2 * Check a SQLite database for a password within edit distance one.
4 * This file implements yet another variation on dictionary lookups.
5 * Passwords are checked against a SQLite database (generally created with the
6 * krb5-strength-wordlist utility) that holds words and reversed words, and
7 * all passwords within edit distance one of a word in the database are
10 * To find passwords within edit distance one, this algorithm checks, for each
11 * dictionary word, whether the length of longest common prefix plus the
12 * length of the longest common suffix between that word and the password is
13 * within 1 of the length of the password. It will be one less if a letter
14 * has been removed or replaced, and equal if the password is an exact match.
16 * To do this, the SQLite database contains one row for each dictionary word,
17 * containing both the word and the reversed version of the word. The
18 * password is divided into two components, a prefix and a suffix. It is
19 * checked against all dictionary words that fall lexicographically between
20 * the prefix and the prefix with its last character incremented, and then
21 * against all words where the word reversed falls lexicographically between
22 * the suffix reversed and the suffix reversed with its last character
25 * If the password matches a dictionary word, the edit must either be in the
26 * first half of the password or the last half of the password. If in the
27 * first half, the word it will match will fall in the prefix range. If in
28 * the last half, the word it will match will fall in the suffix range.
30 * Written by Russ Allbery <eagle@eyrie.org>
31 * Based on work by David Mazières
32 * Copyright 2016 Russ Allbery <eagle@eyrie.org>
34 * The Board of Trustees of the Leland Stanford Junior University
36 * See LICENSE for licensing terms.
40 #include <portable/kadmin.h>
41 #include <portable/krb5.h>
42 #include <portable/system.h>
48 #include <plugin/internal.h>
49 #include <util/macros.h>
52 * The prefix and suffix SQLite query. Finds all candidate words in range of
53 * the prefix or suffix. The prefix query should get bind variables for the
54 * prefix and the prefix with the last character incremented; the suffix query
55 * gets the same, but the suffix should be reversed.
57 #define PREFIX_QUERY \
58 "SELECT password, drowssap FROM passwords WHERE password BETWEEN ? AND ?;"
59 #define SUFFIX_QUERY \
60 "SELECT password, drowssap FROM passwords WHERE drowssap BETWEEN ? AND ?;"
64 * Stub for strength_init_sqlite if not built with SQLite support.
68 strength_init_sqlite(krb5_context ctx, krb5_pwqual_moddata data UNUSED)
72 /* Get CDB dictionary path from krb5.conf. */
73 strength_config_string(ctx, "password_dictionary_sqlite", &path);
75 /* If it was set, report an error, since we don't have CDB support. */
79 krb5_set_error_message(ctx, KADM5_BAD_SERVER_PARAMS, "SQLite dictionary"
80 " requested but not built with SQLite support");
81 return KADM5_BAD_SERVER_PARAMS;
86 /* Skip the rest of this file if SQLite is not available. */
90 * Report a SQLite error. Takes the module data (used to access the SQLite
91 * object) and the Kerberos context, stores the SQLite error in the Kerberos
92 * context, and returns the generic KADM5_FAILURE code, since there doesn't
93 * appear to be anything better.
95 static krb5_error_code __attribute__((__format__(printf, 3, 4)))
96 error_sqlite(krb5_context ctx, krb5_pwqual_moddata data, const char *format,
104 errmsg = sqlite3_errmsg(data->sqlite);
105 va_start(args, format);
106 length = vasprintf(&message, format, args);
109 return strength_error_system(ctx, "cannot allocate memory");
110 krb5_set_error_message(ctx, KADM5_FAILURE, "%s: %s", message, errmsg);
112 return KADM5_FAILURE;
117 * Given a string, returns a reversed version of that string in newly
118 * allocated memory. The caller is responsible for freeing. Returns NULL on
119 * memory allocation failure.
122 reverse_string(const char *string)
127 length = strlen(string);
128 reversed = malloc(length + 1);
129 if (reversed == NULL)
131 reversed[length] = '\0';
132 for (i = 0; i < length; i++)
133 reversed[length - i - 1] = string[i];
139 * Given two strings, return the length of their common prefix, not counting
140 * the nul character that terminates either string.
143 common_prefix_length(const char *a, const char *b)
147 for (i = 0; a[i] == b[i] && a[i] != '\0' && b[i] != '\0'; i++)
154 * Given the length of the password, the password, the reversed password, and
155 * an executed SQLite statement that contains the word and reversed word as
156 * the first two column texts, determine whether this password is a match
157 * within edit distance one.
159 * It will be a match if the length of the common prefix of the password and
160 * word plus the length of the common prefix of the reversed password and the
161 * reversed word (which is the length of the common suffix) is greater than or
162 * equal to the length of the password minus one.
164 * To see why the sum of the prefix and suffix length can be longer than the
165 * length of the password when the password doesn't match the word, consider
166 * the password "aaaa" and the word "aaaaaaaaa"
167 * (The prefix length plus the
168 * suffix length may be greater than the length of the password if the
169 * password is an exact match for the word or
172 match(size_t length, const char *password, const char *drowssap,
175 const char *word, *drow;
176 size_t prefix_length, suffix_length, match_length, word_length;
178 /* Discard all words whose length is too different. */
179 word = (const char *) sqlite3_column_text(query, 0);
180 word_length = strlen(word);
181 if (length > word_length + 1 || length + 1 < word_length)
185 * Get the common prefix length and check if the password is an exact
188 prefix_length = common_prefix_length(password, word);
189 if (prefix_length == length)
193 * Ensure there aren't too many different characters for this to be a
194 * match. If the common prefix and the common suffix together have a
195 * length that's more than one character shorter than the password length,
196 * this is different by at least edit distance two. The sum of the
197 * lengths of the common prefix and suffix can be greater than length in
198 * cases of an edit in the middle of repeated passwords, such as the
199 * password "baaab" and the word "baab", but those are all matches.
201 drow = (const char *) sqlite3_column_text(query, 1);
202 suffix_length = common_prefix_length(drowssap, drow);
203 match_length = prefix_length + suffix_length;
204 return (match_length > length || length - match_length <= 1);
209 * Initialize the SQLite dictionary. Opens the database and compiles the two
210 * queries that we'll use. Returns 0 on success, non-zero on failure (and
211 * sets the error in the Kerberos context).
214 strength_init_sqlite(krb5_context ctx, krb5_pwqual_moddata data)
219 /* Get SQLite dictionary path from krb5.conf. */
220 strength_config_string(ctx, "password_dictionary_sqlite", &path);
222 /* If there is no configured dictionary, nothing to do. */
226 /* Open the database. */
227 status = sqlite3_open_v2(path, &data->sqlite, SQLITE_OPEN_READONLY, NULL);
229 return error_sqlite(ctx, data, "cannot open dictionary %s", path);
231 /* Precompile the queries we'll use. */
232 status = sqlite3_prepare_v2(data->sqlite, PREFIX_QUERY, -1,
233 &data->prefix_query, NULL);
235 return error_sqlite(ctx, data, "cannot prepare prefix query");
236 status = sqlite3_prepare_v2(data->sqlite, SUFFIX_QUERY, -1,
237 &data->suffix_query, NULL);
239 return error_sqlite(ctx, data, "cannot prepare suffix query");
241 /* Finished. Return success. */
248 * Given a password, look for a word in the database within edit distance one.
249 * The full algorithm used here is described in the comment at the start of
250 * this file. Returns a Kerberos status code, which will be KADM5_PASS_Q_DICT
251 * if the password was found in the dictionary.
