S/MIME email encryption library for iOS - ios

Question

I am trying to find a S/MIME email encryption library for an iOS email app I am creating in Swift. I have been having trouble trying to find a library for the encryption, has anyone had any experience with this?
I have tried OpenSSL but have run into issues with importing all the files in need in the bridging header, for example I need to use functions in pem.h but if I try import pem.h the bridging header fails to be imported altogether.
Any help with this would be greatly appreciated.

Answer 1

I had a similar requirement. Eventually I had to import openSSL and write my own code to handle the decrypt of PKCS7. I made a small github repo which should help
https://github.com/zkrige/iOS-pkcs7-decrypt
here is the gist of the code
#include <openssl/bio.h>
#include <openssl/cms.h>
#include <openssl/err.h>
#include <openssl/pem.h>
#include <openssl/ssl.h>
#include <openssl/crypto.h>
#include <openssl/rand.h>
X509 *getCert(const char *certificate) {
BIO *membuf = BIO_new(BIO_s_mem());
BIO_puts(membuf, certificate);
X509 *x509 = PEM_read_bio_X509(membuf, NULL, NULL, NULL);
return x509;
}
EVP_PKEY *getKey(const char *privateKey) {
BIO *membuf = BIO_new(BIO_s_mem());
BIO_puts(membuf, privateKey);
EVP_PKEY *key = PEM_read_bio_PrivateKey(membuf, NULL, 0, NULL);
return key;
}
PKCS7 *getContainer(const char *encrypted) {
BIO* membuf = BIO_new(BIO_s_mem());
BIO_set_mem_eof_return(membuf, 0);
BIO_puts(membuf, encrypted);
PKCS7* pkcs7 = SMIME_read_PKCS7(membuf, NULL);
if (!pkcs7) {
fprintf(stderr, "error: %ld\n", ERR_get_error());
}
return pkcs7;
}
char *decrypt(PKCS7 *pkcs7, EVP_PKEY *pkey, X509 *cert) {
BIO *out = BIO_new(BIO_s_mem());
if (PKCS7_decrypt(pkcs7, pkey, cert, out, 0) != 1) {
X509_free(cert);
EVP_PKEY_free(pkey);
PKCS7_free(pkcs7);
fprintf(stderr, "Error decrypting PKCS#7 object: %ld\n", ERR_get_error());
return NULL;
}
BUF_MEM* mem;
BIO_get_mem_ptr(out, &mem);
char *data = malloc(mem->length + 1);
memcpy(data, mem->data, mem->length + 1);
BIO_flush(out);
BIO_free(out);
return data;
}
char *decrypt_smime(const char *encrypted, const char *privateKey, const char *certificate) {
OpenSSL_add_all_algorithms();
ERR_load_crypto_strings();
X509 *cert = getCert(certificate);
if (!cert) {
return NULL;
}
EVP_PKEY *pkey = getKey(privateKey);
if (!pkey) {
X509_free(cert);
return NULL;
}
PKCS7 *pkcs7 = getContainer(encrypted);
if (!pkcs7) {
X509_free(cert);
EVP_PKEY_free(pkey);
return NULL;
}
char *data = decrypt(pkcs7, pkey, cert);
X509_free(cert);
EVP_PKEY_free(pkey);
PKCS7_free(pkcs7);
return data;
}