Binary PSK: Add in support for examples coap-client and coap-server

If the PSK is defined with a leading 0x, then the hex representation
of the following bytes is converted to a binary PSK.

examples/coap-client.c

@@ -586,7 +586,9 @@ usage(const char *program, const char *version) {
  "\t \t\t hint_to_match,use_user,with_key\n"
  "\t \t\tNote: -k and -u still need to be defined for the default\n"
  "\t \t\tin case there is no match\n"
- "\t-k key \t\tPre-shared key for the specified user identity\n"
+ "\t-k key \t\tPre-shared key for the specified user identityt. If the\n"
+ "\t \t\tkey begins with 0x, then the hex text (two [0-9a-f] per\n"
+ "\t \t\tbyte) is converted to binary data\n"
  "\t-u user\t\tUser identity to send for pre-shared key mode\n"
  "PKI Options (if supported by underlying (D)TLS library)\n"
  "\tNote: If any one of '-c certfile', '-j keyfile' or '-C cafile' is in\n"
@@ -1258,6 +1260,12 @@ static ssize_t
 cmdline_read_key(char *arg, unsigned char **buf, size_t maxlen) {
  size_t len = strnlen(arg, maxlen);
  if (len) {
+ /* read hex string alternative when arg starts with "0x" */
+ if (len >= 4 && arg[0] == '0' && arg[1] == 'x') {
+ /* As the command line option is part of our environment we can do
+ * the conversion in place. */
+ len = convert_hex_string(arg + 2, (uint8_t *)arg);
+ }
  *buf = (unsigned char *)arg;
  return len;
  }

examples/coap-server.c

@@ -2233,7 +2233,9 @@ usage(const char *program, const char *version) {
  "\t-k key \t\tPre-Shared Key. This argument requires (D)TLS with PSK\n"
  "\t \t\tto be available. This cannot be empty if defined.\n"
  "\t \t\tNote that both -c and -k need to be defined for both\n"
- "\t \t\tPSK and PKI to be concurrently supported\n"
+ "\t \t\tPSK and PKI to be concurrently supported. If the\n"
+ "\t \t\tkey begins with 0x, then the hex text (two [0-9a-f] per\n"
+ "\t \t\tbyte) is converted to binary data\n"
  "\t-s match_psk_sni_file\n"
  "\t \t\tThis is a file that contains one or more lines of\n"
  "\t \t\treceived Subject Name Identifier (SNI) to match to use\n"
@@ -2512,10 +2514,59 @@ cmdline_tls_engine(char *arg) {
  return 1;
 }
 
+/**
+ * Utility function to convert a hex digit to its corresponding
+ * numerical value.
+ *
+ * param c The hex digit to convert. Must be in [0-9A-Fa-f].
+ *
+ * return The numerical representation of @p c.
+ */
+static uint8_t
+hex2char(char c) {
+ assert(isxdigit(c));
+ if ('a' <= c && c <= 'f')
+ return c - 'a' + 10;
+ else if ('A' <= c && c <= 'F')
+ return c - 'A' + 10;
+ else
+ return c - '0';
+}
+
+/**
+ * Converts the sequence of hex digits in src to a sequence of bytes.
+ *
+ * This function returns the number of bytes that have been written to
+ * @p dst.
+ *
+ * param[in] src The null-terminated hex string to convert.
+ * param[out] dst Conversion result.
+ *
+ * return The length of @p dst.
+ */
+static size_t
+convert_hex_string(const char *src, uint8_t *dst) {
+ uint8_t *p = dst;
+ while (isxdigit((int)src[0]) && isxdigit((int)src[1])) {
+ *p++ = (hex2char(src[0]) << 4) + hex2char(src[1]);
+ src += 2;
+ }
+ if (src[0] != '\0') { /* error in hex input */
+ coap_log_warn("invalid hex string in option '%s'\n", src);
+ }
+ return p - dst;
+}
+
 static ssize_t
 cmdline_read_key(char *arg, unsigned char **buf, size_t maxlen) {
  size_t len = strnlen(arg, maxlen);
  if (len) {
+ /* read hex string alternative when arg starts with "0x" */
+ if (len >= 4 && arg[0] == '0' && arg[1] == 'x') {
+ /* As the command line option is part of our environment we can do
+ * the conversion in place. */
+ len = convert_hex_string(arg + 2, (uint8_t *)arg);
+ }
  *buf = (unsigned char *)arg;
  return len;
  }

man/coap-client.txt.in

@@ -210,7 +210,9 @@ OPTIONS - PSK
  case there is no match.
 
 *-k* key::
- Pre-shared key for the specified user identity (*-u* option also required).
+ Pre-shared key for the specified user identity (*-u* option also required). +
+ If the key begins with 0x, then the hex text (two [0-9a-f] per byte) is
+ converted to binary data.
 
 *-u* user::
  User identity to send for pre-shared key mode (*-k* option also required).

man/coap-server.txt.in

@@ -176,6 +176,8 @@ OPTIONS - PSK
 *-k* key::
  Pre-shared key to use for inbound connections. This cannot be empty if
  defined. +
+ If the key begins with 0x, then the hex text (two [0-9a-f] per byte) is
+ converted to binary data. +
  Note: if *-c cafile* is defined, you need to define *-k key* as well to
  have the server support both PSK and PKI.