254 strength_check_sqlite(krb5_context ctx, krb5_pwqual_moddata data,
255 const char *password)
257 krb5_error_code code;
258 size_t length, prefix_length, suffix_length;
260 char *drowssap = NULL;
264 /* If we have no dictionary, there is nothing to do. */
265 if (data->sqlite == NULL)
269 * Determine the length of the prefix and suffix into which we'll divide
270 * the string. Passwords shorter than two characters cannot be
271 * meaningfully checked using this method and cause boundary condition
274 length = strlen(password);
277 prefix_length = length / 2;
278 suffix_length = length - prefix_length;
280 /* Obtain the reversed password, used for suffix checks. */
281 drowssap = reverse_string(password);
282 if (drowssap == NULL)
283 return strength_error_system(ctx, "cannot allocate memory");
285 /* Set up the query for prefix matching. */
286 prefix = strdup(password);
287 if (prefix == NULL) {
288 code = strength_error_system(ctx, "cannot allocate memory");
291 status = sqlite3_bind_text(data->prefix_query, 1, password, prefix_length,
293 if (status != SQLITE_OK) {
294 code = error_sqlite(ctx, data, "cannot bind prefix start");
297 prefix[prefix_length - 1]++;
298 status = sqlite3_bind_text(data->prefix_query, 2, prefix, prefix_length,
300 if (status != SQLITE_OK) {
301 code = error_sqlite(ctx, data, "cannot bind prefix end");
306 * Do prefix matching. Get the set of all database entries starting with
307 * the same prefix and, for each, check whether our password matches that
308 * entry within edit distance one.
310 while ((status = sqlite3_step(data->prefix_query)) == SQLITE_ROW)
311 if (match(length, password, drowssap, data->prefix_query)) {
315 if (status != SQLITE_DONE && status != SQLITE_ROW) {
316 code = error_sqlite(ctx, data, "error searching by password prefix");
319 status = sqlite3_reset(data->prefix_query);
320 if (status != SQLITE_OK) {
321 code = error_sqlite(ctx, data, "error resetting prefix query");
327 /* Set up the query for suffix matching. */
328 status = sqlite3_bind_text(data->suffix_query, 1, drowssap, suffix_length,
330 if (status != SQLITE_OK) {
331 code = error_sqlite(ctx, data, "cannot bind suffix start");
334 drowssap[prefix_length - 1]++;
335 status = sqlite3_bind_text(data->suffix_query, 2, drowssap, suffix_length,
337 drowssap[prefix_length - 1]--;
338 if (status != SQLITE_OK) {
339 code = error_sqlite(ctx, data, "cannot bind suffix end");
344 * Do suffix matching. Get the set of all database entries starting with
345 * the same prefix and, for each, check whether our password matches that
346 * entry within edit distance one.
348 while ((status = sqlite3_step(data->suffix_query)) == SQLITE_ROW)
349 if (match(length, password, drowssap, data->suffix_query)) {
353 if (status != SQLITE_DONE && status != SQLITE_ROW) {
354 code = error_sqlite(ctx, data, "error searching by password suffix");
357 status = sqlite3_reset(data->suffix_query);
358 if (status != SQLITE_OK) {
359 code = error_sqlite(ctx, data, "error resetting suffix query");
365 /* No match. Clean up and return success. */
366 memset(prefix, 0, length);
367 memset(drowssap, 0, length);
373 /* We found the password in the dictionary. */
374 code = strength_error_dict(ctx, ERROR_DICT);
377 memset(prefix, 0, length);
378 memset(drowssap, 0, length);
386 * Free internal SQLite state and close the SQLite database.
389 strength_close_sqlite(krb5_context ctx UNUSED, krb5_pwqual_moddata data)
391 if (data->prefix_query != NULL)
392 sqlite3_finalize(data->prefix_query);
393 if (data->suffix_query != NULL)
394 sqlite3_finalize(data->suffix_query);
395 if (data->sqlite != NULL)
396 sqlite3_close(data->sqlite);
399 #endif /* HAVE_SQLITE */