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lib/python3.5/site-packages/Crypto/Util/Counter.py

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+# -*- coding: ascii -*-
+#
+#  Util/Counter.py : Fast counter for use with CTR-mode ciphers
+#
+# Written in 2008 by Dwayne C. Litzenberger <dlitz@dlitz.net>
+#
+# ===================================================================
+# The contents of this file are dedicated to the public domain.  To
+# the extent that dedication to the public domain is not available,
+# everyone is granted a worldwide, perpetual, royalty-free,
+# non-exclusive license to exercise all rights associated with the
+# contents of this file for any purpose whatsoever.
+# No rights are reserved.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+# ===================================================================
+"""Fast counter functions for CTR cipher modes.
+
+CTR is a chaining mode for symmetric block encryption or decryption.
+Messages are divideded into blocks, and the cipher operation takes
+place on each block using the secret key and a unique *counter block*.
+
+The most straightforward way to fulfil the uniqueness property is
+to start with an initial, random *counter block* value, and increment it as
+the next block is processed.
+
+The block ciphers from `Crypto.Cipher` (when configured in *MODE_CTR* mode)
+invoke a callable object (the *counter* parameter) to get the next *counter block*.
+Unfortunately, the Python calling protocol leads to major performance degradations.
+
+The counter functions instantiated by this module will be invoked directly
+by the ciphers in `Crypto.Cipher`. The fact that the Python layer is bypassed
+lead to more efficient (and faster) execution of CTR cipher modes.
+
+An example of usage is the following:
+
+    >>> from Crypto.Cipher import AES
+    >>> from Crypto.Util import Counter
+    >>>
+    >>> pt = b'\x00'*1000000
+    >>> ctr = Counter.new(128)
+    >>> cipher = AES.new(b'\x00'*16, AES.MODE_CTR, counter=ctr)
+    >>> ct = cipher.encrypt(pt)
+
+:undocumented: __package__
+"""
+import sys
+if sys.version_info[0] == 2 and sys.version_info[1] == 1:
+    from Crypto.Util.py21compat import *
+from Crypto.Util.py3compat import *
+
+from Crypto.Util import _counter
+import struct
+
+# Factory function
+def new(nbits, prefix=b(""), suffix=b(""), initial_value=1, overflow=0, little_endian=False, allow_wraparound=False, disable_shortcut=False):
+    """Create a stateful counter block function suitable for CTR encryption modes.
+
+    Each call to the function returns the next counter block.
+    Each counter block is made up by three parts::
+ 
+      prefix || counter value || postfix
+
+    The counter value is incremented by one at each call.
+
+    :Parameters:
+      nbits : integer
+        Length of the desired counter, in bits. It must be a multiple of 8.
+      prefix : byte string
+        The constant prefix of the counter block. By default, no prefix is
+        used.
+      suffix : byte string
+        The constant postfix of the counter block. By default, no suffix is
+        used.
+      initial_value : integer
+        The initial value of the counter. Default value is 1.
+      little_endian : boolean
+        If True, the counter number will be encoded in little endian format.
+        If False (default), in big endian format.
+      allow_wraparound : boolean
+        If True, the function will raise an *OverflowError* exception as soon
+        as the counter wraps around. If False (default), the counter will
+        simply restart from zero.
+      disable_shortcut : boolean
+        If True, do not make ciphers from `Crypto.Cipher` bypass the Python
+        layer when invoking the counter block function.
+        If False (default), bypass the Python layer.
+    :Returns:
+      The counter block function.
+    """
+
+    # Sanity-check the message size
+    (nbytes, remainder) = divmod(nbits, 8)
+    if remainder != 0:
+        # In the future, we might support arbitrary bit lengths, but for now we don't.
+        raise ValueError("nbits must be a multiple of 8; got %d" % (nbits,))
+    if nbytes < 1:
+        raise ValueError("nbits too small")
+    elif nbytes > 0xffff:
+        raise ValueError("nbits too large")
+
+    initval = _encode(initial_value, nbytes, little_endian)
+
+    if little_endian:
+        return _counter._newLE(bstr(prefix), bstr(suffix), initval, allow_wraparound=allow_wraparound, disable_shortcut=disable_shortcut)
+    else:
+        return _counter._newBE(bstr(prefix), bstr(suffix), initval, allow_wraparound=allow_wraparound, disable_shortcut=disable_shortcut)
+
+def _encode(n, nbytes, little_endian=False):
+    retval = []
+    n = int(n)
+    for i in range(nbytes):
+        if little_endian:
+            retval.append(bchr(n & 0xff))
+        else:
+            retval.insert(0, bchr(n & 0xff))
+        n >>= 8
+    return b("").join(retval)
+
+# vim:set ts=4 sw=4 sts=4 expandtab: