diff --git a/demos/auth.c b/demos/auth.c
index bcabbb69..f7179e1d 100644
--- a/demos/auth.c
+++ b/demos/auth.c
@@ -44,14 +44,9 @@ auth(void)
 
     puts("Example: crypto_auth\n");
 
-    memset(key, 0, sizeof key);
-    prompt_input("Enter a key > ", (char*)key, sizeof key);
-    puts("Complete key:");
-    print_hex(key, sizeof key);
-    putchar('\n');
+    prompt_input("a key", (char*)key, sizeof key, 0);
 
-    message_len = prompt_input("Enter a message > ",
-                               (char*)message, sizeof message);
+    message_len = prompt_input("a message", (char*)message, sizeof message, 1);
     putchar('\n');
 
     printf("Generating %s authentication...\n", crypto_auth_primitive());
diff --git a/demos/box.c b/demos/box.c
index 09693f42..d81f4543 100644
--- a/demos/box.c
+++ b/demos/box.c
@@ -96,8 +96,7 @@ box(void)
     putchar('\n');
 
     /* read input */
-    message_len = prompt_input("Enter a message > ",
-                               (char*)message, sizeof message);
+    message_len = prompt_input("a message", (char*)message, sizeof message, 1);
 
     print_hex(message, message_len);
     putchar('\n');
diff --git a/demos/box_detached.c b/demos/box_detached.c
index c37307cb..ec3c834a 100644
--- a/demos/box_detached.c
+++ b/demos/box_detached.c
@@ -96,8 +96,7 @@ box_detached(void)
     putchar('\n');
 
     /* read input */
-    message_len = prompt_input("Enter a message > ",
-                               (char*)message, sizeof message);
+    message_len = prompt_input("a message", (char*)message, sizeof message, 1);
 
     print_hex(message, message_len);
     putchar('\n');
diff --git a/demos/generichash.c b/demos/generichash.c
index ecb26d17..1e4d15fc 100644
--- a/demos/generichash.c
+++ b/demos/generichash.c
@@ -11,34 +11,68 @@
 #include "utils.h" /* utility functions shared by demos */
 
 /*
- * Generic hash is intended as a variable output hash with enough strength
- * to ensure data integrity. The hash out put is also able to vary in size.
- * Key is optional and is able to vary in size.
+ * This function computes a fixed-length fingerprint for an arbitrary long message.
  *
- * Note that it is recommended to stay within the range of MIN and MAX
- * output because larger output will produce gaps.
+ * Sample use cases:
+ *
+ * File integrity checking
+ * Creating unique identifiers to index arbitrary long data
+ *
+ * The crypto_generichash() function puts a fingerprint of the
+ * message in whose length is inlen bytes into out. The output size
+ * can be chosen by the application.
+ *
+ * The minimum recommended output size is crypto_generichash_BYTES.
+ * This size makes it practically impossible for two messages to
+ * produce the same fingerprint.
+ *
+ * But for specific use cases, the size can be any value between
+ * crypto_generichash_BYTES_MIN (included) and
+ * crypto_generichash_BYTES_MAX (included).
+ *
+ * key can be NULL and keylen can be 0. In this case, a message will
+ * always have the same fingerprint, similar to the MD5 or SHA-1
+ * functions for which crypto_generichash() is a faster and more
+ * secure alternative.
+ *
+ * But a key can also be specified. A message will always have the
+ * same fingerprint for a given key, but different keys used to hash
+ * the same message are very likely to produce distinct fingerprints.
+ *
+ * In particular, the key can be used to make sure that different
+ * applications generate different fingerprints even if they process
+ * the same data.
+ *
+ * The recommended key size is crypto_generichash_KEYBYTES bytes.
+ *
+ * However, the key size can by any value between
+ * crypto_generichash_KEYBYTES_MIN (included) and
+ * crypto_generichash_KEYBYTES_MAX (included).
  */
 void
 generichash(void)
 {
-    unsigned char k[crypto_generichash_KEYBYTES_MAX]; /* key */
-    unsigned char h[crypto_generichash_BYTES_MIN];    /* hash output */
-    unsigned char m[MAX_INPUT_SIZE];                  /* message */
-    size_t mlen;                                      /* length */
+    unsigned char key[crypto_generichash_KEYBYTES_MAX];
+    unsigned char hash[crypto_generichash_BYTES_MIN];
+    unsigned char message[MAX_INPUT_SIZE];
+    size_t        message_len;
+    size_t        key_len;
 
     puts("Example: crypto_generichash\n");
 
-    memset(k, 0, sizeof k);
-    prompt_input("Input your key > ", (char*)k, sizeof k);
+    key_len = prompt_input("a key", (char*)key, sizeof key, 1);
 
-    mlen = prompt_input("Input your message > ", (char*)m, sizeof m);
+    message_len = prompt_input("a message", (char*)message, sizeof message, 1);
     putchar('\n');
 
     printf("Hashing message with %s\n", crypto_generichash_primitive());
-    crypto_generichash(h, sizeof h, m, mlen, k, sizeof k);
-    fputs("Hash: ", stdout);
-    print_hex(h, sizeof h);
-    putchar('\n');
+    if (crypto_generichash(hash, sizeof hash, message, message_len,
+                           key, key_len) != 0) {
+        puts("Couldn't hash the message, probably due to the key length");
+    } else {
+        fputs("Hash: ", stdout);
+        print_hex(hash, sizeof hash);
+    }
     putchar('\n');
 }
 
diff --git a/demos/generichashstream.c b/demos/generichashstream.c
index 6f8e2119..25cb2b88 100644
--- a/demos/generichashstream.c
+++ b/demos/generichashstream.c
@@ -26,8 +26,7 @@ generichashstream(void)
 
     puts("Example: crypto_generichashstream\n");
 
