Cryptology Programs

Disclaimer: I am interested in cryptology, but by no means an expert.

Source code

What's in the zip file?

• msc1.c - a program to crack monoalphabetic substitution ciphers
• cdx.c - encryption/decryption using CDX-2 (needs sb.c)
• sb.c - S-boxes for CDX-2 (needs cdx.c)
• sbgen.c - generates a new version of sb.c (S-boxes for CDX-2)
• keygen.c - generates a moderately random key

Note: keygen.c is not portable - it requires console functions kbhit() and getch().

Monoalphabetic Substitution Cipher Cracking

msc1.c - MSC1 is a program for cracking a monoalphabetic substition cipher. Click here for a demonstration run.

CDX-2

The CDX-2 cryptographic algorithm works as follows:

• Load entire plaintext into one buffer
• For each byte in the key (note: therefore, there are as many rounds as the key is long):
  • For each byte in the plaintext
  • Substitute this byte with the S-box value corresponding to this byte's value - use the box corresponding to the current key byte (modulo the number of S-boxes)
  • Rotate the entire plaintext by a number of bits corresponding to a lookup value based on the current key byte
  • Perform a Vigenere-style XOR of the key against the buffer.

sb.c contains the S-boxes for CDX-2.

sbgen.c will generate a set of random S-boxes (i.e. a new version of sb.c). Output is to stdout, so you'll need to redirect to a shiny new C file.

Key Generation

keygen.c generates a one-megabyte random(ish!) bitstream. It's not portable, I'm afraid. It was written for Borland C. The idea is to compile it, run it, and hit the keyboard like crazy. Just keep typing. Anything. Lots. Eventually, it'll come out the other end with this one meg file. Use whichever bits you like from that file. Don't feel you need to keep the rest of them, because you can generate a new file whenever you like.

If anyone feels like calculating the entropy involved, do feel free (and I'd be glad to hear from you).

A Simple Authentication Protocol

Authenticating yourself can be a real challenge. Here's one suggested solution (which has NOT been peer reviewed yet, as at 12 Jan 2001).

Cracking a substitution cipher - a case study

Cracking substitution ciphers like this one, however, is no challenge at all.

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