Windows crypto API - c

Question

I'm having trouble with a function I made to create a sha1 hash of some strings. The function generates a valid hash for strings longer than 4 chars, but for chars less than that the hash is wrong. I've been looking all over for a solution but I can't see whats wrong.
int create_hash(char *plaintext, char *digest) {
char digest_buff[3];
DWORD buf_size = SHA1_HASH_LEN;
HCRYPTPROV prov;
HCRYPTHASH hash;
BYTE rgbHash[SHA1_HASH_LEN];
CHAR rgbDigits[] = "0123456789abcdef";
int i;
if(!CryptAcquireContext(&prov, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT)) {
if(DEBUG) {
printf("CryptAcquireContext failed: %d\n", GetLastError());
}
return SHA1_FAILED;
}
if(!CryptCreateHash(prov, CALG_SHA, NULL, NULL, &hash)) {
if(DEBUG) {
printf("CryptCreateHash failed: %d\n", GetLastError());
}
return SHA1_FAILED;
}
if(!CryptHashData(hash, (BYTE *)plaintext, sizeof(hash), NULL)) {
if(DEBUG) {
printf("CryptHashData failed: %d\n", GetLastError());
}
return SHA1_FAILED;
}
if(!CryptGetHashParam(hash, HP_HASHVAL, rgbHash, &buf_size, NULL)) {
if(DEBUG) {
printf("CryptGetHashParam failed: %d\n", GetLastError());
}
return SHA1_FAILED;
}
for(i=0; i < buf_size; i++) {
sprintf(digest_buff, "%c%c", rgbDigits[rgbHash[i] >> 4], rgbDigits[rgbHash[i] & 0xf]);
strcat(digest, digest_buff);
}
if(!CryptDestroyHash(hash)) {
if(DEBUG) {
printf("CryptDestroyHash failed: %d\n", GetLastError());
}
return SHA1_FAILED;
}
if(!CryptReleaseContext(prov, NULL)) {
if(DEBUG) {
printf("CryptReleaseContext failed: %d\n", GetLastError());
}
return SHA1_FAILED;
}
if(DEBUG) {
printf("digest: %s\n", digest);
}
return 0;
}

Answer 1

CryptHashData accepts the length of the data as third parameter, and you pass size of the hash buffer there. You need to pass smth like strlen(plaintext) there.