Added bit_size function remove log clutter

2 files changed, 29 insertions, 19 deletions

diff --git a/rsa/__init__.py b/rsa/__init__.py
index d7fddd5..00c6c8f 100644
--- a/rsa/__init__.py
+++ b/rsa/__init__.py
@@ -13,6 +13,11 @@ import random
 import sys
 import types
 
+def bit_size(number):
+    """Returns the number of bits required to hold a specific long number"""
+
+    return int(math.ceil(math.log(number,2)))
+
 def gcd(p, q):
     """Returns the greatest common divisor of p and q
     >>> gcd(48, 180)
@@ -167,7 +172,7 @@ def fast_exponentiation(a, e, n):
     """
     #Single loop version is faster and uses less memory
     #MSB is always 1 so skip testing it and start with result = a
-    msbe = int(math.ceil(math.log(e,2))) - 2  #Find MSB-1 of exponent
+    msbe = bit_size(e) - 2      #Find MSB-1 of exponent
     test = long(1 << msbe)      #Isolate each expoent bit with test value
     a %= n                      #Throw away any overflow modulo n
     result = a                  #Start with result = a (skip MSB test)
@@ -198,7 +203,7 @@ def randint(minvalue, maxvalue):
     range = maxvalue - minvalue
 
     # Which is this number of bytes
-    rangebytes = int(math.ceil(math.log(range, 2) / 8.))
+    rangebytes = int(bit_size(range) / 8.)
 
     # Convert to bits, but make sure it's always at least min_nbits*2
     rangebits = max(rangebytes * 8, min_nbits * 2)
@@ -402,8 +407,8 @@ def encrypt_int(message, ekey, n):
         raise OverflowError("The message is too long")
 
     #Note: Bit exponents start at zero (bit counts start at 1) this is correct
-    safebit = int(math.floor(math.log(n,2))) - 1 #compute safe bit (MSB - 1)
-    message += (1 << safebit)                    #add safebit to ensure folding
+    safebit = bit_size(n) - 2                   #compute safe bit (MSB - 1)
+    message += (1 << safebit)                   #add safebit to ensure folding
 
     return fast_exponentiation(message, ekey, n)
 
@@ -413,8 +418,8 @@ def decrypt_int(cyphertext, dkey, n):
 
     message = fast_exponentiation(cyphertext, dkey, n)
 
-    safebit = int(math.floor(math.log(n,2))) - 1 #compute safe bit (MSB - 1)
-    message -= (1 << safebit)                    #remove safebit before decode
+    safebit = bit_size(n) - 2                   #compute safe bit (MSB - 1)
+    message -= (1 << safebit)                   #remove safebit before decode
 
     return message
 
@@ -459,8 +464,8 @@ def chopstring(message, key, n, funcref):
 
     msglen = len(message)
     mbits = msglen * 8
-    # floor of log deducts 1 bit of n and the - 1, deducts the second bit.
-    nbits = int(math.floor(math.log(n, 2))) - 1 # leave room for safebit
+    #Bit counts start at 1, bit numbers start at zero (so deduct 2)
+    nbits = bit_size(n) - 2             # leave room for safebit
     nbytes = nbits / 8
     blocks = msglen / nbytes
 
diff --git a/rsa/fastrsa.py b/rsa/fastrsa.py
index f1bd87a..1437da7 100644
--- a/rsa/fastrsa.py
+++ b/rsa/fastrsa.py
@@ -13,6 +13,11 @@ import random
 import sys
 import types
 
+def bit_size(number):
+    """Returns the number of bits required to hold a specific long number"""
+
+    return int(math.ceil(math.log(number,2)))
+
 def gcd(p, q):
     """Returns the greatest common divisor of p and q
     >>> gcd(48, 180)
@@ -167,7 +172,7 @@ def fast_exponentiation(a, e, n):
     """
     #Single loop version is faster and uses less memory
     #MSB is always 1 so skip testing it and start with result = a
-    msbe = int(math.ceil(math.log(e,2))) - 2  #Find MSB-1 of exponent
+    msbe = bit_size(e) - 2      #Find MSB-1 of exponent
     test = long(1 << msbe)      #Isolate each exponent bit with test value
     a %= n                      #Throw away any overflow modulo n
     result = a                  #Start with result = a (skip MSB test)
@@ -198,7 +203,7 @@ def randint(minvalue, maxvalue):
     range = maxvalue - minvalue
 
     # Which is this number of bytes
-    rangebytes = int(math.ceil(math.log(range, 2) / 8.))
+    rangebytes = ((bit_size(range) + 7) / 8)
 
     # Convert to bits, but make sure it's always at least min_nbits*2
     rangebits = max(rangebytes * 8, min_nbits * 2)
@@ -408,8 +413,8 @@ def encrypt_int(message, key):
         raise OverflowError("The message is too long")
 
     #Note: Bit exponents start at zero (bit counts start at 1) this is correct
-    safebit = int(math.floor(math.log(key['n'],2))) - 1 #safe bit is (MSB - 1)
-    message += (1 << safebit)                    #add safebit to ensure folding
+    safebit = bit_size(n) - 2                   #safe bit is (MSB - 1)
+    message += (1 << safebit)                   #add safebit to ensure folding
 
     return fast_exponentiation(message, key['e'], key['n'])
 
@@ -425,8 +430,8 @@ def verify_int(cyphertext, key):
     message = fast_exponentiation(cyphertext, key['e'], key['n'])
 
     #Note: Bit exponents start at zero (bit counts start at 1) this is correct
-    safebit = int(math.floor(math.log(key['n'],2))) - 1 #safe bit is (MSB - 1)
-    message -= (1 << safebit)                  #remove safe bit before decode
+    safebit = bit_size(n) - 2                   #safe bit is (MSB - 1)
+    message -= (1 << safebit)                   #remove safe bit before decode
 
     return message
 
@@ -444,8 +449,8 @@ def decrypt_int(cyphertext, key):
     h = (key['qi'] * dif) % p
     message = m2 + (h * q)
 
-    safebit = int(math.floor(math.log(n,2))) - 1 #safe bit is (MSB - 1)
-    message -= (1 << safebit)                    #remove safebit before decode
+    safebit = bit_size(n) - 2                   #safe bit is (MSB - 1)
+    message -= (1 << safebit)                   #remove safebit before decode
 
     return message
 
@@ -459,14 +464,14 @@ def sign_int(message, key):
     if not type(message) is types.LongType:
         raise TypeError("You must pass a long or int")
 
-    p = key['p']                #Reduce dictionary lookups
+    p = key['p']                                #Reduce dictionary lookups
     q = key['q']
     n = p * q
 
     if message < 0 or message > n:
         raise OverflowError("The message is too long")
 
-    safebit = int(math.floor(math.log(n,2))) - 1 #safe bit is (MSB - 1)
+    safebit = bit_size(n) - 2                   #safe bit is (MSB - 1)
     message += (1 << safebit)                    #add safebit before encrypt
 
     #Encrypt in 2 parts, using faster Chinese Remainder Theorem method
@@ -526,7 +531,7 @@ def chopstring(message, key, funcref):
     msglen = len(message)
     mbits = msglen * 8
     # floor of log deducts 1 bit of n and the - 1, deducts the second bit.
-    nbits = int(math.floor(math.log(n, 2))) - 1 # leave room for safebit
+    nbits = bit_size(n) - 2                 # leave room for safebit
     nbytes = nbits / 8
     blocks = msglen / nbytes