Add function for reading passphrase from terminal

puer.c

@@ -1,8 +1,14 @@
 #include <assert.h>
+#include <fcntl.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
-#include <stdbool.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <termios.h>
+#include <unistd.h>
 
 void xxtea128(uint32_t const key[4], uint32_t block[4]) {
 	// Encryption half of the XXTEA algorithm, with block size limited
@@ -427,11 +433,93 @@ bool ccm_decrypt(unsigned char key[16], uint32_t messageindex, unsigned char mes
 	return true;
 }
 
+size_t passphrase_prompt(unsigned char *passphrase, size_t size, const char *prompt) {
+	// Read from controlling TTY, even if stdion has been redirected
+	int tty = open("/dev/tty", O_RDWR);
+	if (tty == -1) {
+		perror("Failed to open controlling tty");
+		exit(1);
+	}
+
+	if (write(tty, prompt, strlen(prompt)) == -1) {
+		perror("Failed to write to terminal");
+		exit(1);
+	}
+
+	// Turn off echo
+	struct termios saved;
+	if (tcgetattr(tty, &saved) != 0) {
+		perror("tcgetattr");
+		exit(1);
+	}
+	struct termios altered;
+	altered = saved;
+	altered.c_lflag &= ~ECHO;
+	if (tcsetattr(tty, TCSANOW, &altered) != 0) {
+		perror("tcsetattr");
+		exit(1);
+	}
+
+	// Read until newline
+	size_t index = 0;
+	for (;;) {
+		if (index >= size) {
+			fprintf(stderr, "Passphrase too long, maximum size is %zu bytes\n", size - 1);
+			// Clean any line buffer
+			char tmp;
+			for (;;) {
+				if (read(tty, &tmp, 1) <= 0) {
+					break;
+				}
+				if (tmp == '\n') {
+					break;
+				}
+			}
+			tcsetattr(tty, TCSANOW, &saved);
+			exit(1);
+		}
+
+		ssize_t bytes_read = read(tty, &passphrase[index], size - index);
+		if (bytes_read == -1) {
+			perror("Failed to read passphrase");
+			tcsetattr(tty, TCSANOW, &saved);
+			exit(1);
+		} else if (bytes_read == 0) {
+			fprintf(stderr, "Unexpected EOF\n");
+			tcsetattr(tty, TCSANOW, &saved);
+			exit(1);
+		}
+
+		index += bytes_read;
+		if (passphrase[index-1] == '\n') {
+			// Got end of line
+			break;
+		}
+	}
+
+	// Write a newline (since the user's is not visible) and restore
+	// terminal settings
+	if (write(tty, "\n", 1) == -1) {
+		perror("Failed to write to terminal");
+		exit(1);
+	}
+	tcsetattr(tty, TCSANOW, &saved);
+	close(tty);
+
+	return index - 1;
+}
+
 int main(void) {
-	unsigned char key[16] = {0};
-	unsigned char salt[32] = "seasaltrocksalt seasaltrocksalt";
-	unsigned char passphrase[] = "a quick brown fox jumps over the lazy dog";
-	kdf(key, salt, passphrase, sizeof(passphrase) - 1);
+	unsigned char salt[32];
+	if (getentropy(salt, 32) != 0) {
+		perror("getentropy");
+	}
+
+	unsigned char passphrase[128] = {0};
+	size_t passphrase_len = passphrase_prompt(passphrase, sizeof(passphrase), "passphrase: ");
+
+	unsigned char key[16];
+	kdf(key, salt, passphrase, passphrase_len);
 	for (size_t i = 0; i < 16; i++) {
 		printf("%02hhx ", key[i]);
 	}
@@ -455,7 +543,6 @@ int main(void) {
 	}
 	printf("\n");
 
-
 	bool auth = ccm_decrypt(key, 25, buf, strlen(message), &buf[48]);
 	printf("auth %s\n", auth ? "succeeded" : "failed");
 	for (size_t i = 0; i < sizeof(buf); i++) {