If this is some proprietary service using some proprietary protocol, you can consider using public key cryptography. Encrypt data with a one time AES key. Send the encrypted data. Encrypt the AES key with your public key and send it along. The server decrypts the symmetric AES key with your private key and thereafter decrypts the data !.
objective C
for more information and additional help see this link
Swift
for swift you can see the example here
or use
RNCryptor
encrypt
// Encryption
let data: NSData = ...
let password = "Secret password"
let ciphertext = RNCryptor.encryptData(data, password: password)
decrypt
// Decryption
do {
let originalData = try RNCryptor.decryptData(ciphertext, password: password)
// ...
} catch {
print(error)
}