RSA_verify() causes segmentation fault - c

Question

Teh function below causes a segmentation fault when it gets to the RSA_verify() part. I'm a c-beginner so it's hard for me to find the reason for the problem. Maybe someone can point out what I'm doing wrong. It would be very helpful, thanks in advance.
Here's the Code:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <openssl/sha.h>
#include <openssl/dsa.h>
#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/pem.h>
#include <openssl/rsa.h>
#include <openssl/bio.h>
#define BUF 20000
#define DSS 20
int matchCipherToSign(unsigned char *signPath, char *cipherPath) {
unsigned char *bufCipher = malloc(BUF);
unsigned char *sha = malloc(SHA_DIGEST_LENGTH);
unsigned char *bufSign = malloc(BUF);
unsigned int retSign;
int ret=0;
int retCipher;
FILE *key;
RSA *rsa = RSA_new();
EVP_MD_CTX ctx;
key = fopen("key.bin", "rb");
if (key == NULL){
printf("Couldn't open file key.bin.\n");
exit(EXIT_FAILURE);
}
retSign = readFile(signPath, bufSign);
if (retSign == 0){
printf("Couldn't read file %s.\n", signPath);
exit(EXIT_FAILURE);
}
retCipher = readFile(cipherPath, bufCipher);
if (retCipher == 0){
printf("Couldn't open file %s.\n", cipherPath);
exit(EXIT_FAILURE);
}
rsa = PEM_read_RSA_PUBKEY(key, &rsa, NULL, NULL);
fclose(key);
if (1!=EVP_DigestInit(&ctx, EVP_sha1())) {
printf("EVP_DigestInit Error.\n");
exit(EXIT_FAILURE);
}
if (1!=EVP_DigestUpdate(&ctx, bufCipher, retCipher)) {
printf("VP_DigestUpdate Error.\n");
exit(EXIT_FAILURE);
}
if (1!=EVP_DigestFinal(&ctx, sha, NULL)) {
printf("EVP_DigestFinal Error.\n");
exit(EXIT_FAILURE);
}
ret = RSA_verify(NID_sha1, sha, SHA_DIGEST_LENGTH, bufSign, sizeof(bufSign), rsa);
RSA_free(rsa);
free(bufCipher);
free(bufSign);
free(sha);
return ret;
}
Thanks for your help!