Generate keys from passwords using PBKDF2-HMAC-SHA256

pbkdf2_hmac_sha256.m

#import <CommonCrypto/CommonCrypto.h>
// ...

// http://www.opensource.apple.com/source/OpenSSL/OpenSSL-12/openssl/crypto/evp/p5_crpt2.c
// This is the method specified by RSA's PKCS #5 standard.
// Compatible to https://github.com/bitwiseshiftleft/sjcl/blob/master/core/pbkdf2.js
NSData * PBKDF2_HMAC_SHA256(NSData *data,NSData *salt, int iter, int keylen) {
    
    unsigned char digtmp[CC_SHA256_DIGEST_LENGTH], *p, *buffer, itmp[4];
    NSInteger cplen, j, k, tkeylen;
    unsigned long i = 1;
    CCHmacContext hctx;
    tkeylen = keylen;
    
    buffer = calloc(keylen, sizeof(unsigned char));
    p = buffer;
    
    while(tkeylen) {
        if(tkeylen > CC_SHA256_DIGEST_LENGTH) cplen = CC_SHA256_DIGEST_LENGTH;
        else cplen = tkeylen;
        
        /* We are unlikely to ever use more than 256 blocks (5120 bits!)
         * but just in case...
         */
        itmp[0] = (unsigned char)((i >> 24) & 0xff);
        itmp[1] = (unsigned char)((i >> 16) & 0xff);
        itmp[2] = (unsigned char)((i >> 8) & 0xff);
        itmp[3] = (unsigned char)(i & 0xff);
        
        CCHmacInit(&hctx, kCCHmacAlgSHA256, data.bytes, data.length);
        CCHmacUpdate(&hctx, salt.bytes, salt.length);
        CCHmacUpdate(&hctx, itmp, 4);
        CCHmacFinal(&hctx, digtmp);
        memcpy(p, digtmp, cplen);
        
        for(j = 1; j < iter; j++) {
            CCHmac(kCCHmacAlgSHA256, data.bytes, data.length,
                   digtmp, CC_SHA256_DIGEST_LENGTH, digtmp);
            for(k = 0; k < cplen; k++) p[k] ^= digtmp[k];
        }
        
        tkeylen-= cplen;
        i++;
        p+= cplen;
    }
    
    return [NSData dataWithBytesNoCopy:buffer length:keylen];
}