Allow SSH keys on multiple accounts

Update validation and duplicate checks to scope key uniqueness per
account, and clarify the console error message. Document cross-account
key assignment and authentication testing.

docs/user_administration_tests.md

@@ -48,10 +48,12 @@ Verify all of the following fail without changing the account generation or key
 
 - Unsupported RSA, certificate, or unknown key types.
 - Truncated/invalid Base64, mismatched outer and embedded types, trailing blob data, malformed Ed25519 lengths, and an invalid/off-curve ECDSA point.
-- Adding the same key again to the same account or assigning it to a different account.
+- Adding the same key again to the same account.
 - Adding a fourth key to an account that already has three.
 - Deleting an empty/out-of-range slot.
 
+Assign the same public key to a second account and confirm it is accepted, receives an independent slot, and authenticates as the username selected by the SSH client.
+
 Then exercise `user key delete <username> <index> --force` and `user key clear <username> --force`, reboot, and confirm the exact remaining fingerprints persist. Phase 8A stores these keys but does not yet accept SSH key login.
 
 ### 5. Legacy credential rotation boundary and reboot reconciliation

src/user_console.c

@@ -357,7 +357,7 @@ static int add_key(const char *username)
     secure_wipe(line, sizeof(line));
     if (error != ESP_OK) {
         if (error == USER_DATABASE_ERR_DUPLICATE_SSH_KEY) {
-            printf("Could not add SSH key: that public key is already assigned to an account.\n");
+            printf("Could not add SSH key: that public key is already assigned to this account.\n");
         } else if (error == ESP_ERR_NO_MEM) {
             printf("Could not add SSH key: the account already has %u keys.\n",
                    USER_DATABASE_MAX_SSH_KEYS_PER_USER);

src/user_database.c

@@ -387,17 +387,11 @@ static esp_err_t validate_database(const stored_database_t *database)
                           sizeof(key->blob) - key->blob_length)) {
                 return ESP_ERR_INVALID_RESPONSE;
             }
-            for (size_t prior_user = 0U; prior_user <= index; ++prior_user) {
-                size_t prior_key_limit = prior_user == index
-                                             ? key_index
-                                             : USER_DATABASE_MAX_SSH_KEYS_PER_USER;
-                for (size_t prior_key = 0U; prior_key < prior_key_limit; ++prior_key) {
-                    if (stored_keys_equal(key,
-                                          &database->users[prior_user].keys[prior_key])) {
+            for (size_t prior_key = 0U; prior_key < key_index; ++prior_key) {
+                if (stored_keys_equal(key, &user->keys[prior_key])) {
                     return ESP_ERR_INVALID_RESPONSE;
                 }
             }
-            }
             uint8_t fingerprint[USER_DATABASE_SHA256_LENGTH];
             if (mbedtls_sha256(key->blob, key->blob_length, fingerprint, 0) != 0 ||
                 !constant_time_equal(fingerprint, key->fingerprint,
@@ -1157,21 +1151,16 @@ esp_err_t user_database_add_ssh_key(
     if (error == ESP_OK) {
         stored_user_t *user = &s_candidate.users[user_index];
         int free_index = -1;
-        for (size_t candidate_user_index = 0U;
-             candidate_user_index < USER_DATABASE_MAX_USERS; ++candidate_user_index) {
-            stored_user_t *candidate_user =
-                &s_candidate.users[candidate_user_index];
-            if (candidate_user->active == 0U) {
-                continue;
-            }
         for (size_t index = 0U;
              index < USER_DATABASE_MAX_SSH_KEYS_PER_USER; ++index) {
-                stored_key_t *key = &candidate_user->keys[index];
-                if (candidate_user_index == (size_t)user_index &&
-                    key->active == 0U && free_index < 0) {
+            stored_key_t *key = &user->keys[index];
+            if (key->active == 0U) {
+                if (free_index < 0) {
                     free_index = (int)index;
                 }
-                if (key->active != 0U && key->type_length == key_type_length &&
+                continue;
+            }
+            if (key->type_length == key_type_length &&
                 key->blob_length == key_blob_length &&
                 memcmp(key->type, key_type, key_type_length) == 0 &&
                 constant_time_equal(key->blob, key_blob, key_blob_length)) {
@@ -1179,10 +1168,6 @@ esp_err_t user_database_add_ssh_key(
                 break;
             }
         }
-            if (error != ESP_OK) {
-                break;
-            }
-        }
         if (error == ESP_OK && free_index < 0) {
             error = ESP_ERR_NO_MEM;
         }