Cryptography

Number Theory

2026-07-10
divisibilitymodular‑arithmeticprimesrsaeuclidtotientcryptography

number theory studies the integers on their own terms — divisibility, primes, remainders — and for two millennia it was the purest of pure mathematics, prized precisely because it was useless. 𐃏 then in 1977 the RSA cryptosystem turned euclid, fermat and euler into load-bearing internet infrastructure. this page walks the classical results in dependency order and ends at that payoff (Epp, Susanna S., 2019).

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Cryptography Code

2026-06-28
cryptographytoolsc

Caesar

some of the code I’ve written is so stupid I tell you.

this one basically fires up a REPL with a argv[1] as the number of forward caesar cipher steps you want to take.

really not worth running.

#include <stdio.h>
#include <stdlib.h>

int upper(int character, int shift) {
        character += shift;
        if (character > 90) {
                character = (character-90)%26 + 64;
        }
        else if (character < 65) {
                character = (65-character)%26;
                if (character == 0) return 65;
                character = 91 - character;
        }
        return character;
}

int lower(int character, int shift) {
        character += shift;
        if (character > 122) {
                character = (character-122)%26 + 96;
        }
        else if (character < 97) {
                character = (97-character)%26;
                if (character == 0) return 97;
                character = 123 - character;
        }
        return character;
}


int main(int argc, char* argv[]) {
        int shift = atoi(argv[1]);
        int character = getchar();
        while (character != EOF) {
                if (character >= 'A' && character <= 'Z') {
                        putchar(upper(character, shift));
                }
                else if (character >= 'a' && character <= 'z') {
                        putchar(lower(character, shift));
                }
                else putchar(character);
                character = getchar();
        }
}

File XOR

#include <stdio.h>

int main(int argc, char *argv[])
{
    FILE *fi, *fo;
    char *cp;
    int c;

    if ((cp = argv[1]) && *cp!='\0') {
        if ((fi = fopen(argv[2], "rb")) != NULL) {
            if ((fo = fopen(argv[3], "wb")) != NULL) {
                while ((c = getc(fi)) != EOF) {
                    if (!*cp) cp = argv[1];
                    c ^= *(cp++);
                    putc(c,fo);
                }
                fclose(fo);
            }
            fclose(fi);
        }
    }
}

What every computer science major should know

2026-01-04 by https://matt.might.net (updated: 2026-06-27)
architecturenetworksosunixmathematicscryptographytesting

Here are my distillations of Matt’s seminal blog post: What CS Majors Ought to Know.

I have removed the imperative matter, and left the declarative material; marking off that which I have completed.

  • What should every student know to get a good job?

  • What should every student know to maintain lifelong employment?

  • What should every student know to enter graduate school?

  • What should every student know to benefit society?

Portfolio versus resume

  • Every computer science major should build a portfolio.
  • Contributions to open source should be linked and documented.

Technical communication

Specific recommendations

  • Master a presentation tool.