SymCrypt (v0.1)

A symmetric encryption library and program.

Algorithm

SymCrypt generates keys of variable bit length using a Mersenne Twist implementation for generation of high-quality pseudorandom numbers.

Encryption is a basic XOR cipher using a given or generated key. A key whose length is less than that of the data is repeated until each data character is encrypted, and a key whose length is greater than that of the data is only used until each data character is encrypted.

// For an n-byte key 'key' and input text 'txt', produce ciphertext 'enc'

key[0]    key[1]    key[2]  ...  key[n-1]    key[0]    key[1]   ...  etc.
 XOR       XOR       XOR           XOR        XOR       XOR
txt[0]    txt[1]    txt[2]  ...  txt[n-1]    txt[n]   txt[n+1]  ...  etc.
------    ------    ------       --------    ------   --------
enc[0]    enc[1]    enc[2]       enc[n-1]    enc[n]   enc[n+1]  ...  etc.

Since this is a symmetric encryption algorithm, feeding it ciphertext will result in plaintext (provided the same key was used for encryption).

Usage

First produce the binary symcrypt by running make then run ./symcrypt with a valid combination of options as specified below.

NOTE: All output files are given a random integer name followed by either .key or .crypt depending on whether the file is a cipher key or crypted text, respectively. For keys and crypted text generated in the same execution, this integer will be the same.

Options

-f <filename>

• Generate a key of default bit length (128) and use it to cipher the contents of filename. Output the new key and resulting text to files.

-b <bits>

• Specify the bit length of a key to be generated.

-k <filename>

• Use the contents of filename as the key when ciphering.

-g

• Generate a key of default bit length (128) and output it to a file.

-a

• Prompt for the key on the command-line (no echo)

Examples

$ ./symcrypt 
No file to encrypt/decrypt. Use '-f <filename>' to specify
or use '-g' to generate a new key without crypting anything.

$ ./symcrypt -f encrypt.me
Saved 128-bit key to: 1560854361.key
Saved crypted data to: 1560854361.crypt

$ ./symcrypt -f encrypt.me -b 512
Saved 512-bit key to: 1561913202.key
Saved crypted data to: 1561913202.crypt

$ ./symcrypt -f encrypt.me -a
Key: 
Saved crypted data to: 2039284423.crypt

$ ./symcrypt -g
Saved 128-bit key to: 1562249342.key

$ ./symcrypt -f encrypt.me -k 1561913202.key
Saved crypted data to: 1562787166.crypt

$ ./symcrypt -f encrypt.me -k 1561913202.key -b 512
Key file provided and number of bits specified.
Number of bits used only for generating new keys.
These options are mutually exclusive; please pick only one.

Future Plans

• Refactor for more logical code organization
  • Move anything dealing with KEY structs from main to symcrypt.h and symcrypt.c ?
• Encrypt image files
• Have main() return the integer used in output filenames on successful execution

Known Bugs

• When the current key character and data character are the same, the result of XORing them is 0. This represents the null terminator ('\0') and fools C into believing it is the end of the string, which truncates everything after that character in the crypted text.
• Temporary fix is to detect a 0 result of the XOR operation and set that character equal to something else instead (currently '?'). This way, the entire message is considered but single characters may be incorrect in the decryption, e.g. This is some t/xt..

Author

• Matt Mokary (mxm6060@rit.edu)

Contributors

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