add support for fallback keys

2 files changed, 135 insertions, 1 deletions

diff --git a/src/account.cpp b/src/account.cpp
index 05e8134..a747677 100644
--- a/src/account.cpp
+++ b/src/account.cpp
@@ -21,6 +21,11 @@
 olm::Account::Account(
 ) : next_one_time_key_id(0),
     last_error(OlmErrorCode::OLM_SUCCESS) {
+    // since we don't need to keep track of whether the fallback keys are
+    // published, use the published flag as in indication for whether the keys
+    // were generated
+    current_fallback_key.published = false;
+    prev_fallback_key.published = false;
 }
 
 
@@ -32,6 +37,14 @@ olm::OneTimeKey const * olm::Account::lookup_key(
             return &key;
         }
     }
+    if (current_fallback_key.published
+        && olm::array_equal(current_fallback_key.key.public_key.public_key, public_key.public_key)) {
+        return &current_fallback_key;
+    }
+    if (prev_fallback_key.published
+        && olm::array_equal(prev_fallback_key.key.public_key.public_key, public_key.public_key)) {
+        return &prev_fallback_key;
+    }
     return 0;
 }
 
@@ -46,6 +59,16 @@ std::size_t olm::Account::remove_key(
             return id;
         }
     }
+    // check if the key is a fallback key, to avoid returning an error, but
+    // don't actually remove it
+    if (current_fallback_key.published
+        && olm::array_equal(current_fallback_key.key.public_key.public_key, public_key.public_key)) {
+        return current_fallback_key.id;
+    }
+    if (prev_fallback_key.published
+        && olm::array_equal(prev_fallback_key.key.public_key.public_key, public_key.public_key)) {
+        return prev_fallback_key.id;
+    }
     return std::size_t(-1);
 }
 
@@ -260,6 +283,67 @@ std::size_t olm::Account::generate_one_time_keys(
     return number_of_keys;
 }
 
+std::size_t olm::Account::generate_fallback_key_random_length() {
+    return CURVE25519_RANDOM_LENGTH;
+}
+
+std::size_t olm::Account::generate_fallback_key(
+    std::uint8_t const * random, std::size_t random_length
+) {
+    if (random_length < generate_fallback_key_random_length()) {
+        last_error = OlmErrorCode::OLM_NOT_ENOUGH_RANDOM;
+        return std::size_t(-1);
+    }
+    prev_fallback_key = current_fallback_key;
+    current_fallback_key.id = ++next_one_time_key_id;
+    current_fallback_key.published = true;
+    _olm_crypto_curve25519_generate_key(random, &current_fallback_key.key);
+    return 1;
+}
+
+
+std::size_t olm::Account::get_fallback_key_json_length(
+) {
+    std::size_t length = 4 + sizeof(KEY_JSON_CURVE25519); /* {"curve25519":{}} */
+    OneTimeKey & key = current_fallback_key;
+    if (key.published) {
+        length += 1; /* " */
+        length += olm::encode_base64_length(_olm_pickle_uint32_length(key.id));
+        length += 3; /* ":" */
+        length += olm::encode_base64_length(sizeof(key.key.public_key));
+        length += 1; /* " */
+    }
+    return length;
+}
+
+std::size_t olm::Account::get_fallback_key_json(
+    std::uint8_t * fallback_json, std::size_t fallback_json_length
+) {
+    std::uint8_t * pos = fallback_json;
+    if (fallback_json_length < get_fallback_key_json_length()) {
+        last_error = OlmErrorCode::OLM_OUTPUT_BUFFER_TOO_SMALL;
+        return std::size_t(-1);
+    }
+    *(pos++) = '{';
+    pos = write_string(pos, KEY_JSON_CURVE25519);
+    *(pos++) = '{';
+    OneTimeKey & key = current_fallback_key;
+    if (key.published) {
+        *(pos++) = '\"';
+        std::uint8_t key_id[_olm_pickle_uint32_length(key.id)];
+        _olm_pickle_uint32(key_id, key.id);
+        pos = olm::encode_base64(key_id, sizeof(key_id), pos);
+        *(pos++) = '\"'; *(pos++) = ':'; *(pos++) = '\"';
+        pos = olm::encode_base64(
+            key.key.public_key.public_key, sizeof(key.key.public_key.public_key), pos
+        );
+        *(pos++) = '\"';
+    }
+    *(pos++) = '}';
+    *(pos++) = '}';
+    return pos - fallback_json;
+}
+
 namespace olm {
 
