问题
I'm trying to figure out how to encrypt a columnar transposition cipher in Python given a plaintext uppercase string and a number key of any length. For example, if the key is 3124 and the string is 'IHAVETWOCATS', it would organize the string like so:
3124
IHAV
ETWO
CATS
and then return the characters in column 1 first, then column 2, etc, until finally returning the encrypted string 'HTAAWTIECVOS'. So far I know that I'll need to use an accumulator, and I've been toying with the idea of using a dictionary, but I'm just completely stuck. These are some of the functions I've tried:
def columnar(plaintext,key):
cipher=''
acc=0
for i in range(len(key)):
while acc<(len(plaintext)/len(key)):
cipher=cipher+plaintext[i+acc*5]
acc=acc+1
return(cipher)
^This only returns a few letters, not a string of appropriate length.
def columnar(plaintext,key)
values={}
seqlist=[]
nextvalue=1
indices=rand(len(key))
for letter in plaintext:
for i in indices:
if letter==key[i]:
values[i]=nextvalue
nextvalue=nextvalue+1
for i in indices:
seqlist.append(values[i])
return seqlist
^The above function returns a KeyError: 0 error. Thank you very much for any help!
回答1:
def split_len(seq, length):
return [seq[i:i + length] for i in range(0, len(seq), length)]
def encode(key, plaintext):
order = {
int(val): num for num, val in enumerate(key)
}
ciphertext = ''
for index in sorted(order.keys()):
for part in split_len(plaintext, len(key)):
try:
ciphertext += part[order[index]]
except IndexError:
continue
return ciphertext
print(encode('3214', 'IHAVETWOCATS'))
#>>> HTAAWTIECVOS
split_len is by Ian Bicking
So i split the code into chunks with split_len then use dictionary comprehension to just get correct order of indexes and finally i concatanate the letters in that order.
回答2:
def encode(txt,key):
sz = len(key) # how big are the columns
cols = list(map("".join,zip(*zip(*[iter(txt)]*sz)))) # list partitioned into columns
return "".join([cols[key.index(str(c))] for c in range(1,sz+1)])
encoded = encode("IHAVETWOCATS","3124")
print encoded
is probably how I would do it
来源：https://stackoverflow.com/questions/24872411/encrypting-a-columnar-transposition-cipher