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Turn dkim into a package.

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William Grant
2011-03-09 18:07:27 +11:00
parent 25d3294b30
commit df2c20d4b3
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dkim/__init__.py
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# This software is provided 'as-is', without any express or implied
# warranty. In no event will the author be held liable for any damages
# arising from the use of this software.
#
# Permission is granted to anyone to use this software for any purpose,
# including commercial applications, and to alter it and redistribute it
# freely, subject to the following restrictions:
#
# 1. The origin of this software must not be misrepresented; you must not
# claim that you wrote the original software. If you use this software
# in a product, an acknowledgment in the product documentation would be
# appreciated but is not required.
# 2. Altered source versions must be plainly marked as such, and must not be
# misrepresented as being the original software.
# 3. This notice may not be removed or altered from any source distribution.
#
# Copyright (c) 2008 Greg Hewgill http://hewgill.com
import base64
import hashlib
import re
import time
import dns.resolver
__all__ = [
"Simple",
"Relaxed",
"InternalError",
"KeyFormatError",
"MessageFormatError",
"ParameterError",
"sign",
"verify",
]
class Simple:
"""Class that represents the "simple" canonicalization algorithm."""
name = "simple"
@staticmethod
def canonicalize_headers(headers):
# No changes to headers.
return headers
@staticmethod
def canonicalize_body(body):
# Ignore all empty lines at the end of the message body.
return re.sub("(\r\n)*$", "\r\n", body)
class Relaxed:
"""Class that represents the "relaxed" canonicalization algorithm."""
name = "relaxed"
@staticmethod
def canonicalize_headers(headers):
# Convert all header field names to lowercase.
# Unfold all header lines.
# Compress WSP to single space.
# Remove all WSP at the start or end of the field value (strip).
return [(x[0].lower(), re.sub(r"\s+", " ", re.sub("\r\n", "", x[1])).strip()+"\r\n") for x in headers]
@staticmethod
def canonicalize_body(body):
# Remove all trailing WSP at end of lines.
# Compress non-line-ending WSP to single space.
# Ignore all empty lines at the end of the message body.
return re.sub("(\r\n)*$", "\r\n", re.sub(r"[\x09\x20]+", " ", re.sub("[\\x09\\x20]+\r\n", "\r\n", body)))
class DKIMException(Exception):
"""Base class for DKIM errors."""
pass
class InternalError(DKIMException):
"""Internal error in dkim module. Should never happen."""
pass
class KeyFormatError(DKIMException):
"""Key format error while parsing an RSA public or private key."""
pass
class MessageFormatError(DKIMException):
"""RFC822 message format error."""
pass
class ParameterError(DKIMException):
"""Input parameter error."""
pass
def _remove(s, t):
i = s.find(t)
assert i >= 0
return s[:i] + s[i+len(t):]
INTEGER = 0x02
BIT_STRING = 0x03
OCTET_STRING = 0x04
NULL = 0x05
OBJECT_IDENTIFIER = 0x06
SEQUENCE = 0x30
ASN1_Object = [
(SEQUENCE, [
(SEQUENCE, [
(OBJECT_IDENTIFIER,),
(NULL,),
]),
(BIT_STRING,),
])
]
ASN1_RSAPublicKey = [
(SEQUENCE, [
(INTEGER,),
(INTEGER,),
])
]
ASN1_RSAPrivateKey = [
(SEQUENCE, [
(INTEGER,),
(INTEGER,),
(INTEGER,),
(INTEGER,),
(INTEGER,),
(INTEGER,),
(INTEGER,),
(INTEGER,),
(INTEGER,),
])
]
def asn1_parse(template, data):
"""Parse a data structure according to ASN.1 template.
@param template: A list of tuples comprising the ASN.1 template.
@param data: A list of bytes to parse.
