Demo code for StackOverflow question: https://stackoverflow.com/questions/64477634/cryptoswift-getting-nil-when-using-makeencryptor-for-string-of-less-than-16

gistfile1.txt

//
//  main.swift
//
//  Created by idz on 10/23/20.
//

import Foundation
import CryptoSwift
import CommonCrypto

extension String
{
    func base64Encoded() -> Data?
    {
        self.data(using: .utf8)?.base64EncodedData()
    }
}

func enc(plainText: [String], key: [UInt8], iv: [UInt8]) throws -> (cipherText: [UInt8], authenticationTag: [UInt8]?)
{
    let gcmEnc = GCM(iv: iv, mode: .detached)
    var enc = try AES(key: key, blockMode: gcmEnc, padding: .noPadding).makeEncryptor()
    var ciphertext = Array<UInt8>()
    for txt in plainText {
         ciphertext  += try enc.update(withBytes: Array(txt.utf8))
    }
    ciphertext += try enc.finish()
    return (ciphertext, gcmEnc.authenticationTag)
}

func dec(cipherText: [UInt8], authenticationTag: [UInt8]?, key: [UInt8], iv: [UInt8]) throws -> [UInt8]? {
    let tagLength = authenticationTag?.count ?? 0
    let gcmDec = GCM.init(iv: iv, additionalAuthenticatedData: nil, tagLength: tagLength, mode: .detached)
    gcmDec.authenticationTag = authenticationTag
    var aesDec = try AES(key: key, blockMode: gcmDec, padding: .noPadding).makeDecryptor()
    var decData = try aesDec.update(withBytes: cipherText)
    decData += try aesDec.finish()
    return decData
}

func encAndDec(){
    do {
        guard let key =  Data.init(base64Encoded: "12345678901234567890123456789012".base64Encoded()!)
            else {
                fatalError("Failed to create key")
        }
        let iv : Array<UInt8> = [0,0,0,0,
                                 0,0,0,0,
                                 0,0,0,0]

        //let arrStr = ["My name is tarun"] // Working
        let arrStr = ["tarun"] // Not working for this string
        
        let (cipherText, authenticationTag) = try enc(plainText: arrStr, key: key.bytes, iv: iv)
        guard let decrypedPlainText = try dec(cipherText: cipherText,
                                              authenticationTag: authenticationTag, key: key.bytes, iv: iv) else {
            fatalError("Decryption return nil")
        }
        guard let decryptedString = String(bytes: decrypedPlainText, encoding: .utf8) else {
            fatalError("Failed to convert plaintext to string using UTF8 encoding.")
        }

        print("Decrypted Plaintext: \(decryptedString)")

    }
    catch let e {
        print("EXCEPTION: \(e)")
    }
}

encAndDec()