Solve the garbled problem of AES algorithm CBC mode encryption string after decryption

Problem

After encrypting the string with AES CBC mode, the decrypted string is garbled, usually the first dozens of bytes are garbled;

Repetition

Because the department CGI is used? Aesencryptutil Library, after finding the problem, it is not easy to reproduce here, the use of Python to reproduce, can be easily reproduced.

#!/usr/bin/env python#coding =utf-8 fromCrypto.cipherImportAes? PADDING= ' + ' if __name__ == "__main__":? ? ? Pad_it= LambdaS:s+( - - Len(s)% -)*PADDING??? Key= ' 0123456789abcdef '? ? ? Data=Pad_it (' Luffichen0123456789abcdef ') ? ? Aes_util=Aes.new (Key, AES. MODE_CBC, b' 0000000000000000 ')?? ?? Crypt=Aes_util.encrypt (data)??Print(Base64.b64decode (crypt))??Print(data)??# using the same aes_util can cause garbled characters? ? Recovery=Aes_util.decrypt (crypt)??Print(recovery)

Output Result:

Conclusion

In the CBC mode, the same AES object, all previous encryption and decryption will affect the next encryption and decryption (encryption and decryption who first the impact is the same), because CBC is Block chained, encryption is serial, it is necessary to rely on the last block of the encryption and decryption results, it can be understood as it is the process of recording the middle State, So the next encryption and decryption using the original AES object has problems; The ECB model does not have this problem; more details can be seen wiki:https://en.wikipedia.org/wiki/block_cipher_mode_of_operation

Workaround

Each decryption requires a re-generation of an AES object.

Solve the garbled problem of AES algorithm CBC mode encryption string after decryption