# Encrypted Payment Comments

Overview of encrypted payment comments and its technical details related to symmetric and asymmetric encryption.

### Introduction to Comment Encryption

As part of Celo’s identity protocol, a public encryption key is stored along with a user’s address in the `Accounts`

contract.

Both the address key pair and the encryption key pair are derived from the backup phrase. When sending a transaction the encryption key of the recipient is retrieved when getting his or her address. The comment is then encrypted using a 128 bit hybrid encryption scheme (ECDH on secp256k1 with AES-128-CTR). This system ensures that comments can only be read by the sending and receiving parties and that messages will be recovered when restoring a wallet from its backup phrase.

### Comment Encryption Technical Details

A 128 bit randomly generated session key, sk, is generated and used to symmetrically encrypt the comment. sk is asymmetrically encrypted to the sender and to the recipient.

`Encrypted = ECIES(sk, to=pubSelf) | ECIES(sk, to=pubOther) | AES(ke=sk, km=sk, comment)`

#### Symmetric Encryption (AES-128-CTR)

- Takes encryption key, ke, and MAC key, km, and the data to encrypt, plaintext
- Cipher: AES-128-CTR using a randomly generated iv
- Authenticate iv | ciphertext using HMAC with SHA-256 and km
- Return iv | ciphertext | mac

#### Asymmetric Encryption (ECIES)

- Takes data to encrypt, plaintext, and the public key of the recipient, pubKeyTo
- Generate an ephemeral keypair, ephemPubKey and ephemPrivKey
- Derive 32 bytes of key material, k, from ECDH between ephemPrivKey and pubKeyTousing ConcatKDF (specified as NIST 800-56C Rev 1 One Step KDF) with SHA-256 for H(x)
- The encryption key, ke, is the first 128 bits of k
- The MAC key, km, is SHA-256 of the second 128 bits of k
- Encrypt the plaintext symmetrically with AES-128-CTR using ke, km, and a random iv
- Return ephemPubKey | AES-128-CTR-HMAC(ke, km, plaintext) where the public key needs to be uncompressed (current limitation with decrypt).