There are two different memo concepts on the HIVE blockchain that can lead to some confusion.
The Memo field used in Hive transactions and the Memo key used to encrypt info in the blockchain
Memo: This is an optional field in some transactions. It could be optional if you were not asked to specify a memo.
Memo Key: This is one of the 4 Private keys linked to your account and the one with the lowest authority level. The Memo Key allows encrypting and decrypting messages in the blockchain.
How do I encrypt a message in the blockchain?
You can encrypt a message in the blockchain by sending HIVE or HBD to a HIVE account. This message goes in the Memo field and contains a message that always starts with a "#".
e.g., #This is an encrypted message to @b37
I will send a transfer to @b37 with the memo # This is an encrypted message to @b37 using Hive Keychain with both, Active and Memo private keys imported on it.
The active private key is needed to confirm the transfer, and the Memo private key is required to encode the message.
Once you sign the transaction with your private active key and the transaction is completed can be seen in the blockchain.
Tx 2e14c80e.. | Hive Block 64691338 | Hive
Our Message was encoded as:
#4ocWaGp99XTrcaFxnrcNhyJtYX7zuZgyypik6TiFp1Q4TRZc1PHrH8xXztQxTrCtQ5vk5njLR7Hcy3yMh18Si7SEfu4D7AsNbAtgMEsCKDbpD4KfBGHPYYdvGwWws2ZJ4yJ4QCrJcKsQhb6giRNTFJq1FNBMAY9yKPoydBcrh9fqo
How do I decrypt a message in the blockchain?
You don't need to. Only you and the recipient will be able to read the message. But you will need the Private Memo Key to be able to see the message.
To see this memo, I am using the Hive Keychain extension.
@b37 wallet history
This means. To read the Memo, you would need the sender’s private memo key or the receiver’s private memo key.
There are various sites to look up your transaction history, like PeakD or Hive.blog changing your username in the URL to your actual username. You will need to log into your account using at least your private memo key to see the encrypted memo.