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I am trying to build a Objective-C client for KeyBase.io. Some of the String specifics are getting the best of me however.

Consider this pseudocode from the Keybase api docs:

pwh = scrypt(passphrase, hex2bin(salt), N=215, r=8, p=1, dkLen=224)[192:224]

I believe that a half correct Objective-C mapping would be (scrypt function from Tarsnap and helper category) :

uint8_t *passphrase = (uint8_t *)[@"AVeryCleverPassword1" dataUsingEncoding:NSASCIIStringEncoding].bytes;
uint8_t *salt = (uint8_t *)@"32355c2e7843513463263...".decodeFromHexidecimal.bytes;
uint8_t buffer[224];

crypto_scrypt(passphrase, 20, salt, 24, pow(2, 15), 8, 1, buffer, 224);
uint8_t *pwh = buffer + 192;

The buffer looks like this which doesn't make sense.

[0] = '\xa4'
[1] = '\xb4'
[2] = '\xca'
[3] = '\x17'
[4] = '\xcc'
[5] = '\xa6'
[6] = '\xc7'
[7] = 'n'

Going further consider this next step:

hmac_pwh = HMAC-SHA512(pwh, base64decode(login_session))//hmac_pwh is hex encoded

I believe a correct mapping looks like this with a helper category:

NSData *decodedData = [NSData dataFromBase64String:@"lgHZIDhlY2I0..."];
uint8_t *decodedCString = (uint8_t *)decodedData.bytes;
unsigned char cHMAC[CC_SHA512_DIGEST_LENGTH];

CCHmac(kCCHmacAlgSHA512, pwh, 224 - 192, decodedCString, 108, cHMAC);

the cHMAC buffer doesn't look right either: "\xca\x93 \xc2\xd7"\xfe\xd7\xc9\xa8..."

When converting the cHMAC string back into a NSString, my suspicions are confirmed as

NSData *HMAC = [[NSData alloc] initWithBytes:cHMAC length:sizeof(cHMAC)];
NSString *HMACString = [[NSString alloc] initWithData:HMAC encoding:NSASCIIStringEncoding];

Results in HMACString looking like this "Ê Â×"þ×ɨ©..."

I believe this is a encoding/decoding issue since NSString is UTF-8 encoded but I can officially say that it's above my head. Does cHMAC need to be decoded from binary into hexadecimal before being encoded as a string?

Any help, however small, would be appreciated.

Answer:

The answer is that I wasn't encoding/decoding to and from hex to binary as pointed out by Max.

To get from cHMAC to a NSString representation of HMAC you can do something like this. My interpretation:

NSString* hmac(unsigned int size, unsigned char cHMAC[size])
{

  NSMutableString *result = [NSMutableString string];
  for (int i = 0; i < size; i++)
  {
     [result appendFormat:@"%02hhx", cHMAC[i]];
  }

  return result;
}

And invoked as such:

NSString *result = hmac(CC_SHA512_DIGEST_LENGTH, cHMAC);
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Cezar
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    Do decode hex into NSData (for the salt), you need to run a little function like the one shown here: http://stackoverflow.com/questions/2338975/convert-hex-data-string-to-nsdata-in-objective-c-cocoa – Max Krohn Mar 31 '14 at 19:09
  • That looks correct, question updated! Thank you! I still do not understand why HMACString comes back all garbled after encoding. Does it need to be converted to hexidecimal from binary? – Cezar Mar 31 '14 at 21:17
  • Yes! You'd use the reverse of the code from that stackoverflow answer. – Max Krohn Mar 31 '14 at 21:24
  • Thats it! You are correct! It worked!!! – Cezar Mar 31 '14 at 21:56
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    @MaxKrohn You should post that as an answer so you can get the appropriate credit for it. :) – Itai Ferber Apr 01 '14 at 03:51

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