problems with base64
Byron
DesertLinux at netscape.net
Sun Jul 11 11:37:34 EDT 2004
Hi Karl,
I don't know if this is much help for you, but have you tried using the
following:
import base64
print base64.decodestring("UGxlYXNlLCBoZWxwIG1lIQ==")
print base64.encodestring("Please, help me!")
Byron
---
Karl Pech wrote:
> Hi all,
>
> I'm trying to write a program which can read in files in the following
> format:
> sos_encoded.txt:
> ---
> begin-base64 644 sos.txt
> UGxlYXNlLCBoZWxwIG1lIQ==
> ---
>
> and convert them to "clear byte code". For example if you take the file
> sos_encoded.txt and use my program on it you should get the following:
> sos.txt:
> ---
> Please, help me!
> ---
>
> Unfortunately if I try to convert files which didn't have any "human
> readable text" when they were encoded and if these files are large (> 1.5MB)
> I get back corrupted files.
>
> This is the source of my program:
> ---
> import string
>
> def extract_base64(source):
> source_ = source
> all_chars = string.maketrans('','') # create 256-ASCII-char-table
>
> # delete all base64-chars from source-copy
> source_without_base64_signs = source_.translate(all_chars, string.letters+string.digits+"+/=")
>
> # delete all chars, which remained in the changed source-copy, from the first copy
> # and return this new copy
> # --> all base64-chars remain
> return source_.translate(all_chars, source_without_base64_signs)
>
> def convert_to_8bits(source):
> base64_table = {'A' : 0, 'N' : 13, 'a' : 26, 'n' : 39, '0' : 52,
> 'B' : 1, 'O' : 14, 'b' : 27, 'o' : 40, '1' : 53,
> 'C' : 2, 'P' : 15, 'c' : 28, 'p' : 41, '2' : 54,
> 'D' : 3, 'Q' : 16, 'd' : 29, 'q' : 42, '3' : 55,
> 'E' : 4, 'R' : 17, 'e' : 30, 'r' : 43, '4' : 56,
> 'F' : 5, 'S' : 18, 'f' : 31, 's' : 44, '5' : 57,
> 'G' : 6, 'T' : 19, 'g' : 32, 't' : 45, '6' : 58,
> 'H' : 7, 'U' : 20, 'h' : 33, 'u' : 46, '7' : 59,
> 'I' : 8, 'V' : 21, 'i' : 34, 'v' : 47, '8' : 60,
> 'J' : 9, 'W' : 22, 'j' : 35, 'w' : 48, '9' : 61,
> 'K' : 10, 'X' : 23, 'k' : 36, 'x' : 49, '+' : 62,
> 'L' : 11, 'Y' : 24, 'l' : 37, 'y' : 50, '/' : 63,
> 'M' : 12, 'Z' : 25, 'm' : 38, 'z' : 51, '=' : 0}
>
> result_ = []
>
> # fill an integer with four 6-bit-blocks from left to right
> box_ = int( (base64_table[source[0]] << 26)\
> + (base64_table[source[1]] << 20)\
> + (base64_table[source[2]] << 14)\
> + (base64_table[source[3]] << 8) )
>
> # get 8-bit-blocks out of the integer starting with the first 6-bit-block we have
> # inserted plus the two highest bits from the second 6-bit-block
> result_ += chr((box_ >> 24) & 255) + chr((box_ >> 16) & 255) + chr((box_ >> 8) & 255)
>
> # strip possible zeros from decoded result
> del result_[len(result_)-source.count('='):len(result_)]
>
> return result_
>
> #open source file in binary-mode
> fsource = open(raw_input("Please specify the source file that should be decoded: "), "rb")
>
> # read in first line of the file and split it in 2+n "whitespace-blocks"
> _1stline = fsource.readline().split()
>
> # delete the first two blocks ("begin ..." and "644 ...")
> del _1stline[0:2]
>
> # join the other blocks to the target-filename
> targetname = string.join(_1stline)
> ftarget = open(targetname, "wb")
>
> # read in the remainder of the file in 4-byte-blocks and write the results in 3-byte-blocks
> # into the target file
>
> while 1 == 1:
> source = ''
> while len(source) < 4:
> source += fsource.read(4)
> if source == '':
> break
>
> # reduce byte-code to base64-chars
> source = extract_base64(source)
>
> if source == '':
> break
>
> # convert 6-bit-blocks to 8-bit-blocks
> clear_text = convert_to_8bits(source)
>
> ftarget.writelines(clear_text)
>
> ftarget.close()
> fsource.close()
>
> print "file "+targetname+" has been written!"
> ---
>
> Unfortunately I can't use python's standard base64-module since
> this whole task is an exercise. :(
>
> And I don't see any logical problems in my code. I think I really
> need some more eyes to watch over this. So you are my "last hope"! ;)
> Perhaps you can give me a hint.
>
> Thank you very much!
>
> Regards
> Karl
>
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