"""
r = []
i = 0
for t in template:
tag = ord(data[i])
i += 1
if tag == t[0]:
length = ord(data[i])
i += 1
if length & 0x80:
n = length & 0x7f
length = 0
for j in range(n):
length = (length << 8) | ord(data[i])
i += 1
if tag == INTEGER:
n = 0
for j in range(length):
n = (n << 8) | ord(data[i])
i += 1
r.append(n)
elif tag == BIT_STRING:
r.append(data[i:i+length])
i += length
elif tag == NULL:
assert length == 0
r.append(None)
elif tag == OBJECT_IDENTIFIER:
r.append(data[i:i+length])
i += length
elif tag == SEQUENCE:
r.append(asn1_parse(t[1], data[i:i+length]))
i += length
else:
raise KeyFormatError("Unexpected tag in template: %02x" % tag)
else:
raise KeyFormatError("Unexpected tag (got %02x, expecting %02x)" % (tag, t[0]))
return r
def asn1_length(n):
"""Return a string representing a field length in ASN.1 format."""
assert n >= 0
if n < 0x7f:
return chr(n)
r = ""
while n > 0:
r = chr(n & 0xff) + r
n >>= 8
return r
def asn1_build(node):
"""Build an ASN.1 data structure based on pairs of (type, data)."""
if node[0] == OCTET_STRING:
return chr(OCTET_STRING) + asn1_length(len(node[1])) + node[1]
if node[0] == NULL:
assert node[1] is None
return chr(NULL) + asn1_length(0)
elif node[0] == OBJECT_IDENTIFIER:
return chr(OBJECT_IDENTIFIER) + asn1_length(len(node[1])) + node[1]
elif node[0] == SEQUENCE:
r = ""
for x in node[1]:
r += asn1_build(x)
return chr(SEQUENCE) + asn1_length(len(r)) + r
else:
raise InternalError("Unexpected tag in template: %02x" % node[0])
# These values come from RFC 3447, section 9.2 Notes, page 43.
HASHID_SHA1 = "\x2b\x0e\x03\x02\x1a"
HASHID_SHA256 = "\x60\x86\x48\x01\x65\x03\x04\x02\x01"
def str2int(s):
"""Convert an octet string to an integer. Octet string assumed to represent a positive integer."""
r = 0
for c in s:
r = (r << 8) | ord(c)
return r
def int2str(n, length = -1):
"""Convert an integer to an octet string. Number must be positive.
@param n: Number to convert.
@param length: Minimum length, or -1 to return the smallest number of bytes that represent the integer.
"""
assert n >= 0
r = []
while length < 0 or len(r) < length:
r.append(chr(n & 0xff))
n >>= 8
if length < 0 and n == 0: break
r.reverse()
assert length < 0 or len(r) == length
return r
def rfc822_parse(message):
"""Parse a message in RFC822 format.
@param message: The message in RFC822 format. Either CRLF or LF is an accepted line separator.
@return Returns a tuple of (headers, body) where headers is a list of (name, value) pairs.
The body is a CRLF-separated string.
"""
headers = []
lines = re.split("\r?\n", message)
i = 0
while i < len(lines):
if len(lines[i]) == 0:
# End of headers, return what we have plus the body, excluding the blank line.
i += 1
break
if re.match(r"[\x09\x20]", lines[i][0]):
headers[-1][1] += lines[i]+"\r\n"
else:
m = re.match(r"([\x21-\x7e]+?):", lines[i])
if m is not None:
headers.append([m.group(1), lines[i][m.end(0):]+"\r\n"])
elif lines[i].startswith("From "):
pass
else:
raise MessageFormatError("Unexpected characters in RFC822 header: %s" % lines[i])
i += 1
return (headers, "\r\n".join(lines[i:]))
def dnstxt(name):
"""Return a TXT record associated with a DNS name."""
a = dns.resolver.query(name, dns.rdatatype.TXT)
for r in a.response.answer:
if r.rdtype == dns.rdatatype.TXT:
return "".join(r.items[0].strings)
return None
def fold(header):
"""Fold a header line into multiple crlf-separated lines at column 72."""
i = header.rfind("\r\n ")
if i == -1:
pre = ""
else:
i += 3
pre = header[:i]
header = header[i:]
while len(header) > 72:
i = header[:72].rfind(" ")
if i == -1:
j = i
else:
j = i + 1
pre += header[:i] + "\r\n "
header = header[j:]
return pre + header
def sign(message, selector, domain, privkey, identity=None, canonicalize=(Simple, Simple), include_headers=None, length=False, debuglog=None):
"""Sign an RFC822 message and return the DKIM-Signature header line.
