Reinstated some of the backed out changeset 812d745b6bef:

    - added support for block size
    - improved some exceptions

2 files changed, 45 insertions, 3 deletions

diff --git a/rsa/core.py b/rsa/core.py
index 8f989df..2df4be5 100755
--- a/rsa/core.py
+++ b/rsa/core.py
@@ -17,8 +17,11 @@ def encrypt_int(message, ekey, n):
     if not type(message) is types.LongType:
         raise TypeError("You must pass a long or int")
 
-    if message < 0 or message > n:
-        raise OverflowError("The message is too long")
+    if message < 0:
+        raise ValueError('Only non-negative numbers are supported')
+         
+    if message > n:
+        raise OverflowError("The message %i is too long for n=%i" % (message, n))
 
     #Note: Bit exponents start at zero (bit counts start at 1) this is correct
     safebit = rsa.common.bit_size(n) - 2        # compute safe bit (MSB - 1)
diff --git a/rsa/transform.py b/rsa/transform.py
index 35b7461..076c385 100755
--- a/rsa/transform.py
+++ b/rsa/transform.py
@@ -9,8 +9,22 @@ import types
 def bit_size(number):
     """Returns the number of bits required to hold a specific long number"""
 
+    if number < 0:
+        raise ValueError('Only nonnegative numbers possible: %s' % number)
+
+    if number == 0:
+        return 1
+    
     return int(math.ceil(math.log(number, 2)))
 
+def byte_size(number):
+    """Returns the number of bytes required to hold a specific long number.
+    
+    The number of bytes is rounded up.
+    """
+
+    return int(math.ceil(bit_size(number) / 8.0))
+
 def bytes2int(bytes):
     """Converts a list of bytes or an 8-bit string to an integer.
 
@@ -72,7 +86,7 @@ def int2bytes(number, block_size=None):
 
     # Do some bounds checking
     if block_size is not None:
-        needed_bytes = int(math.ceil(bit_size(number) / 8.0))
+        needed_bytes = byte_size(number)
         if needed_bytes > block_size:
             raise OverflowError('Needed %i bytes for number, but block size '
                 'is %i' % (needed_bytes, block_size))
@@ -92,6 +106,31 @@ def int2bytes(number, block_size=None):
     return padding + ''.join(bytes)
 
 
+def block_op(block_provider, block_size, operation):
+    r'''Generator, applies the operation on each block and yields the result
+    
+    Each block is converted to a number, the given operation is applied and then
+    the resulting number is converted back to a block of data. The resulting
+    block is yielded.
+    
+    @param block_provider: an iterable that iterates over the data blocks.
+    @param block_size: the used block size
+    @param operation: a function that accepts an integer and returns an integer 
+    
+    >>> blocks = ['\x00\x01\x02', '\x03\x04\x05']
+    >>> list(block_op(blocks, 3, lambda x: (x + 6)))
+    ['\x00\x01\x08', '\x03\x04\x0b']
+    
+    '''
+
+    for block in block_provider:
+        number = bytes2int(block)
+        print 'In : %i (%i bytes)' % (number, byte_size(number))
+        after_op = operation(number)
+        print 'Out: %i (%i bytes)' % (after_op, byte_size(after_op))
+        yield int2bytes(after_op, block_size)
+
+
 def to64(number):
     """Converts a number in the range of 0 to 63 into base 64 digit
     character in the range of '0'-'9', 'A'-'Z', 'a'-'z','-','_'.