53 lines
2.8 KiB
TeX
53 lines
2.8 KiB
TeX
\section{Generic Digest Interface}
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All hash algorithms in \texttt{libft\_ssl} are exposed through a single,
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uniform interface built around the \texttt{struct digest\_algo} type. This
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structure holds the algorithm's metadata (its name, digest size and block size)
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along with three function pointers: \texttt{init}, \texttt{update} and
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\texttt{final}. This design allows any algorithm to be driven through the same
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calling convention without the caller needing to know which one is in use. The
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associated context is held in a \texttt{union digest\_ctx}, which overlays the
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per-algorithm state structures so that a single allocation covers all supported
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algorithms.
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\vspace{1em}
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\texttt{struct digest\_algo} describes a hash algorithm as a set of metadata
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and three function pointers. The \texttt{name} field identifies the algorithm.
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The \texttt{digest\_size} and \texttt{block\_size} fields express its output
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length and internal block size in bytes. The three function pointers
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\texttt{init}, \texttt{update} and \texttt{final} define the algorithm's
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lifecycle: \texttt{init} sets the context to its initial state, \texttt{update}
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feeds an arbitrary amount of data into it, and \texttt{final} produces the
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digest and resets the context. All three operate on a \texttt{void~*} context
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pointer, which allows the interface to remain algorithm-agnostic.
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\vspace{1em}
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The \texttt{union digest\_ctx} type provides a single allocation large enough
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to hold the context of any supported algorithm. Because only one algorithm is
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active at a time, overlaying the per-algorithm structures in a union avoids the
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overhead of a separate heap allocation while keeping the calling code uniform.
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The active member is always the one matching the \texttt{struct digest\_algo}
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being used.
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\vspace{1em}
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Each supported algorithm is registered in \texttt{digest\_algos.h} through an
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X-macro list. This file defines a single macro \texttt{DIGEST\_ALGOS(X)} that
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expands \texttt{X} once per algorithm, passing its name, digest size and block
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size. Consuming this list with a different definition of \texttt{X} generates
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the corresponding code or data without repetition --- the global \texttt{struct
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digest\_algo} instances in \texttt{libft\_ssl.c} are produced this way. Adding
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a new algorithm to the library reduces to adding one line to this list.
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\vspace{1em}
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All three algorithms follow the Merkle-Damgård construction. The message is
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split into fixed-size blocks and processed sequentially. After each block, the
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compressed output is combined with the previous state to produce the new state
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--- this chaining ensures that the final digest depends on every bit of the
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input. The exact combination operation is algorithm-specific: MD5 and SHA-256
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use an additive feedforward, while Whirlpool uses the Miyaguchi-Preneel scheme.
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\newpage
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