Problem - Kevin Tafuro

Figure 6-1. The Davies-Meyer construct
Traditionally, hash functions pad by appending a bit with a value of 1, then however
many zeros are necessary to align to the next block of input. Input is typically
strengthened by adding a block of data to the end that encodes the message length.
Nonetheless, such strengthening does not protect against length-extension attacks.
(To prevent against those, see Recipe 6.7.)
Matyas-Meyer-Oseas is a similar construction that is preferable in that the plaintext
itself is not used as the key to a block cipher (this could make related-key attacks on
Davies-Meyer easier); we’ll present that as a component when we show how to
implement MDC-2 in Recipe 6.16.
Here is an example API for using Davies-Meyer wihtout a nonce:
void spc_dm_init(SPC_DM_CTX *c);
void spc_dm_update(SPC_DM_CTX *c, unsigned char *msg, size_t len);
void spc_dm_final(SPC_DM_CTX *c, unsigned char out[SPC_BLOCK_SZ]);
The following is an implementation using AES-128. This code requires linking
against an AES implementation, and it also requires that the macros developed in
Recipe 5.5 be defined (they bridge the API of your AES implementation with this
book’s API).
#include
#include
#ifndef WIN32
#include
#include
#include
#else
#include
#include
#endif
#define SPC_KEY_SZ 16
P i
typedef struct {
unsigned char h[SPC_BLOCK_SZ];
292 | Chapter 6: Hashes and Message Authentication
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