Problem - Kevin Tafuro

Libraries with cryptographic hash functions tend to support incremental operation
using a standard structure. In fact, this structure is standardized for cryptographic
hardware APIs in PKCS (Public Key Cryptography Standard) #11. There are four
steps:
1. Allocate a context object. The context object holds the internal state of the hash
until data processing is complete. The type can be specific to the hash function,
or it can be a single type that works for all hash functions in a library (such as
the EVP_MD_CTX type in the OpenSSLlibrary or HCRYPTHASH in Microsoft’s CryptoAPI).
2. Initialize the context object, resetting internal parameters of the hash function.
Generally, this function takes no arguments other than a pointer to the context
object, unless you’re using a generic API, in which case you will need to specify
which hash algorithm to use.
3. “Update” the context object by passing in data to be hashed and the associated
length of that input. The results of the hash will be dependent on the order of
the data you pass, but you can pass in all the partial data you wish. That is, calling
the update routine with the string “he” then “llo” would produce the same
results as calling it once with the string “hello”. The update function generally
takes the context object, the data to process, and the associated length of that
data as arguments.
4. “Finalize” the context object and produce the message digest. Most APIs take as
arguments the context object and a buffer into which the message digest is
placed.
The OpenSSLAPI has both a single generic interface to all its hash functions and a
separate API for each hash function. Here’s an example using the SHA1 API:
#include
#include
#include
int main(int argc, char *argv[ ]) {
int i;
SHA_CTX ctx;
unsigned char result[SHA_DIGEST_LENGTH]; /* SHA1 has a 20-byte digest. */
unsigned char *s1 = "Testing";
unsigned char *s2 = "...1...2...3...";
SHA1_Init(&ctx);
SHA1_Update(&ctx, s1, strlen(s1));
SHA1_Update(&ctx, s2, strlen(s2));
/* Yes, the context object is last. */
SHA1_Final(result, &ctx);
printf("SHA1(\"%s%s\") = ", s1, s2);
for (i = 0; i < SHA_DIGEST_LENGTH; i++) printf("%02x", result[i]);
printf("\n");
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