-    memset(k, 0, sizeof k);
-    prompt_input("Input your key > ", (char*)k, sizeof k);
+    prompt_input("a key", (char*)k, sizeof k, 0);
     putchar('\n');
 
     printf("Hashing message with %s\n", crypto_generichash_primitive());
@@ -35,8 +34,8 @@ generichashstream(void)
     /* initialize the stream */
     crypto_generichash_init(&state, k, sizeof k, sizeof h);
 
-    while (1) {
-        mlen = prompt_input("> ", (char*)m, sizeof m);
+    for(;;) {
+        mlen = prompt_input("the next part of the message", (char*)m, sizeof m);
         if (mlen == 0)
             break;
 
diff --git a/demos/shorthash.c b/demos/shorthash.c
index 11d54c16..b526397d 100644
--- a/demos/shorthash.c
+++ b/demos/shorthash.c
@@ -42,11 +42,9 @@ shorthash(void)
 
     puts("Example: crypto_shorthash\n");
 
-    memset(key, 0, sizeof key);
-    prompt_input("Enter a key > ", (char*)key, sizeof key);
+    prompt_input("a key", (char*)key, sizeof key, 0);
 
-    message_len = prompt_input("Enter a message > ",
-                               (char*)message, sizeof message);
+    message_len = prompt_input("a message", (char*)message, sizeof message, 1);
     putchar('\n');
 
     printf("Hashing the message with %s\n", crypto_shorthash_primitive());
diff --git a/demos/sign.c b/demos/sign.c
index 6107c642..49b6568e 100644
--- a/demos/sign.c
+++ b/demos/sign.c
@@ -43,8 +43,8 @@ sign(void)
     puts("\n");
 
     /* read input */
-    mlen = prompt_input("Input your message > ", (char*)m,
-                        sizeof m - crypto_sign_BYTES);
+    mlen = prompt_input("a message", (char*)m,
+                        sizeof m - crypto_sign_BYTES, 1);
     putc('\n', stdout);
 
     puts("Notice the message has no prepended padding");
diff --git a/demos/utils.c b/demos/utils.c
index ee7f6fee..428955f0 100644
--- a/demos/utils.c
+++ b/demos/utils.c
@@ -45,22 +45,44 @@ print_hex(const void *bin, const size_t bin_len)
  * trailing newline characters.
  */
 size_t
-prompt_input(char *prompt, char *input, const size_t max_input_len)
+prompt_input(const char *prompt, char *input, const size_t max_input_len,
+             int variable_length)
 {
+    char   input_tmp[MAX_INPUT_SIZE + 1U];
     size_t actual_input_len;
 
-    fputs(prompt, stdout);
+    if (variable_length != 0) {
+        printf("Enter %s (%zu bytes max) > ", prompt, max_input_len);
+    } else {
+        printf("Enter %s (%zu bytes) > ", prompt, max_input_len);
+    }
     fflush(stdout);
-    fgets(input, max_input_len, stdin); /* grab input with room for \0 */
+    fgets(input_tmp, sizeof input_tmp, stdin);
+    actual_input_len = strlen(input_tmp);
 
-    actual_input_len = strlen(input);
-
-    /* trim excess new line */
-    if (actual_input_len > 0 && input[actual_input_len - 1] == '\n') {
-        input[actual_input_len - 1] = '\0';
+    /* trim \n */
+    if (actual_input_len > 0 && input_tmp[actual_input_len - 1] == '\n') {
+        input_tmp[actual_input_len - 1] = '\0';
         --actual_input_len;
     }
-    return actual_input_len;
+
+    if (actual_input_len > max_input_len) {
+        printf("Warning: truncating input to %zu bytes\n", max_input_len);
+        actual_input_len = max_input_len;
+    } else if (actual_input_len < max_input_len && variable_length == 0) {
+        printf("Warning: %zu bytes expected, %zu bytes given: padding with zeros\n",
+               max_input_len, actual_input_len);
+        memset(input, 0, max_input_len);
+    } else {
+        printf("Length: %zu bytes\n", actual_input_len);
+    }
+
+    memcpy(input, input_tmp, actual_input_len);
+    if (variable_length == 0) {
+        return max_input_len;
+    } else {
+        return actual_input_len;
+    }
 }
 
 /*
diff --git a/demos/utils.h b/demos/utils.h
index d6890d16..b88347fd 100644
--- a/demos/utils.h
+++ b/demos/utils.h
@@ -10,7 +10,8 @@
 
 void print_hex(const void *bin, const size_t bin_len);
 
-size_t prompt_input(char *prompt, char *input, const size_t max_input_len);
+size_t prompt_input(const char *prompt, char *input, const size_t max_input_len,
+                    int variable_length);
 
 void print_verification(int r);