diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..89e3504
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+totp
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..cb94cba
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,18 @@
+CC     = cc
+CFLAGS = -Wall -O2
+prefix = /usr/local
+
+totp:
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) totp.c -o totp
+
+install: totp
+	mkdir -p $(DESTDIR)$(prefix)/bin
+	mv -f totp $(DESTDIR)$(prefix)/bin
+
+uninstall:
+	rm -f $(DESTDIR)$(prefix)/bin/totp
+
+clean:
+	rm -f totp
+
+.PHONY: totp install uninstall clean
diff --git a/README.md b/README.md
index a0f8fae..3918b31 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,5 @@
 # totp
 A tiny command line utility to generate OTP tokens
+
+    $ echo -n JBSWY3DPEHPK3PXP | base32 -d | ./totp
+    506471
diff --git a/totp.c b/totp.c
new file mode 100644
index 0000000..bac4812
--- /dev/null
+++ b/totp.c
@@ -0,0 +1,121 @@
+// Copyright (c) 2020 by angt <adrien@gallouet.fr>
+// Don't try to reuse the HMAC-SHA1 implementation of this project,
+// you will suffer unproductively :P
+
+#include <inttypes.h>
+#include <stdio.h>
+#include <string.h>
+#include <time.h>
+#include <unistd.h>
+
+static inline void
+ebe32(uint8_t *dst, uint32_t x)
+{
+    dst[0] = (uint8_t)(x >> 24);
+    dst[1] = (uint8_t)(x >> 16);
+    dst[2] = (uint8_t)(x >>  8);
+    dst[3] = (uint8_t)(x      );
+}
+
+static inline uint32_t
+dbe32(const uint8_t *src)
+{
+    return ((uint32_t)src[0] << 24)
+         | ((uint32_t)src[1] << 16)
+         | ((uint32_t)src[2] <<  8)
+         | ((uint32_t)src[3]      );
+}
+
+static inline void
+ebe64(uint8_t *dst, uint64_t x)
+{
+    ebe32(dst,     (uint32_t)(x >> 32));
+    ebe32(dst + 4, (uint32_t)(x      ));
+}
+
+static inline uint32_t
+rotl(int n, uint32_t x)
+{
+    return (x << n) | (x >> (32 - n));
+}
+
+static void
+sha1_process(const uint8_t *buf, uint32_t x[5])
+{
+    uint32_t w[80];
+    uint32_t a = x[0], b = x[1], c = x[2], d = x[3], e = x[4];
+
+    for (int i = 0; i < 16; i++)
+        w[i] = dbe32(&buf[i << 2]);
+
+    for (int i = 16; i < 80; i++)
+        w[i] = rotl(1, w[i - 3] ^ w[i - 8] ^ w[i - 14] ^ w[i - 16]);
+
+    for (int i = 0; i < 80; i++) {
+        uint32_t t = rotl(5, a) + e + w[i];
+             if (i < 20) t += 0x5A827999 + ((b & c) | ((~b) & d));
+        else if (i < 40) t += 0x6ED9EBA1 + (b ^ c ^ d);
+        else if (i < 60) t += 0x8F1BBCDC + ((b & c) | (b & d) | (c & d));
+        else             t += 0xCA62C1D6 + (b ^ c ^ d);
+        e = d; d = c; c = rotl(30, b); b = a; a = t;
+    }
+    x[0] += a; x[1] += b; x[2] += c; x[3] += d; x[4] += e;
+}
+
+static void
+sha1(uint8_t *digest, uint8_t *buf, size_t len)
+{
+    uint8_t tmp[64] = {0};
+    uint32_t x[] = {0x67452301, 0xEFCDAB89, 0x98BADCFE, 0x10325476, 0xC3D2E1F0};
+    size_t p = 0;
+
+    for (; p + 64 <= len; p += 64)
+        sha1_process(buf + p, x);
+
+    if (len > p)
+        memcpy(tmp, buf + p, len - p);
+
+    p = len - p;
+    tmp[p++] = 0x80;
+
+    if (p > 56) {
+        sha1_process(tmp, x);
+        memset(tmp, 0, sizeof(tmp));
+    }
+    ebe64(tmp + 56, len << 3);
+    sha1_process(tmp, x);
+
+    for (int i = 0; i < 5; i++)
+        ebe32(&digest[i << 2], x[i]);
+}
+
+int
+main(int argc, char **argv)
+{
+    uint8_t key[64];
+    ssize_t len = read(0, key, sizeof(key));
+
+    if (len <= 0)
+        return -1;
+
+    uint8_t h[20];
+    uint8_t ki[64 +  8] = {0};
+    uint8_t ko[64 + 20] = {0};
+
+    memcpy(ki, key, len);
+    memcpy(ko, key, len);
+
+    for (int i = 0; i < 64; i++) {
+        ki[i] ^= 0x36;
+        ko[i] ^= 0x5c;
+    }
+
+    ebe64(&ki[64], ((uint64_t)time(NULL)) / 30);
+    sha1(&ko[64], ki, sizeof(ki));
+    sha1(h, ko, sizeof(ko));
+
+    uint32_t ret = (dbe32(&h[h[19] & 0xF]) & ~(UINT32_C(1) << 31))
+                 % UINT32_C(1000000);
+
+    printf("%06" PRIu32 "\n", ret);
+}