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up vote 4 down vote favorite

I am working on ARM-based modem Linux host machine and I need to read a bin file as an 8-,16- or 32- bits array. I basically need to encrypt the file in question and was thinking of just XOR-ing the bits with a random bit sequence (I don't need a very complex system). The Linux host doesn't have C or PERL modules in it so I cannot use them and hence I need to do it using a shell script. Can anyone give me some pointers on how to proceed with this?

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@DeerHunter, awk doesn't do XOR native. The shell's $((...)) does though. How is that not encryption? –  Stéphane Chazelas Jun 19 '13 at 12:02
    
I'm curious - if you need to encrypt a file, why not just use openssl or gpg/pgp? –  Drake Clarris Jun 19 '13 at 14:49
    
“I don't need a very complex system” Do you need the files to be encrypted or not? If yes: you need a proper encryption algorithm, which xor on its own is not (you need a one-time-pad, or a stream cipher). If not: use cat. –  Gilles Jun 19 '13 at 23:24

3 Answers 3

up vote 3 down vote accepted

Use tr (here rot128):

LC_ALL=C tr '\0-\377' '\200-\377\0-\177' < infile > outfile

For XOR 123, you'd need to compute the corresponding tr line:

LC_ALL=C tr '\0-\377' '\173\172\171\170\177\176\175\174\163\162\161\160\167\166\165\164\153\152\151\150\157\156\155\154\143\142\141\140\147\146\145\144\133\132\131\130\137\136\135\134\123\122\121\120\127\126\125\124\113\112\111\110\117\116\115\114\103\102\101\100\107\106\105\104\73\72\71\70\77\76\75\74\63\62\61\60\67\66\65\64\53\52\51\50\57\56\55\54\43\42\41\40\47\46\45\44\33\32\31\30\37\36\35\34\23\22\21\20\27\26\25\24\13\12\11\10\17\16\15\14\3\2\1\0\7\6\5\4\373\372\371\370\377\376\375\374\363\362\361\360\367\366\365\364\353\352\351\350\357\356\355\354\343\342\341\340\347\346\345\344\333\332\331\330\337\336\335\334\323\322\321\320\327\326\325\324\313\312\311\310\317\316\315\314\303\302\301\300\307\306\305\304\273\272\271\270\277\276\275\274\263\262\261\260\267\266\265\264\253\252\251\250\257\256\255\254\243\242\241\240\247\246\245\244\233\232\231\230\237\236\235\234\223\222\221\220\227\226\225\224\213\212\211\210\217\216\215\214\203\202\201\200\207\206\205\204'

More generally, to answer the question, to convert a file into an array of numbers to be used by the shell with standard commands:

set -- $(od -An -vtu1 < infile)

You can then apply the transformations you want and convert back to a file with awk's printf("%c").

That's going to be very inefficient though.

Like:

for i in $(od -An -vtu1 < infile); do
  echo "$(($i ^ 123))"
done | awk '{printf "%c", $0}' > outfile

You can use od -An -vtu2, to get 16bit numbers, but note that it is in the local endianness, so when converting back to characters, you have to take that into account.

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That doesn't do any useful encryption. You have to vary the transformation with every character. –  Gilles Jun 19 '13 at 23:24
    
@Gilles, please define "useful" in this context. –  Stéphane Chazelas Jun 20 '13 at 6:30
    
Useful = that can't trivially be broken. Encryption that's trivially broken is worse than no encryption: it's added complexity and gives the illusion of security without providing any security. –  Gilles Jun 20 '13 at 7:15
up vote 2 down vote

You can't use null terminators in most shell's variables (or rather, you can, but they will be terminated as, in general, they are stored as such), so that's a fairly major caveat of any binary read.

If you have xxd:

xxd -b | awk '{ for(i=2 ; i<=NF-1 ; i++) { print $i } }'

For example:

echo -n mayonnaise | xxd -b | awk '{ for(i=2 ; i<=NF-1 ; i++) { print $i } }'

01101101
01100001
01111001
01101111
01101110
01101110
01100001
01101001
01110011
01100101
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Note that your first sentence doesn't apply to zsh which can store NUL in its variable. zsh is unlikely to be installed on the OP's machine though. –  Stéphane Chazelas Jun 20 '13 at 7:11
up vote 2 down vote

I used this very crude function to xor two 128 byte files:

one_time_pad() {
    asrc="$1"
    bsrc="$2"
    dst="$3"

    atmp="`mktemp`"
    btmp="`mktemp`"

    hexdump -v -e '1/1 "%02x" " "' "$asrc" > "$atmp"
    hexdump -v -e '1/1 "%02x" " "' "$bsrc" > "$btmp"

    rm "$dst"
    touch "$dst"

    for i in `seq 1 128`
    do
        a=`awk '{print $'$i';}' < "$atmp"`
        b=`awk '{print $'$i';}' < "$btmp"`

        echo -n -e '\x'`printf "%x" $((0x$a^0x$b))` >> "$dst"
    done
}

one_time_pad in1 in2 out

Performance wise this is utterly horrible (it's calling awk twice for every byte!). In my case, performance simply wasn't an issue. I'm sure you can find a much better way.

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