Issues talking between Arduino SHA1-HMAC and base64 encoding and Python

Question

So, what I'm after is a base64-encoded, SHA1 HMAC. I've found the Adafruit Cryptosuite fork which is a stripped-down SHA1 only fork of Cathedrow's library. I want to be able to chat between my Arduino and a Python app, using base64 encoded SHA1-HMAC signatures. However, it doesn't seem to generate consistent results:

On the Arduino, the following should generate the SHA1-HMAC for the string 'testing':

  signingKey[] = "testKey";

  Sha1.initHmac_P((uint8_t *)signingKey, sizeof(signingKey) - 1);
  Sha1.print("testing");

I now want to base64 encode this, which Adafruit's Tweet Example appears to have a tidy function for doing (I realise this expects only a SHA1-HMAC, nothing else):

  // base64-encode SHA-1 hash output.  This is NOT a general-purpose base64
  // encoder!  It's stripped down for the fixed-length hash -- always 20
  // bytes input, always 27 chars output + '='.
  for(in = Sha1.resultHmac(), out=0; ; in += 3) { // octets to sextets
    b64[out++] =   in[0] >> 2;
    b64[out++] = ((in[0] & 0x03) << 4) | (in[1] >> 4);
    if(out >= 26) break;
    b64[out++] = ((in[1] & 0x0f) << 2) | (in[2] >> 6);
    b64[out++] =   in[2] & 0x3f;
  }
  b64[out] = (in[1] & 0x0f) << 2;
  // Remap sextets to base64 ASCII chars
  for(i=0; i<=out; i++) b64[i] = pgm_read_byte(&b64chars[b64[i]]);
  b64[i++] = '=';
  b64[i++] = 0;

Outputs:

qn9vJmo4Q6KhPgbn5nVOSOUCU5Q=

But, all is not well when I look at this from the python side:

hm = hmac.new('testKey', 'testing', hashlib.sha1)
print binascii.b2a_base64(hm.digest())[:-1]

Outputs:

YNQScdQ7h1t5Hi1Uw0vz8Biil2M=

Clearly these two are different. Unfortunately I haven't used Arduino/C++ much and am more familiar with the Python side. Is there an obvious mistake in what I'm trying to do, or are the libraries I'm using inadequate for purpose?

Thanks!

In Crptosuite, it is sha1.initHmac and not sha1.initHmac_P. What's the difference? — Albert
– Albert, Commented Oct 18, 2013 at 9:39
Gah - great shout. That fixes the error. I'd just taken the code from the Adafruit example, specifically here, I've gone back to the original, and I'm not sure what the difference is, but correcting it gives the expected output. — jvc26
– jvc26, Commented Oct 18, 2013 at 9:48

Albert · Accepted Answer · 2013-10-18 09:49:50Z

2

Putting the answer here: The correct function is Sha1.initHmac, not Sha1.initHmac_P.

answered Oct 18, 2013 at 9:49

Albert

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