Reputation: 4250
What is the cause of certain characters to be blank when using XOR encryption? Furthermore, how can this be compensated for when decrypting?
For instance:
....
void basic_encrypt(char *to_encrypt) {
char c;
while (*to_encrypt) {
*to_encrypt = *to_encrypt ^ 20;
to_encrypt++;
}
}
will return "nothing" for the character k
. Clearly, character decay is problematic for decryption.
I assume this is caused by the bit operator, but I am not very good with binary so I was wondering if anyone could explain.
Is it converting an element, k
, in this case, to some spaceless ASCII character? Can this be compensated for by choosing some y < x < z operator where x is the operator?
Lastly, if it hasn't been compensated for, is there a realistic decryption strategy for filling in blanks besides guess and check?
Upvotes: 1
Views: 2060
Reputation: 94038
You will have to know the difference between bytes and characters to understand which is happening. On the one hand you have the C char
type, which is simply a presentation of a byte, not a character.
In the old days each character was mapped to one byte or octet value in a character encoding table, or code page. Nowadays we have encodings that take more bytes for certain characters, e.g. UTF-8, or even encodings that always take more than one byte such as UTF-16. The last two are unicode encodings, which means that each character has a certain number value and the encoding is used to encode this number into bytes.
Many computers will interpret bytes in ISO/IEC 8859-1 or Latin-1, sometimes extended by Windows-1252. These code pages have holes for control characters, or byte values that are simply not used. Now it depends on the runtime system how these values are handled. Java by default substitutes an ?
character in place of the missing character. Other runtimes will simply drop the value or - of course - execute the control code. Some terminals may use the ESC control code to set the color or to switch to another code page (making a mess of the screen).
This is why ciphertext should be converted to another encoding, such as hexadecimals or Base64. These encodings should make sure that the result is readable text. This takes care of the cipher text. You will have to choose a character set for your plain text too, e.g. simply perform ASCII or UTF-8 encoding before encryption.
Upvotes: 3
Reputation: 94779
Getting a zero value from encryption does not matter because once you re-xor with the same xor key you get the original value.
value == value
value XOR value == 0 [encryption]
( value XOR value ) XOR value == value [decryption]
If you're using a zero-terminated string mechanism, then you have two main strategies for preventing 'character degradation'
Upvotes: 1
Reputation: 183968
'k'
has the ASCII value 107 = 0x6B
. 20
is 0x14
, so
'k' ^ 20 == 0x7F == 127
if your character set is ASCII compatible. 127
is \DEL
in ASCII, which is a non-printable character, so won't be displayed if you print it out.
Upvotes: 6