not a key, it is a comment or an empty line.')
+		if line.split(' ')[0] not in key_types:
+			# The hard case: there are options at the beginning.
+			# It is too simple to code the state machine myself, so no library :-)
+			in_quotes = False
+			backslash_preceding = False
+			for idx, char in enumerate(line):
+				if char == '\"' and not backslash_preceeding:
+						in_quotes = not in_quotes
+				elif char == ' ' and not in_quotes:
+						break
+				else:
+					backslash_preceeding = char == '\\'
+			else:
+				raise ValueError('Badly formatted options not followed by a key.')
+
+			if line[idx] != ' ':
+				raise ValueError('I am just broken.')
+			self.options = line[:idx]
+			line = line[idx+1:]
+			line = line.strip()	# In case there are multiple spaces
+		else:
+			self.options = None
+
+		# Now only the key follows, so this is simple
+		self.options = None
+		split = line.split(' ', maxsplit=2)
+		self.type = split[0]
+		self.key_b64 = split[1]
+		self.coment = split[2] if len(split) >= 3 else None
+
+def parse_file(f: IO[str]) -> list[AuthorizedKey | str]:
+	result = []
+	for line in f:
+		stripped = line.strip()
+		if stripped.startswith('#') or stripped == '':
+			# This is a comment / empty line, should be preserved
+			result.append(line)
+		else:
+			result.append(AuthorizedKey(line))
+	return result
+
+# TODO: Implement option parsing, key validation and decoding to bytes.