Ciphersaber

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This is in response to a correspondant of mine whose mail I managed to lose inbetween inconsistent e-mail filtering passes, asking after the useful General Frenetics page that I once linked to. That page, giving a simple "how to" has vanished into Google's cache, aand this page is a sort-of replacement. It is most definitely not meant as a substitute for the Ciphersaber FAQ.

I'll assume that if you're reading this, you have some idea of what encryption is, and why it is useful to everyone (unless you're the sort of person with a webcam running in your bathroom).

Description of Ciphersaber version 1

Ciphersaber (v1) is a simple program based on turning a shared secret pass phrase - longer and harder to guess than a password - into a key for the ARCFOUR algorithm. ARCFOUR is the name used for the reverse engineered algorithm that appears to interoperate with the RSA Security algorithm called RC4.

ARCFOUR is one of the simplest forms of strong (hard to decrypt without having the key) symmetric (same key used to encrypt and decrypt) cipher. The fact that you have to use the same key at both ends means that you have to find some means to agree a key with your correspondent, unlike the well known PGP (for which you can give away the encryption key, without permitting decryption of messages sent to you).

In the implementation we shall be dealing with three chunks of information:

• the message text, which we are going to encrypt (assumed to be a file, which may contain either text or binary data)
• the passphrase, entered by the user, needed to encrypt and decrypt the message
• the initialization vector (or more strictly speaking, the salt), which is simply a string of 10 characters, chosen at random every time a message is encrypted.

The passphrase

First, the program must allow the user to enter the passphrase. The passphrase is a line of characters - it could be made up of one or more words, including alphabetic characters, digits, spaces and punctuation marks.

At this point we get into the murky world of character sets and their representations, because ARCFOUR only understands the manipulation of byte values (i.e. numbers in the range 0-255; or alternatively from -128 to +127, if numbers that would be grater than 127 are treated as if they were 256 less, for those using Java). The simplest representation is the American ASCII standard which gives character meanings to numeric values 32 to 126, for the alphabet, both lower (a-z) and upper (A-Z) case, numbers and common punctuation. For Western European languages, the ISO-Latin-1 character set defines values for common accented characters, and covers the range up to 255. Eastern European languages give different values to the higher values. Oriental languages like Japanese of Chinese have to use more than one byte to represent a character.

Unless you can be sure that your correspondent is using the same character set as you, then it is safest to stick to ASCII for a passphrase. Note that your passphrase will still be case sensitive - the letter 'a' is a different value too the letter 'A'; so "SECRET" is not the same value as "SecreT".

The only limitation on the passphrase is that it and the salt must together be no more than 256 bytes (so 246 characters in ASCII or ISO-Latin variants, Greek and Russian, 123 characters in multi-byte languages) ; and the longer it is, the better - English text has about 1 bit of entropy per character, so that anything much less than 64 characters would be fairly easy to break. The original author of CipherSaber recommended that the length of the passphrase should not exceed 54 letters, to ensure it gets well-mixed with the ten bytes of the initialization vector.

In 'C' the byte values will just be the char[] read in; for Java you'll need to use String.toByteArray() and agree a character encoding. For other languages, you're on your own - use the Ciphersaber test messages.

The salt

Next, your program needs to generate the initialization vector or salt - this is simply a sequence of ten random bytes. Since the values will be made public, it is probably not necessary to use anything more heavy-duty than is already available as part of the language environment for pseudo-random number generation (java.util.Random, or rand(), or rnd() will do just fine for generating it in the encryption stage).

The purpose of the salt is to allow a fixed passphrase to be re-used. If part of the key were not changed every time, then having two different messages would allow the common encryption to be factored out. The salt is simply appended to the byte values extracted from passphrase to form the key. When encrypting, you write these 10 bytes as the first values in the encrypted file; when decrypting you read it as the first ten bytes of the file.

Since these values are themselves reasonably random, they are not distinguishable from the rest of the file - a third party will have to know that this is a ciphersaber encrypted file; it is not apparent from the file itself (unlike ZIP or GIF, or even PGP files, for example). This means your program will have to enable the user to tell it whether it is encrypting or decrypting a given file. One way of doing this is to have the encrypted files have a ".CS1" suffix, and associate files of this type and the decryption mode of operation.

