Challenge

Write an encryption and decryption program which would
 Read file A and use its contents as bitstring which would
be input for this transformation(Its XOR btw), assuming the initial "previous bit" as 0:
If previous bit is equal to next bit write 0 into File B, else write 1 into File B.
The decryption program must take File B and produce File A.

Whoever does it in least characters wins.

Don't start all your homework threads with "Challenge".

Anyway, encoder:

import sys,array
p=0;f=array.array('B',sys.stdin.read())
for i in xrange(len(f)*8):
 if f[i/8]>>(i&7)&1==p:f[i/8]&=~(1<<(i&7))
 else:f[i/8]|=1<<(i&7);p=p+1&1
f.tofile(sys.stdout)

Decoder:

import sys,array
p=0;f=array.array('B',sys.stdin.read())
for i in xrange(len(f)*8):
 if f[i/8]>>(i&7)&1:p=~p
 if p:f[i/8]|=1<<(i&7)
 else:f[i/8]&=~(1<<(i&7))
f.tofile(sys.stdout)

Both are exactly 178 bytes, which makes me ｆｅｅｌ　ｋｉｎｄ　ｏｆ　ｇｏｏｄ　ａｂｏｕｔ　ｉｔ　：）. It's not a winning entry by any means, but I'd appreciate if some Guido's bitch could show me how to do this better (as in smaller), as I'm pretty new to this language.

xrange -> range doesn't count