BOBCryptoKit

This library performs the decryption that is the reverse of ECIESwithSHA256 in the BC library using Apples CryptoKit and some additional code.

ECIESwithSHA256 encryption/decryption

This scheme relies on the following things

• Basic ECDH to establish a shared secret between the two parties
• The shared secret is used with ANSI X9.63 Key Derivation Function (KDF) using SHA256 to generate a key which has the same length as the message + the public key that is sent as part of the message.
• The message itself is encrypted/decrypted by a simple xor with each byte of the message and the corresponding byte in the key.
• An encrypted message consists of public key + encrypted message + mac
• The mac uses an 128-bit key (taken from the generated key) and SHA256 for the digest.
• The mac is calculated on the encrypted data except for the MAC and at the end it is also updated with 8 zero bytes.