# Tag Info

11

There are some serious problems with this design that would preclude it from being standardized, so it probably does not have a name. The 2 visibly main flaws are as follows: If the plaintext follows a pattern similar to the block counter, the block cipher inputs may repeat, exposing information about the plaintext (exact same issue as reuse of nonce, but ...

9

Short answer: There would be nothing (that isn't already wrong with TLS) necessarily wrong with a CTR + HMAC cipher suite, but the technical merits are only one factor in a technical feature getting to RFC status in the TLS working group. Without being discourteous to the TLS Working Group (WG) participants or process, other reasons can be: political ...

7

I would like to ask if that is true for every AES CTR mode implementation?, Doesn't have to be. You can store the nonce anywhere. You could even send it to the recipient via a different channel (e.g., email the ciphertext and use SMS to transmit the nonce). Storing it at the beginning has its advantages. For example, if streaming the data, you can ...

6

First, the obvious advice is not to use this in practice. Rolling your own is fine for learning, but you should use standard primitives when you need actual security. E.g. one from SP 800-90A which poncho linked in comments. Now, some observations. I haven't read all your code, so I may misunderstand things. Is this a good way to whiten the data? Is ...

6

From the diagram on CTR mode you can notice that there are no dependencies between any of the phases of the pipeline. If you have more than one block-size worth of data, you can process each block-size chunk completely independently of the others by calculating $\mathrm{ciphertext}_i = E(\mathrm{key}, \mathrm{nonce} \, || \, \mathrm{counter}_i) \oplus ... 6 There are probably quite a few good reasons for this, although I don't expect that a scientific answer can be composed (as you would need to use a survey, and I've never heard of such a thing for modes of operation). Let me list a few possible reasons: Developers don't know about CTR mode of operation; most questions on StackOverflow are about ECB and CBC ... 5 Yes, there are secure alternatives to support random-access based encryption. I did not come up with a way to break the proposed combination. Still, instead of inventing a new mode, I would recommend to take consider existing modes for this kind of operation, such as XTS mode. The existing modes are more studied, and (in some ways) more efficient. XTS mode ... 5 The reference for this is NIST SP800-38A, especially its appendix B. Basically we consider the IV a binary value of the width of the block cipher (64-bit for DES, 128-bit for AES), and add 1 to that, except for one detail: there is no carry at some application-specified rank, defining the maximum number of blocks that can be enciphered with a single IV; if ... 5 If you want strict indistinguishability, then yes, you need to store the IV (initial counter) somewhere. However, there are some relaxed modes that are used in practice for things like disk encryption, where it is often very useful to decrypt things "in the middle" like you say. For instance, XEX uses a counter which is derived from the sector and offset ... 4 Every paragraph ends with : "this operation is invertible", I suppose the whole salsa20 algorithm is. The Salsa20 quarterround, and thus rowround, columnround and doubleround are invertible. However, the whole Salsa20 core is not because the initial state is added to the state after iterating the rounds (cf. page 6 in the spec). If I use salsa20 ... 4 The sum of PRPs is a secure PRF. That paper gives as a security bound for a sum of two independent PRPs$q^3/2^{2n-1}$, where$q$is the number of queries and$n$the block size (i.e. 128 for AES). That means that your construction, correctly used, is more secure than a single PRP, for which the bound is$q^2/2^n$. If you wanted to give an adversary an ... 4 No, this is safe. In fact, if you show a way of distinguish the stream$AES_{k_1}(C) \oplus AES_{k_2}(C)$from a random stream with fewer than$2^{64}$outputs, you have just demonstrated a way of distinguishing AES from a random permutation. Here is how this works: suppose we are given Oracle assess to a permutation$P$, which might be$AES_{k_1}$for ... 4 Like the other answers say, it does not always have to be the case. One other case where it is often not stored is when you have a single use key, for example as part of some hybrid encryption scheme. Then there is no need to use a nonce at all and it is usually taken to have zero value. 3 Generally, it depends on the architecture. If you have$n$processors available, the obvious way to parallelize CTR mode encryption is to distribute each chunk of$n$consecutive blocks among the processors, so that processor$0 \le i < n$computes: $$C_j = E_K(c_j) \oplus P_j, \quad j = i + kn, k = 0,1,2,\dotsc$$ where$c_j$is the$j$-th counter ... 3 You can get up to around$2^{64}$random bits from counter mode (before you hit the birthday bound due to the lack of a collision), simply by running it as is. If you've got a full implementation of counter mode, the plaintext can be anything you like, because the stream (ie the output of the$E_k(ctr)$calls), should appear uniformly sampled from$2^{128}$. ... 3 Let$2^m$be the average message length in blocks. When using an independent random nonce for the whole 128-bit IV of each block, you would expect a collision after$2^{64}$blocks, i.e.$2^{64-m}$messages. (But you double the data size.) When using a 96-bit nonce and a 32-bit counter, you would expect a nonce collision after$2^{48}$messages. This is ... 3 How does Salsa20 work? The basic building block of salsa20 is a fixed 512 bit permutation. This is similar to a block cipher with a fixed and publicly know key (or a zero bit key if you prefer). Since it has no key input, you can't use it with block cipher modes of operation. The next step in Salsa20 is a feed-forward by adding the input into the output, ... 3 AES-CTR mode can be used to generate stream of random numbers. For generating random numbers, the plaintext is indeed irrelevant. It can be even full of zero (the NIST recommended way to generate random numbers uses such plaintext.) NIST has recommendation on how to generate DRBG (deterministic random bit generator) based on CTR mode. NIST has defined how ... 3 In CTR, you can use any operation which has a full cycle through the space of the IV with the counter. You could use the plus operator like the example:$69dda8455c7dd4254bf353b773304eec + 1 = 69dda8455c7dd4254bf353b773304eed$To calculate the next value, just again add 1. You could also use a increasing counter and xor it with the original IV: ... 3 A message encrypted with AES-GCM can be decrypted with an AES-CTR library IF the authentication tag is stripped from the message. If you are encrypting with AES-GCM and then adding an HMAC tag, you need to strip the HMAC and the GTAG off the message in order to decrypt it, assuming the IV section of the message is in the correct location for each library to ... 2 Your combined mode of operation is not as easy to attack as a two-times-pad (i.e. stream-cipher with fixed IV used twice), but it still has some weaknesses. For example, an attacker which did read your file before and after the change can easily find out which 128-bit-blocks of the file did change and which ones stayed the same. Depending on the file format ... 2 I would advise a different solution. You either generate a master-key (or key set) or derive one from a user password (e.g. via PBKDF2 or SCrypt). For each file to encrypt you generate a random key (file key) and nonce ad-hoc, and encrypt the file with that key, using an AEAD scheme. The random file key is encrypted with you master key and put at the ... 2 It probably doesn't. Yes. For strings$x,m0,m1$such that$\text {length}(x) = \text{length}(nonce)$and$\text{length}(m0) = \text{length}(i) = \text{length}(m1)( nonce || i ) \oplus ( x || m0 ) = ( nonce || j ) \oplus ( x || m1 ) \iff m0 \oplus m1 = i \oplus j$. I'm not aware of any. 2 This will not be secure in general, and is not recommended. The reason is that a PRG is guaranteed to produce a pseudorandom output only if its input is uniformly random (or pseudorandom). Moreover, several PRG outputs are jointly pseudorandom only if the PRG is run on independent seeds. But here you are invoking the PRG on "structured" inputs, i.e., the ... 2 Like said in the comments, a 256-bit message is two blocks of AES, no matter the keysize. The main issue with ECB mode (i.e. using AES directly on 128-bit blocks) is that you leak whether two blocks are equal. When encrypting perfectly random data, that means there's a$2^{-128}\$ chance the two parts of the key are equal, and the attacker knows that. The ...

