peerplays_migrated/draft-docs/extensions/private_memo.md

Status

This spec is a suggestion for a future improvement. It has not yet been implemented and is not yet considered normative for wallets.

private_memo_data specification

The private_memo_data is a data_extension with the following semantics:

• Memo encrypted by ECDH shared secret used as AES key
• All wallets should support displaying and sending memos with accounts' memo keys for asset_issue_operation, transfer_operation, override_transfer_operation and withdraw_permission_claim_operation.

Data extension structure

/**
 *  @brief defines the keys used to derive the shared secret
 *
 *  Because account authorities and keys can change at any time, each memo must
 *  capture the specific keys used to derive the shared secret.  In order to read
 *  the cipher message you will need one of the two private keys.
 *
 *  If @ref from == @ref to and @ref from == 0 then no encryption is used, the memo is public.
 *  If @ref from == @ref to and @ref from != 0 then invalid memo data
 *
 *  The reason we include *both* keys is to allow the wallet to
 *  rebuild the data for any transfer, including the memo, from the
 *  user's private keys and the contents of the blockchain --
 *  regardless of whether the user is the sender or receiver.
 */
struct memo_data
{
   public_key_type from;
   public_key_type to;
   /**
    * 64 bit nonce format:
    * [  8 bits | 56 bits   ]
    * [ entropy | timestamp ]
    * Timestamp is number of microseconds since the epoch
    * Entropy is a byte taken from the hash of a new private key
    *
    * This format is not mandated or verified; it is chosen to ensure uniqueness of key-IV pairs only. This should
    * be unique with high probability as long as the generating host has a high-resolution clock OR a strong source
    * of entropy for generating private keys.
    */
   uint64_t nonce;
   /**
    * This field contains the AES encrypted packed @ref memo_message
    */
   vector<char> message;
};

Encrypting/decrypting

TODO: write in words

NB no public memo (this is separate extension)

void memo_data::set_message(const fc::ecc::private_key& priv, const fc::ecc::public_key& pub,
   const string& msg, uint64_t nonce)
{
   from = priv.get_public_key();
   to = pub;
   auto secret = priv.get_shared_secret(pub);
   auto nonce_plus_secret = fc::sha512::hash(fc::to_string(nonce) + secret.str());
   string text = memo_message(digest_type::hash(msg)._hash[0], msg).serialize();
   message = fc::aes_encrypt( nonce_plus_secret, vector<char>(text.begin(), text.end()) );
}

string memo_data::get_message(const fc::ecc::private_key& priv,
                              const fc::ecc::public_key& pub)const
{
   auto secret = priv.get_shared_secret(pub);
   auto nonce_plus_secret = fc::sha512::hash(fc::to_string(nonce) + secret.str());
   auto plain_text = fc::aes_decrypt( nonce_plus_secret, message );
   auto result = memo_message::deserialize(string(plain_text.begin(), plain_text.end()));
   FC_ASSERT( result.checksum == uint32_t(digest_type::hash(result.text)._hash[0]) );
   return result.text;
}