C: Passwortabfrage im Terminal

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Ich beschreibe hier kurz, wie unter Linux in der Konsole ein Passwort abgefragt werden kann, ohne dass die Eingabe des Users im Terminal erscheint.

Hierzu Definitionen im Header-File passphrase.h:

#ifndef PRJ_PASSPHRASE_H
#define PRJ_PASSPHRASE_H 1

#if defined(__cplusplus)
extern "C" {
#endif

char *
prj_read_passphrase(size_t size);

int
prj_get_passphrase(size_t count, size_t size, char **passphrase);

#if defined(__cplusplus)
}
#endif

#endif /* !PRJ_PASSPHRASE_H */

Daraufhin der eigentliche Code in passphrase.c:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <termios.h>

#include "passphrase.h"

/*
 * \brief   prj_read_passphrase
 *
 * reads from stdin input of user
 *
 * \param   size            maximum length of input
 *
 * \return                  a char array containing input of user
 */
char *
prj_read_passphrase(size_t size) {
    char password_input[size + 1]; 
    char *password = NULL;
    int ch, i;

    for (i = 0; (i < (int) size) && ((ch = getchar()) != EOF) && (ch != '\n') && (ch != '\r'); i++) {
        if (ch == 127 || ch == 8) { /* DEL or BS */
            if (i > 0)
                i = i - 2;
            else
                i = -1; 
        } else if (ch < 33) /* of course no control characters allowed */
            i--;
        else
            password_input[i] = (char) ch; 
    }   
    password_input[i] = '\0'; /* last given hidden character is broken */

    i++;

    password = (char *) malloc(sizeof(char) * i); 
    memcpy(password, password_input, (size_t) i); 
    memset(password_input, 0, (size_t) (i - 1));

    return password;
}

/*
 * \brief   prj_get_passphrase
 *
 * user interface for getting passphrase given by user on terminal
 *
 * \param   count           asking for passphrase one or two times
 * \param   size            maximum length of passphrase allowed
 * \param   passphrase      on success given passphrase is stored here, otherwise NULL
 *
 * \return                  on technical error -1, otherwise 0
 */
int
prj_get_passphrase(size_t count, size_t size, char **passphrase) {
    size_t j, x = 1;
    char *pass_first = NULL, *pass_second = NULL;
    size_t len_first = 0, len_second = 0;
    struct termios oldState, newState;

    if (tcgetattr(STDIN_FILENO, &oldState) == -1) {
        printf("\nError: could not hide terminal input!\n");
        return -1; 
    }   
    newState = oldState;
    newState.c_lflag &= ~(ICANON | ECHO);

    if (tcsetattr(STDIN_FILENO, TCSANOW, &newState) == -1) {
        printf("\nError: could not hide terminal input!\n");
        return -1; 
    }   

    if (count != 1) /* default is asking two times */
        count = 2;

retry:
    if (x > 3) /* three chances */
        goto failed;

    if (x > 1)
        printf("Passphrase mismatch. Please try it again.\n\n");

    x++;
    for (j = 0; j < count; j++) {
        if (j == 0) {
            printf("Please enter your passphrase: ");
            pass_first = prj_read_passphrase(size);
        } else {
            printf("Please re-enter your passphrase: ");
            pass_second = prj_read_passphrase(size);
        }
        printf("\n");
    }

    if (pass_first == NULL)
        goto retry;

    len_first = strlen(pass_first);

    if (pass_first != NULL && pass_second != NULL && count == 2) {
        len_second = strlen(pass_second);

        if (len_first != len_second)
            goto retry;

        if (memcmp(pass_first, pass_second, len_first) != 0)
            goto retry;
    }
    len_first++;

    *passphrase = (char *) malloc(sizeof(char) * len_first);
    memcpy(*passphrase, pass_first, len_first);

failed:
    if (tcsetattr(STDIN_FILENO, TCSANOW, &oldState) == -1) {
        printf("\nError: could not unhide terminal input!\n");
        printf("\nPlease try to reset your terminal by typing command 'reset -x'\n");
    }

    if (pass_first != NULL) {
        memset(pass_first, 0, len_first--);
        free(pass_first);
    }

    if (pass_second != NULL) {
        memset(pass_second, 0, len_second);
        free(pass_second);
    }

    return 0;
}

Jetzt kann die Funktionen prj_get_passphrase() wie in diesem Beispiel verwendet werden (get_passphrase.c):

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

#include "passphrase.h"

#define GPG_MAX_PASSPHRASE_LENGTH 100

/* ********************************************************************** */

int
main(int argc, char *argv[]) {
    (void) argc; /* avoid compiler warnings */
    (void) argv;

    char *passphrase = NULL;

    if (prj_get_passphrase(2, GPG_MAX_PASSPHRASE_LENGTH + 1, &passphrase) != 0) {
        printf("An error occured. Aborting.\n");
        return 1;
    }   

    if (passphrase == NULL) {
        printf("An error occured. Aborting.\n");
        return 1;
    }   

    printf("\nYour passphrase: '%s'\n", passphrase);

    memset(passphrase, 0, strlen(passphrase)); /* overwrite passphrase in memory */
    free(passphrase);

    return 0;
}

Danach kann wie gewohnt das Binary mit zum Beispiel gcc erstellt werden:

gcc -Wall -Wextra -Werror -o get_passphrase passphrase.c get_passphrase.c

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