Are Very Short Messages Ok With HMAC-SHA256
Elpidio Heart
Say I'm designing a library to sign/verify messages with SHA-256 HMAC. If the end user uses a weak shared key and sends a lot of short messages, I assume there would be risk of an attacker discovering the key.
At least, to this date[Feb. 2020], there are no known vulnerabilities relating to short messages with sufficiently sized keys. If this is all you cared about, feel free to stop reading here. The rest of the answer just go into details as to why it's not a problem.
The hashing mechanism should be "unbreakable" for a really short time of only 1 minute - the final purpose is creating a hash-chain for an OTP with a dumb client that stores the whole list and each minute sends the previous "password" in the list (its hash gives the "password" that was sent a minute ago).The memory on the client is limited (every byte counts), but I don't want it to make complex calculations than a lookup from an array.
Slicing the output is shouldn't be a problem even when it comes to finding collisions.There are 65536 times more collisions that would match output[:14] than the full output, so it would be 65536 times "easier" to find one (i.e. $\frac202^16$ years $\approx 2.7$ hours).Saying that the password is 14-bytes is equivalent to saying that it is a 16-bytes password that ends with \00\00 (this is how HMAC treats short passwords anyway). This will only make it harder for the attacker to find a collision, because the collision must end with \00\00 too.
BTW, when working with time estimate like 20 years, you need to remember that processing power double every two years or so (Moore's law). For that reason, if something takes 20 years, large part of processing is done within the last three years. Also, 20 years may expect that there will be some new ways to crypto analyze SHA256 can be found during the time frame (but not full break.).
Hash objects with different digest sizes have completely different outputs(shorter hashes are not prefixes of longer hashes); BLAKE2b and BLAKE2sproduce different outputs even if the output length is the same:
The digest() method of the SubtleCrypto interface generates a digest of the given data. A digest is a short fixed-length value derived from some variable-length input. Cryptographic digests should exhibit collision-resistance, meaning that it's hard to come up with two different inputs that have the same digest value.
\n The digest() method of the SubtleCrypto\n interface generates a digest of the given data. A digest is a short\n fixed-length value derived from some variable-length input. Cryptographic digests should\n exhibit collision-resistance, meaning that it's hard to come up with two different\n inputs that have the same digest value.\n
Aside from its ability to enable data integrity and message authentication, another reason why HMAC is an excellent file transfer data integrity-checking mechanism is its efficiency. As discussed in the article Understanding Hashing, hash functions can take a message of arbitrary length and transform it into a fixed-length digest. That means, even if you have relatively long messages, their corresponding message digests can remain short, allowing you to maximize bandwidth.
The way AES-GCM is initialized is stupid: You encrypt an all-zero block with your AES key (in ECB mode) and store it in a variable called . This value is used for authenticating all messages authenticated under that AES key, rather than for a given (key, nonce) pair.
Although the AES block size is 16 bytes, AES-GCM nonces are only 12 bytes. The latter 4 bytes are dedicated to an internal counter, which is used with AES in Counter Mode to actually encrypt/decrypt messages.
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I recently went through the processing of creating SDKs for an in house API. The API required signing every REST request with HMAC SHA256 signatures. Those signatures then needed to be converted to base64. Amazon S3 uses base64 strings for their hashes. There are some good reasons to use base64 encoding. See the stackOverflow question What is the use of base 64 encoding?
HMACs provides client and server with a shared private key that is known only to them. The client makes a unique hash (HMAC) for every request. When the client requests the server, it hashes the requested data with a private key and sends it as a part of the request. Both the message and key are hashed in separate steps making it secure. When the server receives the request, it makes its own HMAC. Both the HMACS are compared and if both are equal, the client is considered legitimate.
Now we have to calculate S bits
K+ is EXORed with ipad and the result is S1 bits which is equivalent to b bits since both K+ and ipad are b bits. We have to append S1 with plain text messages. Let P be the plain text message.
S1, p0, p1 upto Pm each is b bits. m is the number of plain text blocks. P0 is plain text block and b is plain text block size. After appending S1 to Plain text we have to apply HASH algorithm (any variant). Simultaneously we have to apply initialization vector (IV) which is a buffer of size n-bits. The result produced is therefore n-bit hashcode i.e H( S1 M ).
