CBC Decrypter decrypts the encrypted text but some of the text is replaced with random characters-CodePudding

Lets say I've this base64 encrypted text: (Please understand this is all test data) 0Ns8cv7CHb0Pn/L1RPQEJjAaPJbBuhJxf9RCm3k/qG6DoNgd06jSgUwiaM/ym1Xc9DOBMyZvMNZQKwv/cgr9WGP4ogmqNguaM/zIOIh/GpMGCbTh 5UmP3x La5eQKCmhim nxVIBVzcaepUGUyjyPncu2RPQB8maA2AjgmqFb2tk3rcRozj8rTjcQcykxk2i7omhmt1FHBj4YBJoT4oc6B9bCUj8nhIZPCOj8T1z7A3MvZJwSmMa5rFnnozMCQ3no7lxORJQnN9d7gkLge1BAGwrxbXZapkuBx/juqTK3sJ46oiPDCdB0IzZ6Q1PZTrJPcWghOlQwfvHfKgu8kHtW2y2QskP 1uzVQBainXxcCHX7ZBq7Vhdl7n3WQjjok3P/A/6ELJa6qI9eL/NTfEavr7m0PVyrm6Zci4piK2Gjdz/LJZWEKbX6/wUq3pnDClQXAv1DV6g8P46H0p2ZCb0/WsbyZmgxuWYMx/KV8236IUQwY4ylyxnhCHpgHZAeM/CC9qgnMw6zjEtud uWCx3iWZpQdOmxvaEVMQOQFte/PkUkXgH/9Z5nqJWoftKRcQSyC2 gl5ORgjA1GjbFcB5cr36DFj4su whgKfyDHtCVVqFK lIkcu2uLJQX7MwImuvKEAY48raCt8eE3qj2dfjK dNhOfB8TbU4qAsRTVh7FtD0Hfto7sdVRLWAZRnqo7tHqD0R1LWkXg/VhxFGMnJkUxdJonlEFYLadUTI2PIr4acnpHcr6P91dTU onXdFaYumeJrayrxBAujmXmAzZki KrYyviORMlxmpK87ZJ2jP0psRx0hD SlnHnX3o5nJVMCEPy1Gh9lbU3F4O2PtZ5edc7GEq8TFMoAXtRqL3qg2m/vfyrgzjbU9127pHvk4HNpF0ow9PlC3oi827npONncormAml5Ii 4sz5KF8aN1JEiqt5jvrRnbAOOcOCdCysl1jyDyapchdxcHKk/wmzFVrykkaehfoYYDIAMqzPaBCmGFjZ53eXwvjP2XrKaJDvS2DbIkHnT G0/lv58fYzAGwDlyxqA/oetN Y5WolmS6gJVaws4qSYKaqVmIdz4pP7xOQNP22qnnjAoB49A0aKpQfGkL3W8zppB3Qa3rtvK0piPemX3HbNLAdKFFBEh1 AEbYNyUE/EjdkkVEwIDayw6L3WurLTytPX 05kKSkfG9AmJzqYmd1y50UQ5GxQu7V65nRL73LBoVWxkUO/T rnyILmQ8dh91HuMgXdLlH41gfiRzk4axZr7LlSnsXVODoBYUHLOZ273WtFnixmKYCyzdI7gPnwN/BPBpVScpymKO0RZcQjFEp3TBOpF14W0 2RqVl/d OSmXzeA3j7yvZVeUU59KNSMZzebBI4U9kTu0koHDBJzNHItRkrqsjw3NobpK6JvWjWXatPU9ytJ0gX2smtWGuuzjr/asV0X4omKmovOCllMYh xRICkSIYB8C6rZ/a8ToVusmA62E6W 61t27e5wN4TCrtq9w3e12kNhZ73T/l2GHKa1gEqV6hBFm/VBEgxdiCtJeerqkbuotmJVMzFpJ s NM xfw2w8uyKUjJFX75nwwI4idhbTl

I've this IV: db64fa140a888b41 And this secret key: 62448f7b7128e8b55224dd667b7c1a7e

If I use this site https://www.devglan.com/online-tools/aes-encryption-decryption to decrypt it using CBC, 256 key size and base64 decrypt, I get the correct string which is: "{\"totaldistance\":7.116600000000001,\"totaltraveltime\":9,\"fare\":{\"vehicle_list\":[{\"name\":\"Electric Limousine\",\"category_image\":\"image-1667907996681.png\",\"seat_count\":\"4\",\"unique_category_id\":6,\"available_for\":\"2\",\"schedule_before\":\"20\",\"schedule_upto\":\"10800\",\"nearest_driver\":{\"fare\":52,\"min_fare\":35},\"schedule_only\":true},{\"name\":\"Limousine\",\"category_image\":\"image-1667908122302.png\",\"seat_count\":\"4\",\"unique_category_id\":7,\"available_for\":\"2\",\"schedule_before\":\"20\",\"schedule_upto\":\"10800\",\"nearest_driver\":{\"fare\":48,\"min_fare\":32},\"schedule_only\":true},{\"name\":\"XL Limousine\",\"category_image\":\"image-1667907858244.png\",\"seat_count\":\"6\",\"unique_category_id\":13,\"available_for\":\"2\",\"schedule_before\":\"20\",\"schedule_upto\":\"10800\",\"nearest_driver\":{\"fare\":59,\"min_fare\":40},\"schedule_only\":true},{\"name\":\"Ameera Limousine\",\"category_image\":\"image-1667907336189.png\",\"seat_count\":\"4\",\"unique_category_id\":16,\"available_for\":\"2\",\"schedule_before\":\"20\",\"schedule_upto\":\"60\",\"nearest_driver\":{\"fare\":48,\"min_fare\":32},\"schedule_only\":true}]}}"

