72 lines
2.9 KiB
Plaintext
72 lines
2.9 KiB
Plaintext
====================================
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= Usage of streamciphers =
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====================================
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Author: Daniel Otte
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email: bg@nerilex.org
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0. Foreword
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This file will describe how to use the streamcipher implementations provided
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by this library. It will not only show how to call the cryptographic functions
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but also discuss a little how to build security mechanisms from that.
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1. What a streamcipher does
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A streamcipher normally generates a deterministic, random looking stream of
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bits, known as keystream. For encryption purpose this keystream is XORed with
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the data stream. So decryption is exactly the same as encryption. The
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data-stream is XORed with the keystream giving the plaintext. So both sides
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need exactly the same streamcipher in the same state.
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1.1. high frequent parameters:
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output-size: 8 bit, 1 bit
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keysize: 64 bit, 80 bit, 128 bit
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IVsize: 64 bit
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2. Parts of a streamcipher
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* generation algorithm
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* initialization algorithm
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* state
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As we can see all streamciphers seem to utilize an internal state which
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determines the output. This state is initialized by the initialization
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algorithm with a key and an IV (initialization vector). It is very important
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for security that _never_ the same key with the same IV is used again. The
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IV is not required to be kept secret.
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3. streamcipher API
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The API is not always consistent due to the fact that we tried to optimize the
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code for size (flash, heap and stack) and speed (runtime of the different
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components).
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Generally the API of the implemented streamciphers consists of:
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*_init function, which implements the initialization
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*_gen function, which implements the streamcipher algorithm and generates a
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keystream output
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*_ctx_t context type, which contains internal state information
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3.1 look at the prototypes
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Generally the prototypes (defined in the *.h files) will tell you what
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parameter means what.
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3.1.2 sizes in bits and bytes
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Working with cryptographical functions involves working with different lengths.
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Some times you want to know it in bits and sometimes in bytes. To reduce
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frustration and to avoid bugs we suffix a length parameter with either _b or
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_B depending on the meaning. _b means in bits and _B means in bytes
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(big b big word).
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3.2. *_init function
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The *_init function generally takes a pointer to the key as first parameter.
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For ciphers where the keysize is not fixed the second parameter gives the
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keysize (in bits regularly) followed by a pointer to the IV and a length
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parameter for not fixed IV sizes (both are omitted if the algorithm does not
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specify IV handling, in this case a part of the key should be used as IV).
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The last parameter points to the context variable to fill.
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3.3. *_gen function
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The *_gen function updates the internal state to which a pointer is given as
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parameter and returns a fixed length part of the keystream as return value.
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