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Move to a space-separated format

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This commit is contained in:
Juhani Krekelä 2018-08-30 00:16:22 +03:00
parent 5397293f33
commit 3a9c4baa4b
4 changed files with 117 additions and 76 deletions
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42
src/hashing.py
View file

@ -1,3 +1,4 @@
import base64
import hashlib
import os
@ -23,3 +24,44 @@ def hash_host(host):
salt = generate_salt()
hashed_host = hash_with_salt(host, salt)
return salt, hashed_host
def base64enc(b):
"""base64enc(bytes) → bytes
Uses no padding"""
# Base 64 encodes 3 bytes as 4 characters
# /byte 1\/byte 2\/byte 3\
# ABCDEFGHijklmnopQRSTUVWX
# \64 1/\64 2/\64 3/\64 4/
#
# If you have only one or two bytes, you don't have enough bits to
# fill all of the characters. The rest of the bits will be taken to
# be zeroes.
# /byte 1\
# ABCDEFGH0000
# \64 1/\64 2/
# /byte 1\
#
# /byte 1\/byte 2\
# ABCDEFGHijklmnop00
# \64 1/\64 2/\64 3/
#
# This way you end up with only 2 or 3 characters containing info.
# This usually gets padded into a multiple of 4 with =. However,
# since the amount of bytes left over mod 4 is enough to generate
# the padding back, we can strip it out
return base64.b64encode(b).replace(b'=', b'')
def base64dec(b64):
"""base64dec(bytes) → bytes
Can handle lack of padding."""
assert type(b64) == bytes
# Padded base64 is always a multiple of 4 bytes in length. The
# reasoning for this is because base64 decoding operates in groups
# of 4 base64 characters.
# Since we know the length of the string minus the padding, we can
# just pad it to the nearest multiple of 4
missing_padding_len = (4 - len(b64)%4) % 4
padding = b'=' * missing_padding_len
return base64.b64decode(b64 + padding, validate = True)

97
src/read_file.py
View file

@ -1,4 +1,4 @@
import base64
import hashing
import entry
@ -30,88 +30,79 @@ def parse_header(header):
(if any) if it is"""
assert type(header) == bytes
magic = header[0:6]
if magic != b'SSHWOT':
# Check that it ends in a newline
if len(header) == 0 or header[-1] != 0x0a:
raise FileFormatError('No newline after header')
# Split it into fields and make sure we have at least the magic and
# the version
fields = header[:-1].split(b' ', 3)
if len(fields) < 2:
raise FileFormatError('Too few fields in the header, expected at least magic and version')
# Check the magic
if fields[0] != b'SSHWOT':
raise FileFormatError('Invalid magic')
# Version 0 is the current one
version = header[6:7]
if version == b'':
raise FileFormatError('No newline after header')
if version != b'0':
raise VersionMismatch('Version %i not supported' % version[0])
# See if we have a comment
if header[7:8] == b' ':
if len(fields) == 3:
# It says we have
if header[8:9] == b'\n':
if len(fields[2]) == 0:
# No, we don't, but we do have a space telling we
# have. The header is malformed
raise FileFormatError('Missing comment or spurious space in the header')
else:
# Yes, we do
# Check it ends with a newline
if header[-1] != 0x0a:
raise FileFormatError('Missing newline at the end of the header')
# Yes, we do. Extract it
try:
file_comment = header[8:-1].decode('utf-8')
file_comment = fields[2].decode('utf-8')
except UnicodeDecodeError:
raise FileFormatError('Comment is not valid utf-8')
return file_comment
elif header[7:8] == b'\n':
# No, we have newline
return ''
else:
# No, we have something else
raise FileFormatError("Expected a space or a newline but got '%s' instead" % header[7:].decode('utf-8'))
file_comment = ''
return file_comment
def parse_entry(line):
"""parse_entry(bytes) → Entry"""
assert type(line) == bytes
def extract_b64_field(rest):
"""extract_b64_field(bytes) → (bytes: decoded_field, bytes:rest)"""
field_b64 = rest[0:44]
if len(field_b64) != 44:
raise FileFormatError('Unexpected end of line')
def decode_b64_field(b64):
try:
field = base64.b64decode(field_b64, validate = True)
return hashing.base64dec(b64)
except (ValueError, base64.binascii.Error) as err:
raise FileFormatError('Malformed base64 string: %s' % field_b64.decode('utf-8')) from err
raise FileFormatError('Malformed base64 string: %s' % b64.decode('utf-8')) from err
return field, rest[44:]
# Check that it ends in a newline
if len(line) == 0 or line[-1] != 0x0a:
raise FileFormatError('No newline after entry')
salt, rest = extract_b64_field(line)
hashed_host, rest = extract_b64_field(rest)
fingerprint, rest = extract_b64_field(rest)
# Split the line into fields and make sure we have at least the
# salt, the hashed host, and the fingerprint
fields = line[:-1].split(b' ', 3)
if len(fields) < 3:
raise FileFormatError('Too few fields in the entry, expected in the very least salt, hashed host, and fingerprint')
# What do we have after that?
if rest[0:1] == b' ':
# A comment?
if rest[1:2] == b'\n':
# No, but it says we have. It's malformed
salt = decode_b64_field(fields[0])
hashed_host = decode_b64_field(fields[1])
fingerprint = decode_b64_field(fields[2])
# See if we have a comment
if len(fields) == 4:
# It says we have
if len(fields[3]) == 0:
# No, we don't, but we do have a space telling we
# have. The header is malformed
raise FileFormatError('Missing comment or spurious space in the entry')
else:
# Yes. Make sure it ends in a newline
if rest[-1] != 0x0a:
raise FileFormatError('No newline after entry')
# Yes, we do. Extract it
try:
comment = rest[1:-1].decode('utf-8')
comment = fields[3].decode('utf-8')
except UnicodeDecodeError:
raise FileFormatError('Comment is not valid utf-8')
elif rest[0:1] == b'\n':
# A newline
comment = ''
else:
# Something else
raise FileFormatError('Expected a space or a newline but got "%s" instead' % rest.decode('utf-8'))
comment = ''
return entry.Entry(salt, hashed_host, fingerprint, comment)

