Implement RSASSA-PKCS1-v1_5 in dkim.crypto, and use that in verify() and sign(). Move str2int/int2str into dkim.crypto. Verification no longer does a constant-time string compare; there is no private key involved on which a timing attack could be performed.

dkim/__init__.py

@@ -9,6 +9,8 @@ from dkim.crypto import (
     EMSA_PKCS1_v1_5_encode,
     parse_private_key,
     parse_public_key,
+    RSASSA_PKCS1_v1_5_sign,
+    RSASSA_PKCS1_v1_5_verify,
     )
 
 __all__ = [
@@ -152,38 +154,10 @@ def validate_signature_fields(sig, debuglog=None):
             return False
     return True
 
-def perform_rsa(input, exponent, modulus, modlen):
-    return int2str(pow(str2int(input), exponent, modulus), modlen)
-
 # 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.
 
@@ -316,9 +290,8 @@ def sign(message, selector, domain, privkey, identity=None, canonicalize=(Simple
     if debuglog is not None:
         print >>debuglog, "sign digest:", " ".join("%02x" % ord(x) for x in d)
 
-    modlen = len(int2str(pk['modulus']))
-    encoded = EMSA_PKCS1_v1_5_encode(d, modlen, HASHID_SHA256)
-    sig2 = perform_rsa(encoded, pk['privateExponent'], pk['modulus'], modlen)
+    sig2 = RSASSA_PKCS1_v1_5_sign(
+        d, HASHID_SHA256, pk['privateExponent'], pk['modulus'])
     sig += base64.b64encode(''.join(sig2))
 
     return sig + "\r\n"
@@ -430,9 +403,6 @@ def verify(message, debuglog=None, dnsfunc=dnstxt):
                 print >>debuglog, "invalid format in _domainkey txt record"
             return False
     pk = parse_public_key(base64.b64decode(pub['p']))
-    modlen = len(int2str(pk['modulus']))
-    if debuglog is not None:
-        print >>debuglog, "modlen:", modlen
 
     include_headers = re.split(r"\s*:\s*", sig['h'])
     h = hasher()
@@ -441,18 +411,9 @@ def verify(message, debuglog=None, dnsfunc=dnstxt):
     d = h.digest()
     if debuglog is not None:
         print >>debuglog, "verify digest:", " ".join("%02x" % ord(x) for x in d)
+    signature = base64.b64decode(re.sub(r"\s+", "", sig['b']))
     try:
-        sig2 = EMSA_PKCS1_v1_5_encode(d, modlen, hashid)
+        return RSASSA_PKCS1_v1_5_verify(
+            d, hashid, signature, pk['publicExponent'], pk['modulus'])
     except ParameterError:
         return False
-    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'])
-    signature = base64.b64decode(re.sub(r"\s+", "", sig['b']))
-    v = perform_rsa(signature, 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]]

dkim/crypto.py

@@ -21,6 +21,8 @@ __all__ = [
     'EMSA_PKCS1_v1_5_encode',
     'parse_private_key',
     'parse_public_key',
+    'RSASSA_PKCS1_v1_5_sign',
+    'RSASSA_PKCS1_v1_5_verify',
     ]
 
 from dkim.asn1 import (
@@ -108,3 +110,51 @@ def EMSA_PKCS1_v1_5_encode(digest, modlen, hashid):
         raise Exception("Hash too large for modulus") # XXX: DKIMException
     return "\x00\x01"+"\xff"*(modlen-len(dinfo)-3)+"\x00"+dinfo
 
+
+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 ''.join(r)
+
+
+def perform_rsa(input, exponent, modulus, modlen):
+    return int2str(pow(str2int(input), exponent, modulus), modlen)
+
+
+def RSASSA_PKCS1_v1_5_sign(digest, hashid, private_exponent, modulus):
+    modlen = len(int2str(modulus))
+    encoded_digest = EMSA_PKCS1_v1_5_encode(digest, modlen, hashid)
+    return perform_rsa(encoded_digest, private_exponent, modulus, modlen)
+
+
+def RSASSA_PKCS1_v1_5_verify(digest, hashid, signature, public_exponent,
+                             modulus):
+    modlen = len(int2str(modulus))
+    encoded_digest = EMSA_PKCS1_v1_5_encode(digest, modlen, hashid)
+    signed_digest = perform_rsa(signature, public_exponent, modulus, modlen)
+    return encoded_digest == signed_digest