2

Any plaintext will do. If you choose 0x000..., then you can skip the XOR step entirely. One extra step you can take to improve security is to periodically use the stream to choose a new AES key. This provides forward security: if an attacker manages to compromise the system (say, using a heartbleed-type exploit) and learn the current key, he will not be ...

2

The CTR part of CCM is basically the last for loop in the _ctrMode function: for (i=0; i<l; i+=4) { ctr[3]++; enc = prf.encrypt(ctr); data[i] ^= enc[0]; data[i+1] ^= enc[1]; data[i+2] ^= enc[2]; data[i+3] ^= enc[3]; } i.e. CTR is simply: encrypt a counter block with a block cipher, xor the encrypted block into the data, ...

2

There is no real advantage, other than the fact that it allows you to convert a block cipher into a stream cipher securely. Since there has been a large amount of research put into block ciphers and ciphers such as AES are commonly implemented in hardware (such as AES-NI), it allows for reuse of the primitives. Side note: the nonce generally does not need ...

2

The once part inside of the nonce in CTR mode means effectively "once for this particular key". If you use a fresh key for each message (e.g. by encrypting it using public-key crypto or similar), you can use the same nonce for all the messages (or a size-zero nonce). The important part is that the combination of nonce and ctr-value (i.e. what is input into ...

2

I understand the system as follows: data blocks are enciphered per AES-CTR, using key encryption_key, with an IV made by concatenating device_id and a counter held in Flash or EEPROM, incremented at each use; that enciphered data is integrity-protected by a 256-bit mac_tag computed using HMAC-SHA256 and mac_key. That's theoretically sound if device_id ...

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