22
src/write_file.py
View file

@ -1,4 +1,4 @@
import base64
import hashing
def write_header(f, file_comment):
"""write_header(file(wb), str)
@ -6,6 +6,8 @@ def write_header(f, file_comment):
assert type(file_comment) == str
# b'SSHWOT' magic
f.write(b'SSHWOT')
# Separating space
f.write(b' ')
# Version number
f.write(b'0')
# b' ' + file_comment, if there is one
@ -24,14 +26,20 @@ def write_entry(f, salt, hashed_host, fingerprint, comment):
assert type(fingerprint) == bytes and len(fingerprint) == 32
assert type(comment) == str
# base64 encoded (44 bytes): salt
f.write(base64.b64encode(salt))
# base64 encoded salt
f.write(hashing.base64enc(salt))
# base64 encoded (44 bytes): hashed_host
f.write(base64.b64encode(hashed_host))
# Separating space
f.write(b' ')
# base64 encoded (44 bytes): fingerprint
f.write(base64.b64encode(fingerprint))
# base64 encoded hashed_host
f.write(hashing.base64enc(hashed_host))
# Separating space
f.write(b' ')
# base64 encoded fingerprint
f.write(hashing.base64enc(fingerprint))
# b' ' + comment, if there is one
if len(comment) > 0:

32
sshwot-format.text
View file

@ -1,34 +1,34 @@
Please note that all text insire quotes in the EBNF here is to be taken to
mean bytes that would decode as that using either the ASCII or the UTF-8
character encoding. "\n" refers specifically to the byte 0x0a, and no
alternative newlines are acceptable.
mean bytes that would decode as that using the ASCII. character encoding.
"\n" refers specifically to the byte 0x0a, and no alternative newlines are
acceptable.
The file has a header like:
magic = "SSHWOT" ;
version = "0" ;
comment = " ", ? General comment about the file. Valid utf-8, no '\n'. ? ;
header = magic, version, [comment], "\n" ;
comment = ? General comment about the file. Valid utf-8, no '\n'. ? ;
header = magic, " ", version, [" ", comment], "\n" ;
Examples of valid headers would be "SSHWOT0\n" and "SSHWOT0 Emma G. 2018\n".
Examples of valid headers would be "SSHWOT 0\n" and "SSHWOT 0 Emma G. 2018\n".
"SSHWOT0 \n" is not valid, since a space marks that there will be a comment.
"SSHWOT 0 \n" is not valid, since a space marks that there will be a comment.
After the header the entries are laid out as:
salt = ? base64 encoded salt, 44 bytes long ? ;
hashed host = ? base64 encoded sha256(host concat salt), 44 bytes long ? ;
fingerprint = ? base64 encoded sha256-fingerprint, 44 bytes long ? ;
comment = " ", ? Comment about the host/key. Valid utf-8, no '\n'. ? ;
entry = salt, hashed host, fingerprint, [comment], "\n" ;
salt = ? base64(salt) ? ;
hashed host = ? base64(sha256(host concat salt)) ? ;
fingerprint = ? base64(sha256-fingerprint) ? ;
comment = ? Comment about the host/key. Valid utf-8, no '\n'. ? ;
entry = salt, " ", hashed host, " ", fingerprint, [" ", comment], "\n" ;
The version of base64 used uses + for 62 and / for 63, uses = for padding,
and contains no breaks.
The version of base64 used uses + for 62 and / for 63, doesn't use = for
padding, and contains no breaks.
Examples of valid entries are
"Yixx+B6zrFoubPhBddgyx0nXHmbqMW1Wzneo4JqJv0U=yPUACFC/zPt/ENoIluOuWiTXor3r7oHhac63qej637E=QUJDREVGR0hJSktMTU5PUFFSU1RVVldYWVpbXF1eX2A=\n"
"Yixx+B6zrFoubPhBddgyx0nXHmbqMW1Wzneo4JqJv0U yPUACFC/zPt/ENoIluOuWiTXor3r7oHhac63qej637E QUJDREVGR0hJSktMTU5PUFFSU1RVVldYWVpbXF1eX2A\n"
and
"bd/MfFs+DMVqNQQoZGGCvpTopeS0/Jt6GS5vg7J+638=cbbdTnuIh0ZwnM+/r3sAu4iHgaN3mpkcP9kJND4vBUo=YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXp7fH1+f4A= The old one\n"
"bd/MfFs+DMVqNQQoZGGCvpTopeS0/Jt6GS5vg7J+638 cbbdTnuIh0ZwnM+/r3sAu4iHgaN3mpkcP9kJND4vBUo YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXp7fH1+f4A The old one\n"
Again, if there is a space following the necessary parts, there must also be
a comment or else the entry is malformed.