py-authorizedkeys/authorizedkeys/parser.py

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 to_string(self):
		result = ''
		if self.options is not None: result += self.options + ' '
		result += f'{self.type} {self.key_b64}'
		if self.comment is not None: result += ' ' + self.comment
		return result

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.

def dump_file(keys: list[AuthorizedKey | str], f: IO[str]) -> None:
	for rec in keys:
		if isinstance(rec, AuthorizedKey):
			rec = rec.to_string()
		f.write(rec)
		f.write('\n')