Similarly, n-bits are padded to b-bits And K+ is EXORed with opad producing output S2 bits. S2 is appended to the b-bits and once again hash function is applied with IV to the block. This further results into n-bit hashcode which is H( S2 H( S1 M )).
JWTs are a convenient way of representing authentication and authorization claims for your application. They are easy to parse, human readable and compact. But the killer features are in the JWS and JWE specs. With JWS and JWE all claims can be conveniently signed and encrypted, while remaining compact enough to be part of every API call. Solutions such as session-ids and server-side tokens seem old and cumbersome when compared to the power of JWTs. If you haven't worked with these technologies yet, we strongly recommend you do so in your next project. You won't be disappointed.
The short answer is that you need to set up the webhook to provide the endpoint with the HTTP request and a unique key that the endpoint can use to verify the data. But, before we get into the details, let's briefly cover hashing.
Note: This is a very short explanation and ignores such things as salting, brute force, iterations and rainbow tables. Do not implement password hashing based on what I write here, there is a lot more to it!
crypt-bf numbers are taken using a simple program that loops over 1000 8-character passwords. That way the speed with different numbers of iterations can be shown. For reference: john -test shows 13506 loops/sec for crypt-bf/5. (The very small difference in results is in accordance with the fact that the crypt-bf implementation in pgcrypto is the same one used in John the Ripper.)
Skipjack is a symmetric key algorithm with 64-bit blocks of plaintext and 80-bit key. It was designed by the NSA with the purpose of encrypting voice transmission, and later declassified for public knowledge. The algorithm is based off a technique of repeatedly splitting the plaintext block and performing bitwise operations with subkeys. Currently, the only security limitation is its theoretical risk to brute force, especially due to its relatively short key.
Security-wise, SWT can only be symmetrically signed by a shared secret using the HMAC algorithm. However, JWT and SAML tokens can use a public/private key pair in the form of a X.509 certificate for signing. Signing XML with XML Digital Signature without introducing obscure security holes is very difficult when compared to the simplicity of signing JSON.
Second, an HMAC is based not only on the secret key, but also on the message itself. If two HMACs are created with the same secret key, but different messages, the resulting codes will be different. When the API sees that its HMAC matches the HMAC in the request, it knows the request was not modified during transport. Since hackers will try to modify messages in between clients and servers, HMAC signatures can be a powerful tool to verify authenticity of requests.
Asymmetric: Digital signatures imply anyone can verify the authenticity of a message, given a public key to a corresponding private key that was used to sign the message. Just like signing a document with your signature - everyone can verify that it was you who signed it. Nor can you deny you signed it.
It adds some extra bits to the message, such that the length is exactly 64 bits short of a multiple of 512. During the addition, the first bit should be one, and the rest of it should be filled with zeroes.
The implementation of crypto.createCipher() derives keys using the OpenSSLfunction EVP_BytesToKey with the digest algorithm set to MD5, oneiteration, and no salt. The lack of salt allows dictionary attacks as the samepassword always creates the same key. The low iteration count andnon-cryptographically secure hash algorithm allow passwords to be tested veryrapidly.
The implementation of crypto.createDecipher() derives keys using the OpenSSLfunction EVP_BytesToKey with the digest algorithm set to MD5, oneiteration, and no salt. The lack of salt allows dictionary attacks as the samepassword always creates the same key. The low iteration count andnon-cryptographically secure hash algorithm allow passwords to be tested veryrapidly.
This passage provides insight into what they were attempting to accomplish with Canonicalization. Moreover, it provides several examples of what Canonicalization algorithms are meant to address with XML messages and digital signatures (in particular, the hashing step) upon them. As one example, if your XML parser represents an empty element as , but another one represents an empty element as , both of which are valid XML syntax that mean the same thing, the hash values (and thus digital signatures) calculated by each will be different. Similar issues arise between platforms that treat the end of line differently ('\n' vs. \r\n, for example). There are plenty of other examples of this. So, the concept of Canonicalization was born--and, with it a lot of complexity.
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