One-time pad

One-time pad is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Implement a One-time pad, for encrypting and decrypting messages.
To keep it simple, we will be using letters only.

Sub-Tasks

• Generate the data for a One-time pad (user needs to specify a filename and length)

The important part is to get "true random" numbers, e.g. from /dev/random

• encryption / decryption ( basically the same operation, much like Rot-13 )

For this step, much of Vigenère cipher could be reused,
with the key to be read from the file containing the One-time pad.

• optional: management of One-time pads: list, mark as used, delete, etc.

Somehow, the users needs to keep track which pad to use for which partner.

To support the management of pad-files:

• Such files have a file-extension ".1tp"
• Lines starting with "#" may contain arbitary meta-data (i.e. comments)
• Lines starting with "-" count as "used"
• Whitespace within the otp-data is ignored

For example, here is the data from Wikipedia:

# Example data - Wikipedia - 2014-11-13
-ZDXWWW EJKAWO FECIFE WSNZIP PXPKIY URMZHI JZTLBC YLGDYJ 
-HTSVTV RRYYEG EXNCGA GGQVRF FHZCIB EWLGGR BZXQDQ DGGIAK 
 YHJYEQ TDLCQT HZBSIZ IRZDYS RBYJFZ AIRCWI UCVXTW YKPQMK 
 CKHVEX VXYVCS WOGAAZ OUVVON GCNEVR LMBLYB SBDCDC PCGVJX 
 QXAUIP PXZQIJ JIUWYH COVWMJ UZOJHL DWHPER UBSRUJ HGAAPR 
 CRWVHI FRNTQW AJVWRT ACAKRD OZKIIB VIQGBK IJCWHF GTTSSE 
 EXFIPJ KICASQ IOUQTP ZSGXGH YTYCTI BAZSTN JKMFXI RERYWE 

C

See snapfractalpop - One-Time-Pad Command-Line-Utility.

This uses straight binary data, for the pad- and the data-file, using simple XOR.
To generate a one-time-pad, dd is used, which implies unix.
The pad-file gets rewritten immediately after the operation, with the used bytes removed.

Tcl

Part 1: random strings

Get true random numbers, and turn them into strings.

With "randInt" from Tcl'ers wiki Cryptographically secure random numbers using /dev/urandom

<lang Tcl>puts "# True random chars for one-time pad"

proc randInt { min max } {

   set randDev [open /dev/urandom rb]
   set random [read $randDev 8]
   binary scan $random H16 random
   set random [expr {([scan $random %x] % (($max-$min) + 1) + $min)}]
   close $randDev
   return $random

}

proc randStr { sLen grp alfa } {

 set aLen [string length $alfa]; incr aLen -1
 set rs ""
 for {set i 0} {$i < $sLen} {incr i} {
   if { [expr {$i % $grp} ] == 0} { append rs " " }
   set r [randInt 0 $aLen]
   set char [string index $alfa $r]
   append rs $char
 ##puts "$i: $r $char"
 }
 return $rs

}

set alfa "ABCDEFGHIJKLMNOPQRSTUVWXYZ" set len 48 set lines 4 set fn "test.1tp"

1. Write file:

set fh [open $fn w] puts $fh "# OTP" for {set ln 0} {$ln < $lines} {incr ln} {

   set line [randStr $len 6 $alfa]
 ##puts "$ln :$line."
   puts $fh $line

} close $fh

1. Read file:

puts "# File $fn:" set fh [open $fn] puts [read $fh [file size $fn]] close $fh

puts "# Done."</lang>

Output:

# True random chars for one-time pad
# File test.1tp:
# OTP
 OWCTEL SGDQEA UKEWCU PUTDEA XICBOL VVMJHD OHAXSE ZFAGDE
 QHDHKQ CCJBYF CMRCMC IXXPVM IOHQDA XIDTPX FGRIJC NPDOAT
 MYYQUV ZVKGDF ZLYKSX MBPLON RMQKQT QDYJVO LNKUFV DNKIQP
 NQOZKU MQOWHS VOQFWL EQWBFA HZQAMG JWNHGZ QERNNV GBKQTM
# Done.

Part 2: Encrypt/Decrypt

See Tcl'ers wiki: vignere Vigenere ...

Part 3: Management

• list padfiles in directory
• list lines / blocks between "comment"-lines in padfile (i.e. remaining usable data)

...