exim-encrypt-dlfunc/src/libexim-encrypt-dlfunc-decrypt-secretbox.c

#define _GNU_SOURCE

#include <stdio.h>
#include <string.h>
#include <getopt.h>
#include <sodium.h>
#include <stdbool.h>
#include "common.c"

#define ENVVAR_PASSWORD_NAME "LIBEXIM_PASSWORD"

void print_usage(char *progname) {
    printf("Usage: %s [OPTIONS] [CIPHERTEXT]\n\n", progname);
    printf("Password:\n");
    printf("  -p, --password PASSWORD   Decrypt using PASSWORD\n");
    printf("\n");
    printf("  If the environment variable LIBEXIM_PASSWORD is set the password is read from there.\n");
    printf("  Setting a password with -p/--password overwrites this mechanism.\n");
    printf("\n");
    printf("Select input:\n");
    printf("  -f, --infile FILE         Decrypt contents of the first line of file FILE (use - for stdin)\n");
    printf("\n");
    printf("Output:\n");
    printf("  -n, --no-newline          Do not append a newline to the output\n");
    printf("\n");
    printf("Password and ciphertext are expected to be base64-encoded (as produced by the library).\n");
    printf("\n");
}

typedef enum {
    NONE = 0,
    PASSARG = 1,
    PASSENV = 2,
    INSTRING = 4,
    INFILE = 8
} seen_args;

int main(int argc, char *argv[]) {
    char *prog_basename = basename(argv[0]);
    int opt;
    char *cipherstring;
    size_t password_len;
    char *password;
    char *password_env;
    bool add_newline = true;

    seen_args mode = NONE;
    seen_args input = NONE;

    if (sodium_init() < 0) {
        fputs("Unable to initialize libsodium", stderr);
        exit(128);
    }

    // define arguments
    const char *shortargs = "p:f:nh";
    static struct option long_options[] = {
            {"password",   required_argument, NULL, 'p'},
            {"infile",     required_argument, NULL, 'f'},
            {"no-newline", no_argument,       NULL, 'n'},
            {"help",       no_argument,       NULL, 'h'},
            {0,            0,                 0,    0}
    };

    // check environment for LIBEXIM_PASSWORD
    password_env = getenv(ENVVAR_PASSWORD_NAME);
    if (password_env != NULL && strlen(password_env) > 0) {
        password = password_env;
        password_len = strlen(password);
        mode |= PASSENV;
    }

    // parse arguments
    int long_index = 0;
    while ((opt = getopt_long(argc, argv, shortargs,
                              long_options, &long_index)) != -1) {
        switch (opt) {
            case 'p':
                password_len = strlen((const char *) optarg);
                password = malloc(password_len + 1);
                strncpy(password, optarg, password_len);
                mode |= PASSARG;
                break;
            case 'f':
                cipherstring = read_first_line(optarg);
                input |= INFILE;
                break;
            case 'n':
                add_newline = false;
                break;
            case 'h':
                print_usage(prog_basename);
                exit(EXIT_SUCCESS);
        }
    }

    // read first non-option argument as ciphertext if present
    if (optind < argc) {
        size_t cipherstring_len = strlen(argv[optind]) + 1;
        cipherstring = malloc(cipherstring_len + 1);
        strncpy(cipherstring, argv[optind], cipherstring_len);
        input |= INSTRING;
    }

    // check if a password was provided
    if (mode == NONE) {
        fprintf(stderr, "[ERROR] Please specify a password.\n\n");
        print_usage(prog_basename);
        exit(EXIT_FAILURE);
    }

    // fail if neither argument nor filename for ciphertext is present
    if (input == NONE) {
        fprintf(stderr, "[ERROR] Please specify a ciphertext source.\n\n");
        print_usage(prog_basename);
        exit(EXIT_FAILURE);
    }

    // Derive key from the password.
    unsigned char keybytes[crypto_secretbox_KEYBYTES];
    sodium_memzero(keybytes, crypto_secretbox_KEYBYTES);
    crypto_generichash(keybytes, crypto_secretbox_KEYBYTES,
                       (unsigned char *) password, password_len, NULL, 0);

    // base64-decode input
    unsigned char *ciphertext;
    size_t ciphertext_len;
    if (base64_decode_string(cipherstring, &ciphertext, &ciphertext_len) != 0) {
        fprintf(stderr, "[ERROR] Unable to base64-decode ciphertext.\n\n");
        exit(EXIT_FAILURE);
    }

    // extract nonce
    unsigned char nonce[crypto_secretbox_NONCEBYTES];
    memcpy(nonce, ciphertext, crypto_secretbox_NONCEBYTES);

    // prepare buffer for cleartext
    if (ciphertext_len < crypto_secretbox_NONCEBYTES + crypto_secretbox_MACBYTES) {
        fprintf(stderr, "[ERROR] Ciphertext is too small to contain any data.\n\n");
        exit(EXIT_FAILURE);
    }
    size_t cleartext_len = ciphertext_len - crypto_secretbox_NONCEBYTES - crypto_secretbox_MACBYTES;
    unsigned char *cleartext = (unsigned char *) malloc(cleartext_len + 1);
    sodium_memzero(cleartext, cleartext_len + 1);

    // decrypt
    if (crypto_secretbox_open_easy(cleartext, &ciphertext[crypto_secretbox_NONCEBYTES],
                                   ciphertext_len - crypto_secretbox_NONCEBYTES, nonce, keybytes) == 0) {
    } else {
        fprintf(stderr, "[ERROR] Unable to decrypt message.\n\n");
        exit(EXIT_FAILURE);
    }

    // print cleartext to stdout
    if (add_newline == true) {
        fprintf(stdout, "%s\n", (const char *) cleartext);
    } else {
        fprintf(stdout, "%s", (const char *) cleartext);
    }

    free(cleartext);
    exit(EXIT_SUCCESS);
}