Hi everyone, I’m currently stumbling across an issue handling decrypting AES encrypted strings from ...

# android

[mng]

Hi everyone, I’m currently stumbling across an issue handling decrypting AES encrypted strings from iOS. Currently the flow would be something like:
1. User on iOS App encrypts data with Common Crypto framework and saves it to back end
2. Android app pulls that encrypted data from endpoint and decrypts it
I ensured that on Android side we’re using the same padding and transformations as iOS.
I’m currently able to decrypt half of the string but losing data in the first half. I was wondering if it was because of the encrypted string wrapping/newlining and tried a few methods of handling it but to no avail.
Can anybody help point me in the right direction?

[mng]

iOS Encrypted String

"yxgNZLv27IP00rtbuy+wAAVT4U6By+u7J1j8Q/QO8Q30+wsjjRruuDorN3Kpbe2E::HSIK62Vp2iLLrAYZvSyvY0e5upEMMHnFMnactC/YMFw="

[mng]

Android Decryption Result

J8CD]KyU1394764",
      "recordingId" : "abcdefg",
      "classId" : "abcdefg"
    }

[mng]

Expected Result

{
      "userId": abcedfg",
      "recordingId" : "abcdefg",
      "classId" : "abcdefg"
    }

[mng]

Decryption Function

fun String.decryptWithAES(key: String, iv: String): String? {
    Security.addProvider(BouncyCastleProvider())

    val skey = SecretKeySpec(key.toByteArray(Charsets.UTF_8), "AES")
    val ivKey = ByteArray(iv.toByteArray(Charsets.UTF_8).size)
    val input = android.util.Base64.decode(this.toByteArray(Charsets.UTF_8), android.util.Base64.CRLF or android.util.Base64.NO_WRAP)

    synchronized(Cipher::class.java) {
        val cipher = Cipher.getInstance("AES/CBC/PKCS5Padding")
        cipher.init(Cipher.DECRYPT_MODE, skey, IvParameterSpec(ivKey))
        val decryptedString = String(cipher.doFinal(input))
        return decryptedString.trim { it <= ' ' }
    }
}

[Sinan Gunes]

@mng Base64 functions may generate different values on Android and iOS. it does not seem to be related encryption, because even 1 bit of change will produce different result completely.

[mng]

Thank you for the response @Sinan Gunes. For my use case, would it be best to have a shared kotlin multiplatform library for encrypting/decrypting?

[Sinan Gunes]

¯\_(ツ)_/¯

[mng]

😂

[Sinan Gunes]

I don’t know details of your use case. but AES should be standard between platforms unlike Base64 implementations.

[mng]

I thought the most peculiar thing was that I was able to decrypt half of the input as shown above

[mng]

I’m not too sure what happened in the first half

[Sinan Gunes]

Base64 happened. AES is good.

[Sinan Gunes]

try to compare these two:

Data("Hello World{}".utf8).base64EncodedString()

and

Base64.encodeToString("Hello World{}".toByteArray(Charsets.UTF_8))

[mng]

What class type is Data?

[Sinan Gunes]

it is Swift vs Kotlin. I don’t know details. of swift part

[mng]

ah i seee

[mng]

let me try really quick

[mng]

Both of those are equal

[mng]

both produce

SGVsbG8gV29ybGR7fQ==

[Sinan Gunes]

can you try

{
      "userId": abcedfg",
      "recordingId" : "abcdefg",
      "classId" : "abcdefg"
    }

as String?

[mng]

both are slightly different, i’m not sure if it’s because i’m doing multiline string in Swift

[mng]

KOTLIN
ewogICAgICBcInVzZXJJZFwiOiBcImFiY2VkZmdcIiwKICAgICAgXCJyZWNvcmRpbmdJZFwiIDogXCJhYmNkZWZnXCIsCiAgICAgIFwiY2xhc3NJZFwiIDogXCJhYmNkZWZnXCIKICAgIH0=

SWIFT
ewogICJ1c2VySWQiOiAiYWJjZWRmZyIsCiAgInJlY29yZGluZ0lkIiA6ICJhYmNkZWZnIiwKICAiY2xhc3NJZCIgOiAiYWJjZGVmZyIKfQ==

[Sinan Gunes]

difference is the escapes on Kotlin code

[Sinan Gunes]

what about

.trim { it <= ' ' }

why do you need it?

[mng]

ah let me try removing

[mng]

same results but i suppose i don’t really need it

[mng]

one thing to note in the encrypted String above, i’m supposed to replace instances of :: with \r\n

[mng]

not sure if that’s part of the issue as well

[Sinan Gunes]

I don’t know its relation.