cmdgen: command-line control of PPK save parameters.

This allows you to manually adjust the Argon2 parameters so that you
can trade off CPU requirements in legitimate use against difficulty of
brute-force attack. It also allows downgrading the key file version
back to the widespread PPK v2, so you can manually back-port a key
that you accidentally generated too new.

1 files changed, 72 insertions, 2 deletions

diff --git a/cmdgen.c b/cmdgen.c
index c7f6edbf..79fd3733 100644
--- a/cmdgen.c
+++ b/cmdgen.c
@@ -150,6 +150,20 @@ void help(void)
            "        proven         numbers that have been proven to be prime\n"
            "        proven-even    also try harder for an even distribution\n"
            "  --strong-rsa         use \"strong\" primes as RSA key factors\n"
+           "  --ppk-param <key>=<value>[,<key>=<value>,...]\n"
+           "        specify parameters when writing PuTTY private key file "
+           "format:\n"
+           "            version       PPK format version (min 2, max 3, "
+           "default 3)\n"
+           "            memory        Kb of memory to use in password hash "
+           "(default 8192)\n"
+           "            time          approx milliseconds to hash for "
+           "(default 100)\n"
+           "            passes        exact number of hash passes to run "
+           "(alternative to 'time')\n"
+           "            parallelism   number of parallelisable threads in the "
+           "hash function\n"
+           "                             (default 1)\n"
            );
 }
 
@@ -227,6 +241,7 @@ int main(int argc, char **argv)
     int exit_status = 0;
     const PrimeGenerationPolicy *primegen = &primegen_probabilistic;
     bool strong_rsa = false;
+    ppk_save_parameters params = ppk_save_default_parameters;
 
     if (is_interactive())
         progress_fp = stderr;
@@ -366,6 +381,62 @@ int main(int argc, char **argv)
                         strong_rsa = true;
                     } else if (!strcmp(opt, "-reencrypt")) {
                         reencrypt = true;
+                    } else if (!strcmp(opt, "-ppk-param") ||
+                               !strcmp(opt, "-ppk-params")) {
+                        if (!val && argc > 1)
+                            --argc, val = *++argv;
+                        if (!val) {
+                            errs = true;
+                            fprintf(stderr, "puttygen: option `-%s'"
+                                    " expects an argument\n", opt);
+                        } else {
+                            char *nextval;
+                            for (; val; val = nextval) {
+                                nextval = strchr(val, ',');
+                                if (nextval)
+                                    *nextval++ = '\0';
+
+                                char *optvalue = strchr(val, '=');
+                                if (!optvalue) {
+                                    errs = true;
+                                    fprintf(stderr, "puttygen: PPK parameter "
+                                            "'%s' expected a value\n", val);
+                                    continue;
+                                }
+
+                                *optvalue++ = '\0';
+                                char *end;
+                                unsigned long n = strtoul(optvalue, &end, 0);
+                                if (!*optvalue || *end) {
+                                    errs = true;
+                                    fprintf(stderr, "puttygen: value '%s' for "
+                                            "PPK parameter '%s': expected a "
+                                            "number\n", optvalue, val);
+                                    continue;
+                                }
+
+                                if (!strcmp(val, "version")) {
+                                    params.fmt_version = n;
+                                } else if (!strcmp(val, "memory") ||
+                                           !strcmp(val, "mem")) {
+                                    params.argon2_mem = n;
+                                } else if (!strcmp(val, "time")) {
+                                    params.argon2_passes_auto = true;
+                                    params.argon2_milliseconds = n;
+                                } else if (!strcmp(val, "passes")) {
+                                    params.argon2_passes_auto = false;
+                                    params.argon2_milliseconds = n;
+                                } else if (!strcmp(val, "parallelism") ||
+                                           !strcmp(val, "parallel")) {
+                                    params.argon2_parallelism = n;
+                                } else {
+                                    errs = true;
+                                    fprintf(stderr, "puttygen: unrecognised "
+                                            "PPK parameter '%s'\n", val);
+                                    continue;
+                                }
+                            }
+                        }
                     } else {
                       errs = true;
                       fprintf(stderr,
@@ -1033,8 +1104,7 @@ int main(int argc, char **argv)
             }
         } else {
             assert(ssh2key);
-            ret = ppk_save_f(outfilename, ssh2key, new_passphrase,
-                             &ppk_save_default_parameters);
+            ret = ppk_save_f(outfilename, ssh2key, new_passphrase, &params);
             if (!ret) {
                 fprintf(stderr, "puttygen: unable to save SSH-2 private key\n");
                 RETURN(1);