Fix Ed25519 keypair generation

Ed25519 private keys, it turns out, have 64 bytes, not 32.

We were previously generating only 32 bytes (which is all that is required to
generate the public key), and then using the public key as the upper 32 bytes
when generating the per-message session key. This meant that everything
appeared to work, but the security of the private key was severely compromised.

By way of fixes:

 * Use the correct algorithm for generating the Ed25519 private key, and store
   all 512 bits of it.

 * Update the account pickle format and refuse to load the old format (since we
   should consider it compromised).

 * Bump the library version, and add a function to retrieve the library
   version, so that applications can verify that they are linked against a
   fixed version of the library.

 * Remove the curve25519_{sign, verify} functions which were unused and of
   dubious quality.

1 files changed, 12 insertions, 4 deletions

diff --git a/src/account.cpp b/src/account.cpp
index c8e6e40..ec763f8 100644
--- a/src/account.cpp
+++ b/src/account.cpp
@@ -326,7 +326,9 @@ static std::uint8_t const * unpickle(
 } // namespace olm
 
 namespace {
-static const std::uint32_t ACCOUNT_PICKLE_VERSION = 1;
+// 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;
 }
 
 
@@ -360,9 +362,15 @@ std::uint8_t const * olm::unpickle(
 ) {
     uint32_t pickle_version;
     pos = olm::unpickle(pos, end, pickle_version);
-    if (pickle_version != ACCOUNT_PICKLE_VERSION) {
-        value.last_error = OlmErrorCode::OLM_UNKNOWN_PICKLE_VERSION;
-        return end;
+    switch (pickle_version) {
+        case ACCOUNT_PICKLE_VERSION:
+            break;
+        case 1:
+            value.last_error = OlmErrorCode::OLM_BAD_LEGACY_ACCOUNT_PICKLE;
+            return end;
+        default:
+            value.last_error = OlmErrorCode::OLM_UNKNOWN_PICKLE_VERSION;
+            return end;
     }
     pos = olm::unpickle(pos, end, value.identity_keys);
     pos = olm::unpickle(pos, end, value.one_time_keys);