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Questions tagged [gcm]

The Galois Counter Mode, an authenticated encryption mode of operation for a block cipher.

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What is the difference between CBC and GCM mode?

I am trying to learn more about GCM mode and how it differs from CBC. I already know that GCM provides a MAC, which is used for message authentication. From what I have read and from the code snippets ...
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AES256-GCM - can someone explain how to use it securely (ruby)

I am looking into using AES256-GCM for encrypting some database fields. I know that for AES256-CBC, I need to generate a new IV for each encrypt, but I can use the same key. The IV can be openly ...
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47 votes
2 answers
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How to choose between AES-CCM and AES-GCM for storage volume encryption

We are using the encryption built into Solaris 11 ZFS, which offers the choice between CCM (CBC counter mode) and GCM (Galois counter mode). What are the pros and cons of choosing each of these cipher ...
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AES-GCM recommended IV size: Why 12 bytes?

When using AES-GCM, a 96-bit IV is generally recommended. Most implementations I've seen also use 96-bit. However, I'm unsure on where this recommendation or convention comes from. Let's assume a ...
Hendrikvh's user avatar
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Ciphertext and tag size and IV transmission with AES in GCM mode

I am completely new to using AES in GCM mode of operation, and I have not a very large background in cryptography as well. I have been playing with OpenSSL trying to encrypt and decrypt some messages. ...
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Practical disadvantages of GCM mode encryption

It seems that GCM mode encryption has a clear advantage over CBC + HMAC in the sense that it only requires a single key. But it seems that there are some experts here that do not trust it enough to ...
Maarten Bodewes's user avatar
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Disadvantage AES-GCM

What are the disadvantages and weaknesses of AES-GCM mode for authenticated encryption? Why does the CAESAR competition say that it’s one of the goals to "find an AE scheme that offers an ...
user2035863's user avatar
27 votes
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Plain text size limits for AES-GCM mode just 64GB?

Based on NIST SP 800-38D section 5.2.1.1, it seems that the maximum length of plaintext is 2^39-256 bits ~ 64 GB. We've got 100+GB files in genomics that need to be GCM encrypted so are concerned ...
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25 votes
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How bad it is using the same IV twice with AES/GCM?

I understand that initialization vectors (IV) should not be used twice when using AES/GCM. I am using a counter as an initialization vector. Every time I send out a new packet (I am developing an UDP ...
Matteo Monti's user avatar
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AES-GCM and its IV/nonce value

I was reading about the differences between the GCM and the CBC modes here and I have a follow up question: In the CBC mode the person who performs the encryption is the one who provides the IV for ...
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23 votes
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Does AAD make GCM encryption more secure?

Does additional authenticated data (AAD) make AES GCM encryption more secure? What if we drop AAD in AES-GCM 256? If we drop it, how will it make the encryption less secure?
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Is GCM still recommended?

I stumbled across a forum thread where security researcher Thomas Ptacek seemed to have negative feelings towards GCM. I had always thought from prior readings that GCM was the current gold standard ...
Anthony Kraft's user avatar
22 votes
2 answers
4k views

Is (AES-)GCM parallelizable?

I recentely faced the issue of random access decryption while AES-GCM was being used. I said this person that the underlying CTR should allow parallelization but I have no idea how authentication ...
SEJPM's user avatar
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AES GCM : is it acceptable to return the wrong plaintext if the tag is incorrect?

Let's start by saying I'm no cryptography expert, I'm just a developer, so feel free to correct me (using words, not downvotes) if what I'm saying is non-sense. Context: I'm doing some crypto as a ...
ShellCode's user avatar
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GCM vs CTR+HMAC tradeoffs

So these days I see everyone using AES-GCM. What are its advantages over simple CTR+HMAC modes? Is it speed? Or ciphertext length? And what are the security tradeoffs, both in terms of practical ...
Samee's user avatar
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Where is the authentication tag stored in file encrypted using AES-GCM?

So I am using cryptopp to encrypt a file in a project and I am using AES-GCM. I have a 256bit key and a 128bit ivec and a 128bit tag that I use to encrypt files. However, I want to know where the ...
Mo Beigi's user avatar
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1 answer
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Does the GCM authentication tag need to be protected?

I am just learning about the GCM mode of AES and I'd like to start using it in my software. However, I have a question about its use. As I understand it the output of a GCM encryption operation is ...
Isaac's user avatar
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Windows 8/Server 2012: Passes FIPS-140-2 despite failing AES-GCM for IV != 96 bits long?