@param message: an RFC822 formatted message (with either \\n or \\r\\n line endings)
@param selector: the DKIM selector value for the signature
@param domain: the DKIM domain value for the signature
@param privkey: a PKCS#1 private key in base64-encoded text form
@param identity: the DKIM identity value for the signature (default "@"+domain)
@param canonicalize: the canonicalization algorithms to use (default (Simple, Simple))
@param include_headers: a list of strings indicating which headers are to be signed (default all headers)
@param length: true if the l= tag should be included to indicate body length (default False)
@param debuglog: a file-like object to which debug info will be written (default None)
"""
(headers, body) = rfc822_parse(message)
m = re.search("--\n(.*?)\n--", privkey, re.DOTALL)
if m is None:
raise KeyFormatError("Private key not found")
try:
pkdata = base64.b64decode(m.group(1))
except TypeError, e:
raise KeyFormatError(str(e))
if debuglog is not None:
print >>debuglog, " ".join("%02x" % ord(x) for x in pkdata)
pka = asn1_parse(ASN1_RSAPrivateKey, pkdata)
pk = {
'version': pka[0][0],
'modulus': pka[0][1],
'publicExponent': pka[0][2],
'privateExponent': pka[0][3],
'prime1': pka[0][4],
'prime2': pka[0][5],
'exponent1': pka[0][6],
'exponent2': pka[0][7],
'coefficient': pka[0][8],
}
if identity is not None and not identity.endswith(domain):
raise ParameterError("identity must end with domain")
headers = canonicalize[0].canonicalize_headers(headers)
if include_headers is None:
include_headers = [x[0].lower() for x in headers]
else:
include_headers = [x.lower() for x in include_headers]
sign_headers = [x for x in headers if x[0].lower() in include_headers]
body = canonicalize[1].canonicalize_body(body)
h = hashlib.sha256()
h.update(body)
bodyhash = base64.b64encode(h.digest())
sigfields = [x for x in [
('v', "1"),
('a', "rsa-sha256"),
('c', "%s/%s" % (canonicalize[0].name, canonicalize[1].name)),
('d', domain),
('i', identity or "@"+domain),
length and ('l', len(body)),
('q', "dns/txt"),
('s', selector),
('t', str(int(time.time()))),
('h', " : ".join(x[0] for x in sign_headers)),
('bh', bodyhash),
('b', ""),
] if x]
sig = "DKIM-Signature: " + "; ".join("%s=%s" % x for x in sigfields)
sig = fold(sig)
if debuglog is not None:
print >>debuglog, "sign headers:", sign_headers + [("DKIM-Signature", " "+"; ".join("%s=%s" % x for x in sigfields))]
h = hashlib.sha256()
for x in sign_headers:
h.update(x[0])
h.update(":")
h.update(x[1])
h.update(sig)
d = h.digest()
if debuglog is not None:
print >>debuglog, "sign digest:", " ".join("%02x" % ord(x) for x in d)
dinfo = asn1_build(
(SEQUENCE, [
(SEQUENCE, [
(OBJECT_IDENTIFIER, HASHID_SHA256),
(NULL, None),
]),
(OCTET_STRING, d),
])
)
modlen = len(int2str(pk['modulus']))
if len(dinfo)+3 > modlen:
raise ParameterError("Hash too large for modulus")
sig2 = int2str(pow(str2int("\x00\x01"+"\xff"*(modlen-len(dinfo)-3)+"\x00"+dinfo), pk['privateExponent'], pk['modulus']), modlen)
sig += base64.b64encode(''.join(sig2))
return sig + "\r\n"
def verify(message, debuglog=None):
"""Verify a DKIM signature on an RFC822 formatted message.
