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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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10
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2answers
4k views

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 ...
19
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3answers
4k views

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 ...
20
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2answers
2k 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 ...
33
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2answers
22k views

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 ...
24
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2answers
4k views

AES-GCM Disadvantage

What is the disadvantage 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 advantage over AES-GCM? What ...
24
votes
1answer
21k views

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 ...
6
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1answer
4k views

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 ...
71
votes
1answer
123k views

What is the difference between CBC and GCM mode?

I am trying to learn more about GCM mode and how it differs between CBC. I already know that GCM provides a MAC which is used for message authentication. From what I have read, and from the code ...
23
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1answer
19k views

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. ...
7
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2answers
10k views

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 ...
6
votes
1answer
369 views

Security of GCM if Implemented with Stream Cipher instead of Block Cipher

Can GCM be implemented with a stream cipher instead of a block cipher? If no, then what security flaw does it have? If yes, then which possibility from below two is better in terms of security? In ...
3
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2answers
201 views

Generate $J_0$ for GCM cipher when $\operatorname{len}(IV) \neq 96$ bits

I have the following questions about the construction of J0 when is not equal to 96 bits, $s= 128\big\lceil \operatorname{len}(\text{IV})/128\big\rceil-\operatorname{len}(\text{IV})$ For example if ...
9
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1answer
280 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: ...
4
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2answers
1k views

GCM encryption for 256-bit and 512-bit block ciphers

The Galois/Counter Mode of Operation (GCM) This paper describe using GCM mode with 128-bit and 64-bit block cipher. Can we use this mode for 256-bit and 512-bit block cipher?
9
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1answer
8k views

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 ...
7
votes
1answer
958 views

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 ...
5
votes
2answers
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 ...
17
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2answers
8k views

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 ...
7
votes
1answer
2k views

Has GMAC mode a future outside GCM?

Once gcm has been implemented for providing both encryption & authentication solution , it appears obvious to consider that such implementation can also provide (if required by new user) a Mac ...
7
votes
1answer
2k views

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 ...
6
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1answer
3k views

Security of authenticated encryption modes GCM & CCM

I have two questions for Clarification for AE mode choice criteria GCM : it appears to be actually the most popular and widely used AE mode of operation. however it is also well-known to be highly ...
5
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1answer
386 views

GCM: Math behind update of AAD after ciphertext has been processed

In the Bouncy Castle libraries, the GCM cipher implementation has an interesting property that does not seem described in the GCM papers (neither the NIST or the original paper): Some AAD was sent ...
1
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1answer
411 views

Execution speed of AES_128 GMAC operation

I have read about the speed performance of AES_128_GCM on specific processors Intel and AMD that support AES New Instruction set. As compared with CMAC and HMAC is it relatively slow for other ...
4
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3answers
357 views

Nonce-misuse-resistance scheme applied after the fact to AES-GCM for defense in depth?

This is a follow-up to a previous question about encrypting IV/MAC results from AEAD ciphers. I have a system I'm working on that needs to use standard (NIST/FIPS) cryptography, at least for its ...
1
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3answers
265 views

Is the Salsa20-GCM composition secure?

AES-GCM seems to be used by everyone, but I have never seen even one post about Salsa20-GCM. Is it secure? Wikipedia mentions ...
10
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1answer
1k views

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 ...
6
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1answer
761 views

Useful additional data to employ in GCM

Are there any requirements for the additional data to be used in the GCM block cipher mode? And are there any "classic" used informations worthwile to be used? Can for example a username or the name ...
6
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1answer
519 views

How to communicate authentication tag for GCM?

I have written some code to do AES in GCM. I currently manually append the tag property to the ciphertext, is this the proper way to communicate the authentication tag?
6
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1answer
256 views

Do Carter–Wegman MACs allow key reuse if the MAC tag is kept secret?

Poly1305 (and GHASH) are secure authenticators, but only for one use. Thus, nonce reuse in Poly1305-AES, ChaCha20-Poly1305, and AES-GCM all reveal the authentication key. However, my understanding ...
5
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1answer
2k views

Safety of random nonce with AES-GCM?

I am encrypting multiple messages using a 256 bit key with AES in GCM mode, generating a random 96 bit nonce for each message. The small nonce size combined with random values is worrying me: AES-...
5
votes
1answer
1k views

Security for IV in AES-GCM mode

I'd like to use AES-GCM instead of HMAC for authentication and privacy in a token used in a web application, but I'm unsure how to securely tag each token with an IV. Is it safe for me to prepend the ...
2
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2answers
255 views

Does AES-GCM without security tag turn into regular CTR Mode?

We are looking to implement a simple password based key agreement scheme. Part of this is deriving a symmetric key from a common passphrase known to both parties involved. This symmetric key is then ...
8
votes
1answer
4k views

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 ...
6
votes
2answers
853 views

Security of encrypting many short values with AES in GCM mode

We intend to encrypt certain values in a database which holds personal information such as addresses so as to prevent data leakage should the database be compromised. The intention is that each field ...
3
votes
1answer
204 views

Why is GCM only usable for ciphers with a 128-bit block size?

All block cipher modes of operation that I understand are generic schemes. They are not restricted by key size or block size. So why is GCM restricted to 128-bit block size?
3
votes
1answer
723 views

Security of integrity protection with HMAC vs AEAD

I'd like to know how the security of the integrity protection compares between: HMAC-SHA-256 with an 32 byte tag or a truncated 16 byte tag AES-GCM with a 16 byte tag AES-CCM with a 16 byte tag or ...
3
votes
1answer
634 views

Is the length limit of AES-GCM per key or per (key, nonce) pair?

AES-GCM has a limit on the length of the plaintext it encrypts: $2^{39} - 256$ bits. But is this limit per key or per (key, nonce)? That is, if I encrypt a message with the maximum length, and encrypt ...
2
votes
1answer
2k views

AES-CBC then SHA vs AES-GCM for encrypting and authenticating a web token

I am trying to have something like JWT but kinda ad hoc and encrypted. The token itself is simply a stringified JSON that contains the user ID and Unix timestamp. Now, I tried to use AES-128-GCM, ...
1
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2answers
1k views

Parallel file encryption and authentication for large files

For a cloud storage upload client I want to encrypt files during upload, so that they cannot be read by the cloud service provider or anyone who gains access to the account. I would like to use some ...
1
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2answers
471 views

Is it safe to use a counter as IV for AES256-GCM

What if I will use 96-bit integer counter starting from 0 as IV for AES256-GCM cipher. Is it safe? Is there any other ways to ensure uniqueness of IV for GCM cipher?
0
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1answer
268 views

Is there a TLS extension for generating unique IVs for AES GCM?

Since reusing IVs is death for AES GCM, it seems like using the fixed IV in the TLS connection state is a phenomenally bad idea. It wouldn't even seem to be a good idea to use the salt + counter ...
0
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2answers
596 views

Which is slower, AES or GCM hash (GHASH), if CPU has no special instructions?

In an answer to this question: Security of GCM if Implemented with Stream Cipher instead of Block Cipher ...it was stated that: The GHASH part of GCM is usually the problematic one when it comes to ...
0
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1answer
145 views

Is there a strong cryptographic reason for GCM's 2^39 - 256 bit limit?

In reading through the original GCM specification (McGrew & Viega '05), the composition of the 128 bit Initialization Vector as a concatenation of a 96b nonce and a 32b unsigned wrapping counter ...