But whenever I use my CBC decrypt function with the same input, I get this decrypted text:

"��(|��\u000bҖ�\b�Dvltraveltime\\\":9,\\\"fare\\\":{\\\"vehicle_list\\\":[{\\\"name\\\":\\\"Electric Limousine\\\",\\\"category_image\\\":\\\"image-1667907996681.png\\\",\\\"seat_count\\\":\\\"4\\\",\\\"unique_category_id\\\":6,\\\"available_for\\\":\\\"2\\\",\\\"schedule_before\\\":\\\"20\\\",\\\"schedule_upto\\\":\\\"10800\\\",\\\"nearest_driver\\\":{\\\"fare\\\":52,\\\"min_fare\\\":35},\\\"schedule_only\\\":true},{\\\"name\\\":\\\"Limousine\\\",\\\"category_image\\\":\\\"image-1667908122302.png\\\",\\\"seat_count\\\":\\\"4\\\",\\\"unique_category_id\\\":7,\\\"available_for\\\":\\\"2\\\",\\\"schedule_before\\\":\\\"20\\\",\\\"schedule_upto\\\":\\\"10800\\\",\\\"nearest_driver\\\":{\\\"fare\\\":48,\\\"min_fare\\\":32},\\\"schedule_only\\\":true},{\\\"name\\\":\\\"XL Limousine\\\",\\\"category_image\\\":\\\"image-1667907858244.png\\\",\\\"seat_count\\\":\\\"6\\\",\\\"unique_category_id\\\":13,\\\"available_for\\\":\\\"2\\\",\\\"schedule_before\\\":\\\"20\\\",\\\"schedule_upto\\\":\\\"10800\\\",\\\"nearest_driver\\\":{\\\"fare\\\":59,\\\"min_fare\\\":40},\\\"schedule_only\\\":true},{\\\"name\\\":\\\"Ameera Limousine\\\",\\\"category_image\\\":\\\"image-1667907336189.png\\\",\\\"seat_count\\\":\\\"4\\\",\\\"unique_category_id\\\":16,\\\"available_for\\\":\\\"2\\\",\\\"schedule_before\\\":\\\"20\\\",\\\"schedule_upto\\\":\\\"60\\\",\\\"nearest_driver\\\":{\\\"fare\\\":47,\\\"min_fare\\\":31},\\\"schedule_only\\\":true}]}}\"\u000b\u000b\u000b\u000b\u000b\u000b\u000b\u000b\u000b\u000b\u000b"

I'm using this decrypt function:

func DecryptCBC(key, ciphertext []byte, iv string) (plaintext []byte, err error) {
    var block cipher.Block
    ciphertext, err = b64.StdEncoding.DecodeString(string(ciphertext))
    if block, err = aes.NewCipher(key); err != nil {
        return
    }

    if len(ciphertext) < aes.BlockSize {
        fmt.Printf("ciphertext too short")
        return
    }

    ciphertext = ciphertext[aes.BlockSize:]

    cbc := cipher.NewCBCDecrypter(block,[]byte(iv))
    cbc.CryptBlocks(ciphertext, ciphertext)

    plaintext = ciphertext

    return
}

What am I doing wrong?

I tried not passing the vector and do this:

    iv := ciphertext[0:aes.BlockSize]

But its returning the same result.

The encrypted text is returned from a third party api, which also provides the secret key and token from a different API.

CodePudding user response：

The fact that the ciphertext is correctly decrypted by entering it, the IV, and the key into the appropriate boxes on the linked website tells me that the IV is not transmitted with the ciphertext, and therefore that decryption should start at the first byte of ciphertext rather than the 16th. So remove the line

    ciphertext = ciphertext[aes.BlockSize:]

and I'd expect you'll get the correct answer. Because the hex-encoded IV and key are used as is rather than hex-decoded, there is at most only half as much entropy in each. I'm not aware of any weakness of having a 256-bit AES key with only 128 bits of entropy but I'd rather not see an IV with only 64 bits of entropy. In any event, this is something that's decided by the encrypting party and not necessarily something you can control.