How to encrypt a memo when transferring Steem

[dantheman]

Normally steem memos are sent as plain text, this article describes how to format your memos for encryption.

CLI Wallet

When using the cli wallet, if you include '#' as the first character of your memo will be encrypted.

unlocked >>> transfer from to "1.000 STEEM" "#encrypted memo" false
{
    "operations": [[ "transfer",{
         "from": "steemit",
         "to": "steem",
         "amount": "1.000 STEEM",
         "memo": "#CqojqJfFwawzgnYSFUWokj...xAfynekJtySMsqK8A9doD4gHA5fDKgQTEfUKZgAm8Dx"
    } ]  ]
}

When the wallet is unlocked and get_account_history is called, the "memo" field will be replaced with "encrypted memo" without the initial '#'. This means that exchanges that upgrade to the latest CLI wallet can transparently support encrypted memos.

Algorithm

The algorithm for encrypting the memo involves the following steps:
1. Generate a memo_data struct containing:

      public_key   from
      public_key   to
      uint64_t     nonce
      uint32_t     check
      vector<char> encrypted

1. Calculate the AES encryption key as:

  shared_secret = from_private_key.get_shared_secret( to );
  /// concatenate nonce and shared secret (binary)
  encryption_key = sha512( nonce + shared_secret )
  ///< check is first 64 bit of sha256 hash treated as uint64_t truncated to 32 bits.
  check = sha256( encryption_key )._hash[0]
  /// pack the memo as a length-prefixed string, length is serialized as varint   
  plain_text = pack( memo_text) 
  encrypted = aes_encrypt( encryption_key, plain_text )

1. Serialize memo_data
   Now convert the memo data into a vector and then convert it to base58

 string result = '#' + to_base58( pack( memodata ) );

The to and from keys should be fetched as the memo_key property on the respective accounts.

Comments

[jsc]

This is a random nonce with a time-based component. It returns a unique 64 bit unsigned number string.

    uniqueNonce() {
        if(unique_nonce_entropy === null) {
            const b = secureRandom.randomUint8Array(2)
            unique_nonce_entropy = parseInt(b[0] << 8 | b[1], 10)
        }
        let long = Long.fromNumber(Date.now())
        const entropy = ++unique_nonce_entropy % 0xFFFF
        long = long.shiftLeft(16).or(Long.fromNumber(entropy));
        return long.toString()
    }
    let unique_nonce_entropy = null

npm libraries used: long and secure-random

[dantheman]

The purpose of the 'nonce' field is to generate a unique encryption key for every transfer between two accounts. I it should be a random number.

[smooth]

Is this working properly? As far as I can tell the same key is used for every transfer. See test (both used plaintext of "abc"): https://steemd.com/@aaa