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I have two devices which communicate via sms.

This communication should now be encrypted.

The devices can only send messages with the lenght of 160 characters.

So i am looking for an encryption system which can limit the length of the encypted message to 160.

Is there such a cryptography system? Which system that meets this requirement is the safest?

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  • $\begingroup$ Does the list of allowed characters have an even number of elements? $\;$ $\endgroup$
    – user991
    Commented Oct 16, 2014 at 6:50
  • $\begingroup$ I passed the question to my provider... $\endgroup$
    – Felix Schmidt
    Commented Oct 16, 2014 at 7:12
  • $\begingroup$ That only affects how easy/hard it will be to do format-preserving encryption $\hspace{1.74 in}$ on the space of 160-character strings. $\;$ $\endgroup$
    – user991
    Commented Oct 16, 2014 at 7:19

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As encryption scheme that could only encrypt fix length message is a important essence in cryptography, e.g. one-time pad. Block cipher is a better example, e.g. AES is a popular block cipher whose block size is 128 bits. While given such block cipher who could only encrypt 128-bit string, you can construct encryption scheme that encrypt arbitrary length string.

The security of the encryption scheme is not determined by the length of message but by the length of the key. For example, AES has variant using 128-bit, 192-bit, 256-bit key. AES-256, who use 256-bit key, is considered to be safer.

Finally, for ultimate security, you should use one-time pad. One-time pad is information-theoretically secure which means the most powerful adversary can not break the cipher. The disadvantage is the key could be only used once. You could store multiple keys in your devices. 10MB space could store more than 60000 pre-shared keys, which sustain 60000 secure SMS message.

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  • $\begingroup$ Sorry, but a one time pad is basically a theoretical construct. Using it for a practical application such as SMS encryption is not a good idea. Using a stream cipher could work, but you would make sure that both sides stay in sync somehow. $\endgroup$
    – Maarten Bodewes ♦
    Commented Oct 19, 2014 at 16:52
  • $\begingroup$ @owlstead One-time pad is not just a theoretical construct, e.g. most stream cipher can be considered as a combination of a pseudorandom generator and one-time pad. $\endgroup$
    – Tianren Liu
    Commented Oct 19, 2014 at 17:17
  • $\begingroup$ @owlstead A block cipher is different from a stream cipher. Block ciphers are based on pseudorandom permutation while stream ciphers are based on pseudorandom generator. In particular, you should view AES as a substitution-permutation network that implement a pseudorandom permutation. A block cipher does not have sync problem as a stream cipher. $\endgroup$
    – Tianren Liu
    Commented Oct 19, 2014 at 17:28
  • $\begingroup$ Thank you for telling me all that, Tianren Lu, I never would have guessed. $\endgroup$
    – Maarten Bodewes ♦
    Commented Oct 20, 2014 at 0:54

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