initial checksum commit

file-integrity/README.md

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+
+# Overview
+
+The program `checksum.py` is very simple: it can be used to calculate checksums
+of different flavors (addition, xor-based, or fletcher) on either random
+values (as generated by the program) or on values you pass in as a series of
+bytes.
+
+When run in the default mode:
+
+```sh
+prompt> ./checksum.py
+
+OPTIONS seed 0
+OPTIONS data_size 4
+OPTIONS data
+
+Decimal:          216        194        107         66
+Hex:             0xd8       0xc2       0x6b       0x42
+Bin:       0b11011000 0b11000010 0b01101011 0b01000010
+
+Add:      ?
+Xor:      ?
+Fletcher: ?
+
+prompt>
+```
+
+In this example, the program produces a random set of four numbers (the
+"data") of 216 194 107 66 (decimal). The numbers are also shown in hex and
+binary. 
+
+The program then asks you to compute the additive, xor-based, and fletcher
+checksums. The addition is just the result of adding each of the bytes
+together, with the result modulo 256 (it's just a single byte checksum).
+The xor-based checksum is the result of xor'ing each byte together (it is also
+a single byte). Finally, fletcher is the result of computing the two parts of
+the fletcher checksum (as described in the chapter), which is two bytes in
+total.
+
+You can change the seed to get a different problem:
+
+```sh
+prompt> ./checksum.py -s 1
+
+OPTIONS seed 1
+OPTIONS data_size 4
+OPTIONS data
+
+Decimal:           34        216        195         65
+Hex:             0x22       0xd8       0xc3       0x41
+Bin:       0b00100010 0b11011000 0b11000011 0b01000001
+
+Add:      ?
+Xor:      ?
+Fletcher: ?
+
+prompt> 
+```
+
+You can specify a different length for the random data:
+
+```sh
+prompt> ./checksum.py -D 2
+
+...
+
+You can also specify your own data string:
+
+prompt> ./checksum.py -D 1,2,3,4
+
+OPTIONS seed 0
+OPTIONS data_size 4
+OPTIONS data 1,2,3,4
+
+Decimal:            1          2          3          4
+Hex:             0x01       0x02       0x03       0x04
+Bin:       0b00000001 0b00000010 0b00000011 0b00000100
+
+Add:      ?
+Xor:      ?
+Fletcher: ?
+
+prompt> 
+```
+
+Finally, you can use `-c` to have the program compute the checksums for you.
+
+```sh
+prompt> ./checksum.py -D 1,2,3,4 -c
+
+OPTIONS seed 0
+OPTIONS data_size 4
+OPTIONS data 1,2,3,4
+
+Decimal:            1          2          3          4
+Hex:             0x01       0x02       0x03       0x04
+Bin:       0b00000001 0b00000010 0b00000011 0b00000100
+
+Add:             10       (0b00001010)
+Xor:              4       (0b00000100)
+Fletcher(a,b):   10, 20   (0b00001010,0b00010100)
+
+prompt> 
+```
+
+Thus ends the worst README in this collection of READMEs.

file-integrity/checksum.py

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+#! /usr/bin/env python
+
+from __future__ import print_function
+import random
+from optparse import OptionParser
+
+# to make Python2 and Python3 act the same -- how dumb
+def random_seed(seed):
+    try:
+        random.seed(seed, version=1)
+    except:
+        random.seed(seed)
+    return
+
+def print_hex(v):
+    if v < 16:
+        return '0x0%x' % v
+    else:
+        return '0x%x' % v
+    
+def print_bin(word):
+    v = bin(word)
+    o = '0b'
+    o += ('0' * (10 - len(str(v))))
+    o += str(v)[2:]
+    return o
+    
+
+parser = OptionParser()
+parser.add_option('-s', '--seed', default='0', help='Random seed', action='store', type='int', dest='seed')
+parser.add_option('-d', '--data_size', default='4', help='Number of bytes in data word', action='store', type='int', dest='data_size')
+parser.add_option('-D', '--data', default='', help='Data in comma separated form', action='store', type='string', dest='data')
+parser.add_option('-c', '--compute', help='compute answers for me', action='store_true', default=False, dest='solve')
+(options, args) = parser.parse_args()
+
+print('')
+print('OPTIONS seed', options.seed)
+print('OPTIONS data_size', options.data_size)
+print('OPTIONS data', options.data)
+print('')
+
+random_seed(options.seed)
+
+values = []
+if options.data != '':
+    tmp = options.data.split(',')
+    for t in tmp:
+        values.append(int(t))
+else:
+    for t in range(int(options.data_size)):
+        values.append(int(random.random() * 256))
+
+
+add = 0
+xor = 0
+fletcher_a, fletcher_b = 0, 0
+
+for value in values:
+    add = (add + value) % 256
+    xor = xor ^ value
+    fletcher_a = (fletcher_a + value) % 255
+    fletcher_b = (fletcher_b + fletcher_a) % 255
+
+print('Decimal:  ', end=' ')
+for word in values:
+    print('%10s' % str(word), end=' ')
+print('')
+
+print('Hex:      ', end=' ')
+for word in values:
+    print('     ', print_hex(word), end=' ')
+print('')
+
+print('Bin:      ', end=' ')
+for word in values:
+    print(print_bin(word), end=' ')
+print('')
+
+print('')
+if options.solve:
+    print('Add:           ', '%3d      ' % add, '(%s)' % print_bin(add))
+    print('Xor:           ', '%3d      ' % xor, '(%s)' % print_bin(xor))
+    print('Fletcher(a,b): ', '%3d,%3d  ' % (fletcher_a, fletcher_b), '(%s,%s)' % (print_bin(fletcher_a), print_bin(fletcher_b)))
+else:
+    print('Add:      ?')
+    print('Xor:      ?')
+    print('Fletcher: ?')
+print('')
+    
+
+