USAGE: caesarion <OPTION>[/<skip>[/<length>]] <infile> <KEY> [<outfile>]
OPTION:
-e[ncode] encrypts <infile> to <outfile>, <infile>-YYYY-MM-DD-hh:mm by default.
-d[ecode] decrypts <infile> to <outfile>, to stdout by default.
-E[NCODE] encrypts <infile> to <outfile>, <infile>-YYYY-MM-DD-hh:mm by default.
          Leaves the first 10 bytes of the message unchanged.
-D[ECODE] decrypts <infile> to <outfile>, to stdout by default.
          Leaves the first 10 bytes of the message unchanged.
WHERE <KEY> is either -, -<keytext>, "-<keytext>" or <keyfile>.
"-" as <KEY> will read from stdin, terminated by Enter.
"-" as <infile> will read from stdin, terminated by Ctrl D or a line "exit".
In case of encoding from stdin, a message is written to stderr.
The first <skip> bytes of the key will not be used. Default: 0 .
The optional <length> will truncate the number of bytes used from the key.
DESCRIPTION:
Encryption by outfile[i]=(infile[i]+sum(j<=i,key[j%(length_of_key]))%256.
Decryption by outfile[i]=(infile[i]-sum(j<=i,key[j%(length_of_key]))%256.
For a 1-character key this reduces to a variant of the Caesarian encryption.
If the effective length of the key(file) is comparable to the length of the
message, the encyption is virtually unbreakable without knowledge of the key.
The program was named after Ptolemy XV Philopator Philometor Caesar (47-30 BC)
a.k.a. Caesarion, son of Julius Caesar (100-44 BC) and Cleopatra (69-30 BC).
RETURN VALUES:
Successful: 0. Too few arguments, USAGE info written to stdout: 1.
Syntax error: 2. File IO error: 3. Error messages are  written to stderr.

Copyright (C) 2014 by Harm J. Schoonhoven

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