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aespycrypto

Why must all inputs to AES be multiples of 16?

I'm using the PyCrypto implementation of AES and I'm trying to encrypt some text (24 bytes) using a 24 byte key. 

aes_ecb = AES.new('\x00'*24, AES.MODE_ECB)
aes_ecb.encrypt("123456"*4)

I get this surprising error ValueError: Input strings must be a multiple of 16 in length
So why is it that my input must be a multiple of 16? It would make more sense to me that the input string length must be a multiple of my key size, because this would allow nice bitwise operations between the key and blocks of plaintext.

Solution

  • AES is a block cipher. Quote from the Wikipedia page: “a block cipher is a deterministic algorithm operating on fixed-length groups of bits”.

    AES can only work with blocks of 128 bits (that is, 16 chars, as you noticed).

    If your input can have lengths others than a multiple of 128, depending on your application, you may have to be extremely careful how you handle padding.