session-ios/SessionMessagingKit/Sending & Receiving/MessageReceiver+Decryption.swift
2021-02-09 15:14:18 +11:00

30 lines
2 KiB
Swift

import CryptoSwift
import SessionUtilitiesKit
import Sodium
extension MessageReceiver {
internal static func decryptWithSessionProtocol(ciphertext: Data, using x25519KeyPair: ECKeyPair) throws -> (plaintext: Data, senderX25519PublicKey: String) {
let recipientX25519PrivateKey = x25519KeyPair.privateKey
let recipientX25519PublicKey = Data(hex: x25519KeyPair.hexEncodedPublicKey.removing05PrefixIfNeeded())
let sodium = Sodium()
let signatureSize = sodium.sign.Bytes
let ed25519PublicKeySize = sodium.sign.PublicKeyBytes
// 1. ) Decrypt the message
guard let plaintextWithMetadata = sodium.box.open(anonymousCipherText: Bytes(ciphertext), recipientPublicKey: Box.PublicKey(Bytes(recipientX25519PublicKey)),
recipientSecretKey: Bytes(recipientX25519PrivateKey)), plaintextWithMetadata.count > (signatureSize + ed25519PublicKeySize) else { throw Error.decryptionFailed }
// 2. ) Get the message parts
let signature = Bytes(plaintextWithMetadata[plaintextWithMetadata.count - signatureSize ..< plaintextWithMetadata.count])
let senderED25519PublicKey = Bytes(plaintextWithMetadata[plaintextWithMetadata.count - (signatureSize + ed25519PublicKeySize) ..< plaintextWithMetadata.count - signatureSize])
let plaintext = Bytes(plaintextWithMetadata[0..<plaintextWithMetadata.count - (signatureSize + ed25519PublicKeySize)])
// 3. ) Verify the signature
let verificationData = plaintext + senderED25519PublicKey + recipientX25519PublicKey
let isValid = sodium.sign.verify(message: verificationData, publicKey: senderED25519PublicKey, signature: signature)
guard isValid else { throw Error.invalidSignature }
// 4. ) Get the sender's X25519 public key
guard let senderX25519PublicKey = sodium.sign.toX25519(ed25519PublicKey: senderED25519PublicKey) else { throw Error.decryptionFailed }
return (Data(plaintext), "05" + senderX25519PublicKey.toHexString())
}
}