Background Microsoft certifies Windows 7/8 as well as Server 2008 R2 and 2012 to be FIPS-140-2 compliant. Actually they certify just a small crypto core, bcrypt.dll (the library, which is unrelated ...
DeepSpace101's user avatar
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What is a safe maximum message size limit when encrypting files to disk with AES-GCM before the need to re-generate the key or NONCE

The general limits from the NIST recommendation are as follows: Maximum Encrypted Plaintext Size ≤ 239 - 256 bits; Maximum Processed Additional Authenticated Data ≤ 264 - 1 bits; This stack ...
Stan Ivanov's user avatar
12 votes
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Why does the Signal protocol use AES/CBC instead of AES/GCM?

AES/GCM has obviously proved itself to be better than AES/CBC. Unless the key is re-used with the same initialization vector (see disadvantages of GCM). More information on its advantages against CBC ...
OughtToPrevail's user avatar
11 votes
1 answer
667 views

how does BearSSL's GCM modular reduction work?

BearSSL (in src/hash/ghash_ctmul.c) seems to be doing a modular reduction that I don't completely understand. Here's the code: ...
neubert's user avatar
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11 votes
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AES GCM vs other encryption modes in embedded environment for network traffic

I'm doing a lot of research since I'm doing an internship for a company where I need to upgrade their security level. Their own protocol needs a cryptographic upgrade and I'm leaning towards AES since ...
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How was the GCM polynomial found?

As far as I understand, there is no general way to enumerate irreducible polynomials in a particular finite field, which are similar in nature to prime numbers over the integers. The GCM mode finite ...
conchild's user avatar
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GCM padding or not?

In this question on StackOverflow https://stackoverflow.com/questions/31248777/can-pkcs5padding-be-in-aes-gcm-mode it's said that GCM doesn't require a padding: why?
M-elman's user avatar
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Encryption of big files in Java with AES/GCM

I have to encrypt big files. Say their size ranges from 500mb to several of gigabytes. I would like to use AES/GCM/NoPadding as provided by Java 1.8 since that gives me automatic authentication and ...
alejo's user avatar
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How to use GCM mode and associated-data properly

I'm currently migrating a project to use the bouncy castle GCM mode. I understand that using an authenticated mode removes the need for a HMAC, however I want to be sure that I'm using this mode ...
hunter's user avatar
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GCM mode of operation with long-term keys

Regarding GCM, NIST specifies the following: The total number of invocations of the authenticated encryption function shall not exceed $2^{32}$, including all IV lengths and all instances of the ...
Sadeq Dousti's user avatar
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Unique GCM/CCM initial counters without recipient side message counters

I am implementing the encryption layer for a communication protocol. The bulk encryption method used is either AES-CCM or AES-GCM. Due to implementation details, encryption of packets is usually, but ...
Henrick Hellström's user avatar
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1 answer
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AES cache-timing attacks and nonce-based operation modes

As I understand it, AES cache-timing attacks exploit the execution time variations due to cache hits/misses, especially the ones depending on the SBox look-up tables which are key-related. To mount ...
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GCM as a general purpose cryptographic hash?

Why are hash functions (e.g SHA-3) so complicated when GCM apparently provides secure hashing and has a relatively simple construction? Is this purely about speed? I imagine that GMAC with a fixed ...
conchild's user avatar
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2 answers
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Leaving authentication data blank less secure for AES GCM?

The AES GCM example for Ruby / OpenSSL shown at: http://ruby-doc.org/stdlib-2.0/libdoc/openssl/rdoc/OpenSSL/Cipher.html has a GCM example of the following: ...
mrmanman's user avatar
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3 answers
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Can a zero nonce be safely used with AES-GCM if the key is random and never used again?

I could generate a random nonce and prepend it to the ciphertext, but storage space is at a premium and the only constraint AES-GCM has on the nonce (if I'm reading correctly) is that the same nonce ...
jnm2's user avatar
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Are lookup tables safe when they fit in a cache line?

The typical cache line size of modern x86 machines is 64 bytes. Is it safe to use lookup tables with secret data if the table does not exceed the cache line size, and is aligned accordingly? I was ...
Myria's user avatar
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9 votes
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Why does AES-GCM need a hash/MAC in TLS?

"TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256" is just one example for a cipher suite, as far as I'm concerned it means this: It uses the TLS protocol, exchanges keys with Elliptic Curve Diffie Hellman ...
hl3mukkel's user avatar
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2 answers
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IV in Galois Counter Mode

I am looking at Wikepedia Galois Counter Mode article. I see this diagram I am trying to figure out where the IV works its way in. Is it used to initialize the counter? If so, how does a sequence of ...
John Frye's user avatar
9 votes
1 answer
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TLS 1.2 Cipher Suites With AES-GCM – What data (if any) is passed to the AES-GCM cipher as the Additional Authentication Data?

TLS 1.2 defines a number of cipher suites that employ AES-GCM, e.g.: ...
weaver's user avatar
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9 votes
1 answer
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Poly1305-AES vs AES-GCM

What are the advantages of Poly1305-AES over AES-GCM? Please note I am not talking about chacha20-Poly1305 that has been widely adopted, including by Google. But I would like to know pros and cons ...
user12480's user avatar
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What are the popular modes-of-operation (AES-GCM, AES-SIV, AES-GCM-SIV, etc.) geared for?

I'm interested in developing software capable of encrypting personal files (which will ultimately be backed up to the cloud) and have been doing my best to follow best practices. There are many forms ...
meci's user avatar
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9 votes
1 answer
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What are the constraints on using GCM with a tag size of 96 and 128 bits?

Say we want to use AES (or any other secure 128 bit block cipher) with GCM and a tag size of 96 or 128 bits. I'm assuming an AES key size of 128 bits and an IV size of 96 bits (the default). NIST SP ...
Maarten Bodewes's user avatar
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9 votes
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GCM with reversed poly

These slides talk about how GCM can be sped up if one uses $x^{128}+x^{127}+x^{126}+x^{121}+1$ as the reduction polynomial instead of $x^{128}+x^7+x^2+x^1+1$. When one is doing that one needs to ...
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8 votes
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Would an encryption-only block cipher be useful at all?

I recently implemented AES block cipher, encryption side only, to be used in QUIC parsing (QUIC uses GCM mode). There are other modes than GCM that use only encryption: for example CTR, OFB, and CFB. ...
juhist's user avatar
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8 votes
1 answer
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Relationship between AES GCM and AES CTR

I have read that AES GCM uses AES CTR for encryption and GMAC for authentication. If that is correct, what is the relationship between the IV used for AES GCM and the IV used for AES CTR? TO put the ...
Tricky Dixon's user avatar
8 votes
1 answer
2k views

Use Initialization Vector as Additional Authenticated Data

Cipher is GCM-AES. Is it acceptable/recommended to store the IV as AAD? Any popular examples?
Bondolin's user avatar
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3 answers
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How to encrypt files using AES256-GCM cipher under Linux? [closed]

I'm struggling to implement AES256-GCM on a Linux machine to encrypt files outgoing to another party. We're usually exchanging files using PKI, but this particular party insists on AES256-GCM and ...
Marek's user avatar
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8 votes
2 answers
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Lack of response to CAESAR competition

Why is there so little response (e.g. implementation in crypto libraries, programs...) after the end of the CAESAR competition? As far as I can see, there is no shift from AES-GCM to any of the CAESAR ...
BeloumiX's user avatar
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1 answer
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How to implement "online" feature of GCM?

I've heard that Galois Counter Mode (GCM) is an "online" encryption algorithm—it is not necessary to know the size of the input ahead of time. But does GCM require that all authenticated-but-not-...
Myria's user avatar
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8 votes
1 answer
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Detection of weak keys for AES-GCM

There are many papers out there that show that a message authenticated and encrypted by AES-GCM can be forged if the used key is weak (e.g. by Handschuh and Preneel, Saarinen or Procter and Cid). With ...
budderick's user avatar
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7 votes
1 answer
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How can there be AES-256-GCM, when GCM is defined for 128-sized blocks?

In both node.js and OpenSSL, I am seeing cipher "aes-256-gcm". However, in NIST Special Publication 800-38D, that defines GCM, I am reading: The underlying block cipher shall be approved, the ...
Karel Bílek's user avatar
7 votes
2 answers
5k views

Should I use XTS or GCM to encrypt my hard drives?

I want to start encrypting all of my hard drives, but I don't know whether to choose XTS or GCM mode. Why is it that XTS is recommended (since the most websites I visit use GCM in their HTTPS ...
blacklight's user avatar
7 votes
2 answers
2k views

Is it possible to tweak AES-GCM so that it is satisfactory for whole-disk encryption (like XTS mode)?

Is it possible to leverage a preexisting implementation of AES-GCM to provide the key security benefits essential for full-disk encryption (similar to AES-XTS)? GCM is a popular encryption mode ...
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