ARCFOUR

The algorithm used is simple yet strong. Its one weakness is that some information about the key does leak out in the first few hundred bytes of the encrypted message. This is generally not of practical use, but in a system like Cipersaber v1 in which similar keys are used (just changing the salt), practical attacks have been mounted, aimed against similar encryption systems used in Wi-Fi networking. We will deal with this problem later in discussing version 2.

The algorithm works in two stages - first you set it up with the key; next you run it to generate the keystream - the actual values used to encrypt your message.

As note above, everything works on bytes. I'll assume you're usign an integer type that can represent 0-255 (for Java people, use anything but byte here, unless you know what you're doing). We shall be doing a lot of arithmetic modulo 256 - this simply means that after doing any arithmetic, if the answer is 256 or more, subtract 256, repeating until we're back in the 0-255 range. For negative numbers keep adding 256 instead. I'll also assume that array indexing works from 0 - if it works from 1, like in Fortran, simply add 1 to each array index.

The ARCFOUR state is an array of 256 byte values. The key is the array generated from the passphrase plus the salt; the key_length is the length of this array. We have two worker values, that we will call A and B.

Set-up

Fill the State array with 256 values from 0 to 255 in ascending order. The first cell of the array will acquire the value of zero, the second cell the value of one etc; the last cell will hold the value of 255.

Initialize both A and B to zero.

The state array will now undergo 256 mixing operations. Use a variable called K to hold the number of the current operation. The first time the loop runs, K will equal zero; the second time through the loop K will equal one, etc.

1. To the variable B (whose initial value was zero but will now change) add the value of the Ath element of the key and the Kth element of state. (Remember that all additions must be performed modulo 256, unless a particular section states otherwise.)
2. Swap the values of the Kth element and the B th element of state - so if state[K] was 23 and state[B] was 42 before, after state[K] will be 42, snd state[B], 23.
3. Increment the value of A by one. If A is above key_length subtract key_length from A

When that is done, reset A and B to zero.

Generating the key stream

For each byte in the data to encrypt or decrypt do this

1. increment A by one (modulo 256, as always)
2. increment B by the value of the Ath element of state
3. Swap the values of the Ath element and the B th element of state
4. Generate our magic number for this time around as the sum of the Ath element and the B th element of state (you can do this before the swap if you want)
5. The next byte of the keystream is the magic number'th element of state
6. XOR that value and the input byte, and write it to the output

If you haven't an obvious XOR operator you need to do it manually like this

1. Extract the 128's place of the state value - it's 1 if the number is > 127, otherwise zero. If it's 1, subtract 128
2. Extract the 64's place - it's 1 if the number is now over 63, else zero. Subtract 64 if the it's 1.
3. Similarly extract the 32s, 16s, 8s, 4s, 2s and 1s places.
4. Do the same for the data value.
5. Compare the values, place by place. If they are the same the output value for the place is 0, otherwise 1.
6. Build up from 0 by adding 128 if the 128's place is 1, 64 if the 64's place is 1 and so forth.

Notes

The whole point of Ciphersaber is that it's a home-build program, using an algorithm that could fit as the slogan on a coffee mug, so I'm not going to supply source - if you have to, I'm sure Google will turn up an ARCFOUR implementation in your chosen language, but that spoils the fun.

If you are new to programming, you can work up to implementing Ciphersaber in stages. Start with a program that just copies a file byte by byte; then one that adds or removes the 10 byte random header; finally perform the encryption or decryption.

Do take care with array indexing - it is so easy to be off-by-one; and since the whole point of encryption is to generate random output, it's hard to tell if you have the right random. Use Google to look for RC4 test vectors, and use the Cipehrsaber test messages.

Description of Ciphersaber version 2

CipherSaber-2 is a modification to Ciphersaber-1 that addresses concerns about weaknesses in RC4. In CipherSaber-2 the entire state array mixing loop is repeated N times, where N is a number that the sender and receiver agree upon. When N=1, CipherSaber-2 is the same as CipherSaber-1. Following the attack, using CipherSaber-2 in place of CipherSaber-1, with a value of N=20 or more is recommended.You should first implement CipherSaber-1 and verify that it works using the text vectors on the main CipherSaber page. It is then a simple matter to modify your program to do CipherSaber-2, and use the test message in the Ciphersaber FAQ to verify the implementation.

In our description above, simply run the loop of 256 mixing operations through N times, without resetting A and B until all N operations are done.

The value of N must be agreed by sender and receiver - it is not written to the file - otherwise the N=1 state would not be the same as Ciphersaber-1.

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