From e7a188d9f8cd105485c5362f525cdd614c03dd3c Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom@tomsmeding.com>
Date: Wed, 10 Feb 2021 12:07:45 +0100
Subject: utilities: Add gen_apikey

---
 utilities/.gitignore   |  1 +
 utilities/gen_apikey.c | 99 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 100 insertions(+)
 create mode 100644 utilities/gen_apikey.c

diff --git a/utilities/.gitignore b/utilities/.gitignore
index 41ea945..c944114 100644
--- a/utilities/.gitignore
+++ b/utilities/.gitignore
@@ -1 +1,2 @@
 hash_pass
+gen_apikey
diff --git a/utilities/gen_apikey.c b/utilities/gen_apikey.c
new file mode 100644
index 0000000..6f8d3f7
--- /dev/null
+++ b/utilities/gen_apikey.c
@@ -0,0 +1,99 @@
+#ifdef NDEBUG
+#error Asserts must work here
+#endif
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <string.h>
+#include <stdint.h>
+#include <assert.h>
+
+
+// This gives log2(64)*24 = 6*24 = 144 bits of entropy.
+#define ALPHABET_SIZE 64
+#define KEY_LENGTH 24
+
+
+__attribute__((noreturn))
+static void die(const char *msg) {
+	fprintf(stderr, "%s\n", msg);
+	exit(1);
+}
+
+
+//////////////////// RANDOM GENERATOR ////////////////////
+
+struct randgen {
+	FILE *urandom;
+	uint8_t buffer[256];
+	size_t cursor;
+};
+
+static void randgen_refresh(struct randgen *gen) {
+	size_t nr = fread(gen->buffer, 1, sizeof gen->buffer, gen->urandom);
+	if (nr < sizeof gen->buffer) die("Cannot read from /dev/urandom");
+	gen->cursor = 0;
+}
+
+static struct randgen randgen_init(void) {
+	FILE *f = fopen("/dev/urandom", "r");
+	if (!f) die("Cannot open /dev/urandom");
+
+	struct randgen gen;
+	gen.urandom = f;
+	randgen_refresh(&gen);
+
+	return gen;
+}
+
+static uint8_t randgen_gen_byte(struct randgen *gen) {
+	if (gen->cursor == sizeof gen->buffer) randgen_refresh(gen);
+	return gen->buffer[gen->cursor++];
+}
+
+static uint64_t randgen_gen(struct randgen *gen, uint64_t below) {
+	if (below == 0) return 0;
+
+	int nbytes = 0;
+	while (nbytes < 8 && (1ULL << (8 * nbytes)) < below) nbytes++;
+
+	// This check allows us to do fearless modulo in the main loop
+	if (nbytes == 8) assert(false && "'below' too large");
+
+	const uint64_t gen_limit = 1ULL << (8 * nbytes);
+
+	while (true) {
+		uint64_t sample = 0;
+		for (int i = 0; i < nbytes; i++) {
+			sample |= (uint64_t)randgen_gen_byte(gen) << (8 * i);
+		}
+
+		if (sample < gen_limit - gen_limit % below) {
+			return sample % below;
+		}
+	}
+}
+
+
+//////////////////// MAIN FUNCTIONALITY ////////////////////
+
+static char gen_key_char(struct randgen *gen) {
+	// 64 characters, so 6 bits
+	static const char alphabet[] =
+		"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_";
+	assert(strlen(alphabet) == ALPHABET_SIZE);
+	return alphabet[randgen_gen(gen, strlen(alphabet))];
+}
+
+int main() {
+	struct randgen gen = randgen_init();
+
+	char key[KEY_LENGTH + 1];
+	for (size_t i = 0; i < KEY_LENGTH; i++) {
+		key[i] = gen_key_char(&gen);
+	}
+	key[KEY_LENGTH] = '\0';
+
+	printf("%s\n", key);
+}
-- 
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