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I have a key for AES encryption and I'm trying to encrypt strings with constant length as well, does the resultant encrypted string will always have the same length?

kelalaka
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htopalov
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  • Does this answer your question? [Size of data after AES/CBC and AES/ECB encryption](https://stackoverflow.com/questions/3283787/size-of-data-after-aes-cbc-and-aes-ecb-encryption) – Martheen Oct 27 '21 at 09:14

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That depends on what you mean by 'the same length'.

The same length as the original string: generally no; the original string will be padded to a multiple of the cipher block-length. Check padding modes for details.

The same length every time you encrypt: yes; as long as you stick to the same mode and padding the encrypted output will have the same length.

clausc
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  • By length I ment the count of characters of string after encryption. Provider key and text to encrypt are with constant number of characters, mode is the same as well. – htopalov Oct 27 '21 at 09:40
  • Then you should expect the encrypted values to have the size of the original strings rounded up to the nearest multiple of 8. Small caveat: encryption works with BYTES - not characters; so you should expect the number of BYTES to be the nearest multiple of 8. If you use multi-byte characters be aware of this. – clausc Oct 27 '21 at 10:53
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    *...nearest multiple of 8...* It's AES, so nearest multiple of **16**. – President James K. Polk Oct 27 '21 at 11:55
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First of all, a block cipher requires a full block to encrypt, therefore AES requires 16-byte ( or 128-bit ) input to encrypt will output 16-byte ciphertext. This is always the case for block ciphers since they are Pseudo-Random Permutations (PRP) - they are always permutations we expect them to be PRP.

Block ciphers, like AES, are primitives and must be used with a proper mode of operation. Here we will talk only about some of them.

  • ECB mode is the default and insecure.

    Padding: In the case of the message size is not a multiple of the 128, we apply padding. There are various paddings, however, the most common one is the PCKS#7 ( update of PCKS#5 that was for 64-bit block ciphers like DES). This padding append characters to the end so that the size is multiple of 128.

    Therefore; the appended number of characters for ECB mode can be from 1 to 16 ( if the block size is already a multiple of 128) then a new block is added with 16 10 bytes.

  • CBC mode requires a random and unpredictable IV so that we can achieve probabilistic encryption. As in ECB, it requires padding.

    Therefore; the appended number of characters for CBC mode can be from 1+16 to 32 since we need to add the IV to the ciphertext.

  • CTR mode requires Initial Value (IV) that can be random or deterministic. CTR mode converts a block cipher (PRP) into a stream cipher. The plaintext is x-ored with the ciphertext where the input was the IV and incremented for each encryption. There is no need for padding in CTR mode.

    Therefore; the appended number of characters for CTR mode is (at most)16.

  • AAED: The above are archaic mode of operations and today we use Authenticated Encryption (with Associated Data) (AE/AEAD). Unlike the archaic modes, these modes can provide us not only confidentiality but also integrity and authentication.

    The authentication part requires a tag (MAC tag) and this really depends on the scheme.

    • AES-GCM is the NIST-approved mode. AES-GCM's recommended IV size 12 since different size requires an additional process. It always produces a 16-byte tag, however, one can reduce the size of the tag if they want to, although not recommended.

      At first one might consider that the size is incremented by 12-16, however, it is not. The GCM always used Associated Data (AD) and allows zeroAD and the size of the AD is always added. For details, see NIST GCM specification

This was the short story of the long list of how the mode of operations behaves in terms of input vs output length.

If you omit the IV size, it is possible with CTR mode.

kelalaka
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