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I using getting the encrypted message from a third party sever.

I am using the tools in enter link description here

to test and its correct

The Encrypted Text = 
"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"

Key = "1234567812345678"

And the result After AES Decrypted Output (Base64): And after Decode Plain Text THe result is :

{"upPacketSN":-1,"upDataSN":-1,"topic":"v1/up/ad","timestamp":1619672797621,"tenantId":"2000034792","serviceId":"","protocol":"lwm2m","productId":"15044315","payload":{"APPdata":"MTIzNDU2NzgsNTM0OCwwNywwOCwyNTUsMTc5LDEyOCwwMDAwLDQyNTksMSw2LDAsMCwwLDAsMCwwLDI0LDMuNzAsMy43MCwxLDEsNiwsMCwwLDAsMCwwLDAsMCwwLDAsMCwtMjU1LC0yNTUsLTI1NSwxLjY5LDEuOCw0OTk4LDYwLDE0NDAsNjA="},"messageType":"dataReport","deviceType":"","deviceId":"523ede8bb7e34dd4a1bd74028d63749e","assocAssetId":"","IMSI":"undefined","IMEI":"864162041961023"}

One of the thing is that IV must be NULL to get the correct answer.

And so if i implement in Python AES CBC using pycryptodome library in the following like:

class AES_CBC:
 
    def add_to_16(self, value):
        while len(value) % 16 != 0:
            value += '\0'
        return str.encode(value)  # 返回bytes
 
   
    
    #解密方法
    def decrypt_oralce(self, key, text):
        # 初始化加密器
        # 偏移量 16个0
        iv = "0000000000000000"
        aes = AES.new(self.add_to_16(key), AES.MODE_CBC, self.add_to_16(iv))
        #优先逆向解密base64成bytes
        base64_decrypted = base64.decodebytes(text.encode(encoding='utf-8'))
        #
        decrypted_text = str(aes.decrypt(base64_decrypted), encoding='utf-8') # 执行解密密并转码返回str
        unpad = lambda s : s[0:-ord(s[-1])]
        #PADDING = '\0'
        #print decrypted_text.rstrip(PADDING)  #zeropadding只见诶去掉结尾\0
        # print(unpad(decrypted_text))
        return unpad(decrypted_text)
 
 
if __name__ == '__main__':
    aes = AES_CBC()
    #加密
    key = "1234567812345678"
    enc_msg = "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"
 
    #解密
    dec_text = aes.decrypt_oralce(key, enc_msg)
    print(key)
    print(dec_text)

EDITED: i got a different result using same IV 16'0' and the result from the web is

{"upPacketSN":-1,"upDataSN":-1,"topic":"v1/up/ad","timestamp":1619687373640,"tenantId":"2000034792","serviceId":"","protocol":"lwm2m","productId":"15044315","payload":{"APPdata":"MTIzNDU2NzgsNTM0OCwwNywwOCwyNTUsMTc5LDEyOCwwMDAwLDQ1MDQsMSw2LDAsMCwwLDAsMCwwLDI0LDMuNzAsMy43MCwxLDEsNiwsMCwwLDAsMCwwLDAsMCwwLDAsMCwtMjU1LC0yNTUsLTI1NSwxLjY5LDEuOCw0OTk4LDYwLDE0NDAsNjA="},"messageType":"dataReport","deviceType":"","deviceId":"523ede8bb7e34dd4a1bd74028d63749e","assocAssetId":"","IMSI":"undefined","IMEI":"864162041961023"}

while the result of my code is :

KE@`QS[UDc~
1,"upDataSN":-1,"topic":"v1/up/ad","timestamp":1619687373640,"tenantId":"2000034792","serviceId":"","protocol":"lwm2m","productId":"15044315","payload":{"APPdata":"MTIzNDU2NzgsNTM0OCwwNywwOCwyNTUsMTc5LDEyOCwwMDAwLDQ1MDQsMSw2LDAsMCwwLDAsMCwwLDI0LDMuNzAsMy43MCwxLDEsNiwsMCwwLDAsMCwwLDAsMCwwLDAsMCwtMjU1LC0yNTUsLTI1NSwxLjY5LDEuOCw0OTk4LDYwLDE0NDAsNjA="},"messageType":"dataReport","deviceType":"","deviceId":"523ede8bb7e34dd4a1bd74028d63749e","assocAssetId":"","IMSI":"undefined","IMEI":"864162041961023"}

can anyone help me which part i get wrong?

Thanks

Man Man Yu
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  • The IV can not be null in AES CBC mode. If a tool / library allows you to not specify an IV in CBC mode, it uses a default value. – Ebbe M. Pedersen Apr 29 '21 at 08:47
  • thanks i saw it. CBC mode is highly recommended and it requires IV to make each message unique. If no IV is entered then default will be used here for CBC mode and that defaults to a zero based byte[16]. – Man Man Yu Apr 29 '21 at 09:00

1 Answers1

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Currently the code is not using a "null-IV", which means an array filled with bytes set to zero. Instead it is using an array filled with '0' characters, which have value 0x30 in hexadecimals or 48 in decimals; distinctly not zero.

To create a null-IV please have a look here on how to indicate byte values within byte arrays in Python. The same trick is used within the add_to_16 loop, where the \0 escape is used to indicate a zero byte.

Note that padding a key or IV is very bad practice. Those need to consist of randomized bytes. Beware that working crypto code is not the same thing as secure crypto code, which should probably be your goal.

Maarten Bodewes
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