@param message: an RFC822 formatted message (with either \\n or \\r\\n line endings)
@param debuglog: a file-like object to which debug info will be written (default None)
"""
(headers, body) = rfc822_parse(message)
sigheaders = [x for x in headers if x[0].lower() == "dkim-signature"]
if len(sigheaders) < 1:
return False
# Currently, we only validate the first DKIM-Signature line found.
a = re.split(r"\s*;\s*", sigheaders[0][1].strip())
if debuglog is not None:
print >>debuglog, "a:", a
sig = {}
for x in a:
if x:
m = re.match(r"(\w+)\s*=\s*(.*)", x, re.DOTALL)
if m is None:
if debuglog is not None:
print >>debuglog, "invalid format of signature part: %s" % x
return False
sig[m.group(1)] = m.group(2)
if debuglog is not None:
print >>debuglog, "sig:", sig
if 'v' not in sig:
if debuglog is not None:
print >>debuglog, "signature missing v="
return False
if sig['v'] != "1":
if debuglog is not None:
print >>debuglog, "v= value is not 1 (%s)" % sig['v']
return False
if 'a' not in sig:
if debuglog is not None:
print >>debuglog, "signature missing a="
return False
if 'b' not in sig:
if debuglog is not None:
print >>debuglog, "signature missing b="
return False
if re.match(r"[\s0-9A-Za-z+/]+=*$", sig['b']) is None:
if debuglog is not None:
print >>debuglog, "b= value is not valid base64 (%s)" % sig['b']
return False
if 'bh' not in sig:
if debuglog is not None:
print >>debuglog, "signature missing bh="
return False
if re.match(r"[\s0-9A-Za-z+/]+=*$", sig['bh']) is None:
if debuglog is not None:
print >>debuglog, "bh= value is not valid base64 (%s)" % sig['bh']
return False
if 'd' not in sig:
if debuglog is not None:
print >>debuglog, "signature missing d="
return False
if 'h' not in sig:
if debuglog is not None:
print >>debuglog, "signature missing h="
return False
if 'i' in sig and (not sig['i'].endswith(sig['d']) or sig['i'][-len(sig['d'])-1] not in "@."):
if debuglog is not None:
print >>debuglog, "i= domain is not a subdomain of d= (i=%s d=%d)" % (sig['i'], sig['d'])
return False
if 'l' in sig and re.match(r"\d{,76}$", sig['l']) is None:
if debuglog is not None:
print >>debuglog, "l= value is not a decimal integer (%s)" % sig['l']
return False
if 'q' in sig and sig['q'] != "dns/txt":
if debuglog is not None:
print >>debuglog, "q= value is not dns/txt (%s)" % sig['q']
return False
if 's' not in sig:
if debuglog is not None:
print >>debuglog, "signature missing s="
return False
if 't' in sig and re.match(r"\d+$", sig['t']) is None:
if debuglog is not None:
print >>debuglog, "t= value is not a decimal integer (%s)" % sig['t']
return False
if 'x' in sig:
if re.match(r"\d+$", sig['x']) is None:
if debuglog is not None:
print >>debuglog, "x= value is not a decimal integer (%s)" % sig['x']
return False
if int(sig['x']) < int(sig['t']):
if debuglog is not None:
print >>debuglog, "x= value is less than t= value (x=%s t=%s)" % (sig['x'], sig['t'])
return False
m = re.match("(\w+)(?:/(\w+))?$", sig['c'])
if m is None:
if debuglog is not None:
print >>debuglog, "c= value is not in format method/method (%s)" % sig['c']
return False
can_headers = m.group(1)
if m.group(2) is not None:
can_body = m.group(2)
else:
can_body = "simple"
if can_headers == "simple":
canonicalize_headers = Simple
elif can_headers == "relaxed":
canonicalize_headers = Relaxed
else:
if debuglog is not None:
print >>debuglog, "Unknown header canonicalization (%s)" % can_headers
return False
headers = canonicalize_headers.canonicalize_headers(headers)
if can_body == "simple":
body = Simple.canonicalize_body(body)
elif can_body == "relaxed":
body = Relaxed.canonicalize_body(body)
else:
if debuglog is not None:
print >>debuglog, "Unknown body canonicalization (%s)" % can_body
return False
if sig['a'] == "rsa-sha1":
hasher = hashlib.sha1
hashid = HASHID_SHA1
elif sig['a'] == "rsa-sha256":
hasher = hashlib.sha256
hashid = HASHID_SHA256
else:
if debuglog is not None:
print >>debuglog, "Unknown signature algorithm (%s)" % sig['a']
return False
if 'l' in sig:
body = body[:int(sig['l'])]
h = hasher()
h.update(body)
bodyhash = h.digest()
if debuglog is not None:
print >>debuglog, "bh:", base64.b64encode(bodyhash)
if bodyhash != base64.b64decode(re.sub(r"\s+", "", sig['bh'])):
if debuglog is not None:
print >>debuglog, "body hash mismatch (got %s, expected %s)" % (base64.b64encode(bodyhash), sig['bh'])
return False
s = dnstxt(sig['s']+"._domainkey."+sig['d']+".")
