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I've been doing a lot of reading on the proper way to implement AES CBC mode with HMAC authentication. I've seen many explanations, however, I've had a hard time finding an actual real example (with code) on all steps including key derivation, encryption, and decryption. My application uses PBKDF2 for key derivation. Here is what I have so far:

Derive Keys

  1. Generate 256 bits of "key material" with KDF

    key_material = pbkdf2(user_password, salt, iterations, size:256, alg:sha256)
    
  2. Split key material in half yielding x2 128 bit keys for encryption and signing

    enc_key = key_material.split(0, 127)
    mac_key = key_material.split(128, 255)
    

Encrypt

  1. Generate 128 bit initialization vector

    iv = secureRandomBits(128)
    
  2. Do AES encrypt

    cipher = aesEncrypt(plaintext, iv, mode:cbc, key:enc_key)
    
  3. Compute MAC with concatenated IV and cipher bits

    mac = hmac(iv + cipher, alg:sha256, key:mac_key)
    
  4. Store resulting values to server (considered unsecure storage)

    storeValuesToServer(cipher, iv, mac)
    

Decrypt

  1. Fetch data from storage

    cipher, iv, mac = getValuesFromServer()
    
  2. Recompute mac

    new_mac = hmac(iv + cipher, alg:sha256, key:mac_key)
    
  3. Compare macs

    if(mac != new_mac)
      throw 'Failed MAC check!'
    
  4. Do AES decrypt

    plaintext = aesDecrypt(cipher, iv, mode:cbc, key:enc_key)
    

Questions

Does this look correct? Main concerns here are:

  • Have I derived the keys correctly, splitting them in half based on the output of the KDF?
  • Am I computing the HMAC correctly by combining the iv + cipher. Does it matter if I had done cipher + iv instead?
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You'll find there's a lot of splitting hairs regarding this topic, especially key derivation. But yes, your pseudocode is fine, although you may want to revise (0, 128) => (0, 127) and (129, 256) => (128, 255) ... (correct me if I'm wrong?). Also, you might want to implement a constant-time comparison function for verifying the mac.

Have I derived the keys correctly, splitting them in half based on the output of the KDF?

This is fine. Some people would disagree. See here here and here for more in-depth discussion.

Am I computing the HMAC correctly by combining the iv + cipher. Does it matter if I had done cipher + iv instead?

Yes, you're computing the HMAC correctly. No, the order of cipher + iv does not matter at all. If it's available to you, you might want to consider a mode like GCM, which takes care of encryption and authentication for you with a single key.

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    $\begingroup$ Why is a constant time compare needed on for the mac? $\endgroup$ – zaph Jul 13 '17 at 2:59
  • $\begingroup$ @zaph to mitigate timing-attacks $\endgroup$ – hunter Jul 13 '17 at 6:28
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    $\begingroup$ The referenced article is general and does not cover or extend to mac compare timings. In the case of using a non-constant compare function to compare a hash output the compare time taken is essentially random since the output is essentially random. The compare time is not a function of the "closeness" of the attempted input to the correct input. The compare time for an attack is however reduced but that is small compared to the mac execution time. $\endgroup$ – zaph Jul 13 '17 at 12:03
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    $\begingroup$ That article is again for another kind of attack, just comparing MACs. This attack requires a hash to be made over the encrypted data, not just a compare of MACs, the attack will not work for this encryption authentication. Do try again, I would love to see a good attack against encrypt then MAC. $\endgroup$ – zaph Jul 13 '17 at 20:30
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    $\begingroup$ I really was hoping you could explain the particular attack. I have heard this before but I have never seen it explained. I am well aware that in many instances a constant time compare is essential. I am not a crypto expert by any means but have had such a person who also does not see the need in this instance. I also feel one should be able to explain their statements. I want to understand the rational. $\endgroup$ – zaph Jul 14 '17 at 12:44
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IMO, you code looks pretty solid. A few things I might suggest taking a closer look at are:

  • You haven't specified what iteration count you're using for PBKDF2. You should make the iteration count as high as practical.

    PKCS #5 suggests a minimum of 1000 iterations, but that recommendation comes from nearly two decades ago. IMO, nowadays there's very little reason to ever use less than a million iterations, and (at least for client-side apps, where denial of service attacks are not a concern) a billion iterations is often more reasonable.

    You should also include the iteration count in the public description of your encryption scheme. Just saying that your software uses PBKDF2, without specifying the iteration count, gives very little useful information to users wishing to evaluate the security of your software.

  • While using PBKDF2 is a lot better than not using any key-stretching KDF at all, there are nowadays even better alternatives like scrypt, Argon2 or Balloon hashing. These new KDFs are designed to consume not just a lot of time but also lots of memory, making them more resistant to massively parallel brute-force password cracking using e.g. GPUs or ASICs.

  • Requesting 256 bits of key material from PBKDF2-HMAC-SHA256, and splitting it into two 128-bit keys, is a perfectly good choice if you're sure that 128 + 128 bits is all you're going to need. If there's a chance that you might some day need more than that, however (say, because you want to switch to AES-256 for post-quantum security, or because you might need to derive more than one set of keys from the same passphrase), then you might want to consider re-deriving your AES and HMAC keys from the PBKDF2 output using another (non-iterated) KDF like HKDF-Expand.

  • If your ciphertexts can be long, first concatenating the IV and the ciphertext and then passing the result to HMAC might be needlessly inefficient. Most HMAC implementations allow you to feed the input in multiple chunks, so you don't have to do any explicit string concatenation. Of course, this is just a performance issue, not a security issue.

    (Alternatively, if you're working in a low-level language like C, you might be able to allocate the memory for the IV and the ciphertext as one contiguous chunk, effectively letting you treat them as a single concatenated string without having to do any actual string copying.)

  • As noted by hunter, you should use a constant-time string comparison routine for the MAC comparison. Otherwise, an attacker who could time your decryption code precisely enough might be able to discover the correct MAC for a forged message byte-by-byte, by guessing different MACs and timing how long it takes for your code to reject them to see how many initial bytes they've guessed correctly.

    In practice, such timing attacks may or may not be actually feasible, depending on just how noisy the timing information available to the attacker is. But using a constant-time string comparison is an easy way to avoid that particular attack entirely.

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    $\begingroup$ Thanks for all the details. As a follow-up: 1) Unfortunately the limitations of platform compatibility restrain me to PBKDF2 for key derivation so I am not able to explore other options such as scrypt or Argon2. 2) Would simply moving to PBKDF2-HMAC-SHA512 allow me to split the key_material into x2 256 bit keys in much the same way? 3) I'll be changing my mac comparisons during decryption to use a constant-time comparison. What do you think about double HMAC verification described here: nccgroup.trust/us/about-us/newsroom-and-events/blog/2011/…? $\endgroup$ – izzle Apr 27 '17 at 17:04
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    $\begingroup$ Yes, switching to SHA-512 would obviously let you obtain two 256-bit keys simply by splitting the 512-bit PBKDF2 output. And yes, the double HMAC trick should be a perfectly fine way to thwart timing attacks. $\endgroup$ – Ilmari Karonen Apr 27 '17 at 18:00

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