 static std::size_t pickle_length(
@@ -329,7 +413,8 @@ static std::uint8_t const * unpickle(
 namespace {
 // pickle version 1 used only 32 bytes for the ed25519 private key.
 // Any keys thus used should be considered compromised.
-static const std::uint32_t ACCOUNT_PICKLE_VERSION = 2;
+// pickle version 2 does not have fallback keys.
+static const std::uint32_t ACCOUNT_PICKLE_VERSION = 3;
 }
 
 
@@ -340,6 +425,8 @@ std::size_t olm::pickle_length(
     length += olm::pickle_length(ACCOUNT_PICKLE_VERSION);
     length += olm::pickle_length(value.identity_keys);
     length += olm::pickle_length(value.one_time_keys);
+    length += olm::pickle_length(value.current_fallback_key);
+    length += olm::pickle_length(value.prev_fallback_key);
     length += olm::pickle_length(value.next_one_time_key_id);
     return length;
 }
@@ -352,6 +439,8 @@ std::uint8_t * olm::pickle(
     pos = olm::pickle(pos, ACCOUNT_PICKLE_VERSION);
     pos = olm::pickle(pos, value.identity_keys);
     pos = olm::pickle(pos, value.one_time_keys);
+    pos = olm::pickle(pos, value.current_fallback_key);
+    pos = olm::pickle(pos, value.prev_fallback_key);
     pos = olm::pickle(pos, value.next_one_time_key_id);
     return pos;
 }
@@ -365,6 +454,7 @@ std::uint8_t const * olm::unpickle(
     pos = olm::unpickle(pos, end, pickle_version);
     switch (pickle_version) {
         case ACCOUNT_PICKLE_VERSION:
+        case 2:
             break;
         case 1:
             value.last_error = OlmErrorCode::OLM_BAD_LEGACY_ACCOUNT_PICKLE;
@@ -375,6 +465,14 @@ std::uint8_t const * olm::unpickle(
     }
     pos = olm::unpickle(pos, end, value.identity_keys);
     pos = olm::unpickle(pos, end, value.one_time_keys);
+    if (pickle_version == 2) {
+        // version 2 did not have fallback keys
+        value.current_fallback_key.published = false;
+        value.prev_fallback_key.published = false;
+    } else {
+        pos = olm::unpickle(pos, end, value.current_fallback_key);
+        pos = olm::unpickle(pos, end, value.prev_fallback_key);
+    }
     pos = olm::unpickle(pos, end, value.next_one_time_key_id);
     return pos;
 }
diff --git a/src/olm.cpp b/src/olm.cpp
index 0333b10..50742cc 100644
--- a/src/olm.cpp
+++ b/src/olm.cpp
@@ -417,6 +417,42 @@ size_t olm_account_generate_one_time_keys(
 }
 
 
+size_t olm_account_generate_fallback_key_random_length(
+    OlmAccount * account
+) {
+    return from_c(account)->generate_fallback_key_random_length();
+}
+
+
+size_t olm_account_generate_fallback_key(
+    OlmAccount * account,
+    void * random, size_t random_length
+) {
+    size_t result = from_c(account)->generate_fallback_key(
+        from_c(random), random_length
+    );
+    olm::unset(random, random_length);
+    return result;
+}
+
+
+size_t olm_account_fallback_key_length(
+    OlmAccount * account
+) {
+    return from_c(account)->get_fallback_key_json_length();
+}
+
+
+size_t olm_account_fallback_key(
+    OlmAccount * account,
+    void * fallback_key_json, size_t fallback_key_json_length
+) {
+    return from_c(account)->get_fallback_key_json(
+        from_c(fallback_key_json), fallback_key_json_length
+    );
+}
+
+
 size_t olm_create_outbound_session_random_length(
     OlmSession * session
 ) {