if not s:
return False
a = re.split(r"\s*;\s*", s)
# Trailing ';' on signature record is valid, see RFC 4871 3.2
# tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ]
if a[-1] == '':
a.pop(-1)
pub = {}
for f in a:
m = re.match(r"(\w+)=(.*)", f)
if m is not None:
pub[m.group(1)] = m.group(2)
else:
if debuglog is not None:
print >>debuglog, "invalid format in _domainkey txt record"
return False
x = asn1_parse(ASN1_Object, base64.b64decode(pub['p']))
# Not sure why the [1:] is necessary to skip a byte.
pkd = asn1_parse(ASN1_RSAPublicKey, x[0][1][1:])
pk = {
'modulus': pkd[0][0],
'publicExponent': pkd[0][1],
}
modlen = len(int2str(pk['modulus']))
if debuglog is not None:
print >>debuglog, "modlen:", modlen
include_headers = re.split(r"\s*:\s*", sig['h'])
if debuglog is not None:
print >>debuglog, "include_headers:", include_headers
sign_headers = []
lastindex = {}
for h in include_headers:
i = lastindex.get(h, len(headers))
while i > 0:
i -= 1
if h.lower() == headers[i][0].lower():
sign_headers.append(headers[i])
break
lastindex[h] = i
# The call to _remove() assumes that the signature b= only appears once in the signature header
sign_headers += [(x[0], x[1].rstrip()) for x in canonicalize_headers.canonicalize_headers([(sigheaders[0][0], _remove(sigheaders[0][1], sig['b']))])]
if debuglog is not None:
print >>debuglog, "verify headers:", sign_headers
h = hasher()
for x in sign_headers:
h.update(x[0])
h.update(":")
h.update(x[1])
d = h.digest()
if debuglog is not None:
print >>debuglog, "verify digest:", " ".join("%02x" % ord(x) for x in d)
dinfo = asn1_build(
(SEQUENCE, [
(SEQUENCE, [
(OBJECT_IDENTIFIER, hashid),
(NULL, None),
]),
(OCTET_STRING, d),
])
)
if debuglog is not None:
print >>debuglog, "dinfo:", " ".join("%02x" % ord(x) for x in dinfo)
if len(dinfo)+3 > modlen:
if debuglog is not None:
print >>debuglog, "Hash too large for modulus"
return False
sig2 = "\x00\x01"+"\xff"*(modlen-len(dinfo)-3)+"\x00"+dinfo
if debuglog is not None:
print >>debuglog, "sig2:", " ".join("%02x" % ord(x) for x in sig2)
print >>debuglog, sig['b']
print >>debuglog, re.sub(r"\s+", "", sig['b'])
v = int2str(pow(str2int(base64.b64decode(re.sub(r"\s+", "", sig['b']))), pk['publicExponent'], pk['modulus']), modlen)
if debuglog is not None:
print >>debuglog, "v:", " ".join("%02x" % ord(x) for x in v)
assert len(v) == len(sig2)
# Byte-by-byte compare of signatures
return not [1 for x in zip(v, sig2) if x[0] != x[1]]
if __name__ == "__main__":
message = """From: greg@hewgill.com\r\nSubject: test\r\n message\r\n\r\nHi.\r\n\r\nWe lost the game. Are you hungry yet?\r\n\r\nJoe.\r\n"""
print rfc822_parse(message)
sig = sign(message, "greg", "hewgill.com", open("/home/greg/.domainkeys/rsa.private").read())
print sig
print verify(sig+message)
#print sign(open("/home/greg/tmp/message").read(), "greg", "hewgill.com", open("/home/greg/.domainkeys/rsa.private").read())