HOTP tutorial stages

Starter has a basic HOTP application with a fixed secret
Milestone one allows for changing of the secret
Milestone two saves the secret in Flash
Complete application allows for multiple secrets (one per button)

examples/tutorials/hotp/hotp_complete/Makefile

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+# Makefile for user application
+
+# Specify this directory relative to the current application.
+TOCK_USERLAND_BASE_DIR = ../../../../
+
+# Which files to compile.
+C_SRCS := $(wildcard *.c)
+
+# Include userland master makefile. Contains rules and flags for actually
+# building the application.
+include $(TOCK_USERLAND_BASE_DIR)/AppMakefile.mk

examples/tutorials/hotp/hotp_complete/README.md

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+
+Additional abilities:
+ * Can program a new key on button hold
+ * Saves key in Flash across reboots
+ * Supports multiple keys (one per button)

examples/tutorials/hotp/hotp_complete/base32.c

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+// Base32 implementation
+//
+// Copyright 2010 Google Inc.
+// Author: Markus Gutschke
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <string.h>
+
+#include "base32.h"
+
+int base32_decode(const uint8_t *encoded, uint8_t *result, int bufSize) {
+  int buffer   = 0;
+  int bitsLeft = 0;
+  int count    = 0;
+  for (const uint8_t *ptr = encoded; count < bufSize && *ptr; ++ptr) {
+    uint8_t ch = *ptr;
+    if (ch == ' ' || ch == '\t' || ch == '\r' || ch == '\n' || ch == '-') {
+      continue;
+    }
+    buffer <<= 5;
+
+    // Deal with commonly mistyped characters
+    if (ch == '0') {
+      ch = 'O';
+    } else if (ch == '1') {
+      ch = 'L';
+    } else if (ch == '8') {
+      ch = 'B';
+    }
+
+    // Look up one base32 digit
+    if ((ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z')) {
+      ch = (ch & 0x1F) - 1;
+    } else if (ch >= '2' && ch <= '7') {
+      ch -= '2' - 26;
+    } else {
+      return -1;
+    }
+
+    buffer   |= ch;
+    bitsLeft += 5;
+    if (bitsLeft >= 8) {
+      result[count++] = buffer >> (bitsLeft - 8);
+      bitsLeft       -= 8;
+    }
+  }
+  if (count < bufSize) {
+    result[count] = '\000';
+  }
+  return count;
+}
+
+int base32_encode(const uint8_t *data, int length, uint8_t *result,
+                  int bufSize) {
+  if (length < 0 || length > (1 << 28)) {
+    return -1;
+  }
+  int count = 0;
+  if (length > 0) {
+    int buffer   = data[0];
+    int next     = 1;
+    int bitsLeft = 8;
+    while (count < bufSize && (bitsLeft > 0 || next < length)) {
+      if (bitsLeft < 5) {
+        if (next < length) {
+          buffer  <<= 8;
+          buffer   |= data[next++] & 0xFF;
+          bitsLeft += 8;
+        } else {
+          int pad = 5 - bitsLeft;
+          buffer  <<= pad;
+          bitsLeft += pad;
+        }
+      }
+      int index = 0x1F & (buffer >> (bitsLeft - 5));
+      bitsLeft       -= 5;
+      result[count++] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"[index];
+    }
+  }
+  if (count < bufSize) {
+    result[count] = '\000';
+  }
+  return count;
+}

examples/tutorials/hotp/hotp_complete/base32.h

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+//
+// Copyright 2010 Google Inc.
+// Author: Markus Gutschke
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Encode and decode from base32 encoding using the following alphabet:
+//   ABCDEFGHIJKLMNOPQRSTUVWXYZ234567
+// This alphabet is documented in RFC 4648/3548
+//
+// We allow white-space and hyphens, but all other characters are considered
+// invalid.
+//
+// All functions return the number of output bytes or -1 on error. If the
+// output buffer is too small, the result will silently be truncated.
+
+#ifndef _BASE32_H_
+#define _BASE32_H_
+
+#include <stdint.h>
+
+int base32_decode(const uint8_t *encoded, uint8_t *result, int bufSize)
+    __attribute__((visibility("hidden")));
+int base32_encode(const uint8_t *data, int length, uint8_t *result,
+                  int bufSize)
+    __attribute__((visibility("hidden")));
+
+#endif /* _BASE32_H_ */