AES-GCM and breaking it on nonce reuse
frereit.de
frereit.de
Also, if the attacker only has c1 and c2, if the nonce is reused then c1 xor c2 will be the same as p1 xor p2. In most cases, two plaintexts xored with each other are trivial to decode.
* https://ctr.var.tailcall.net/
* https://ecb.var.tailcall.net/
* https://cbc.var.tailcall.net/
The goal is a bit different (I use them when teaching university courses, to show that encryption is not authentication), but the same ideas apply.
The vast majority of encryption algorithms must be used in a nonce-respecting scenario. This is part of the contract to achieve the claimed security properties.
Alternatives require multiple passes over the data, which is not applicable to some protocols in addition to having performance implications.
Common protocols such as TLS transparently handle nonces in a safe way. But the primitives used in TLS may require additional steps to be safely used is other contexts, especially in distributed systems.
Whenever applications try to use these primitives directly, using a fixed key and picking nonces at random is a very common practice. Unfortunately, due to their small size, nonces collisions can quickly happen.
We're missing standard constructions with large nonces that would alleviate this problem, because IETF protocols haven't needed them. But there's a lot of evidence that many custom applications and protocols do.
There are multiple great proposals to derive AES-GCM subkeys and nonces from a key and a large nonce. We may expect convergence and adoption in crypto libraries soon.
Until then, constructions such as XSalsa20 and XChaCha20 are widely implemented and deployed. If you don't need NIST compliance, they're excellent choices.
But my recommendation today would be to replace AES-GCM with the AEGIS family of algorithms whenever possible. They have nice properties that AES-GCM doesn't have, including more comfortable usage limits, much better performance and large nonces up to 256 bits.
This page [1] and that draft [2] summarize usage limits of common constructions, including when using random nonces.
[2] https://doc.libsodium.org/secret-key_cryptography/aead
[3] https://datatracker.ietf.org/doc/draft-irtf-cfrg-aead-limits...
I've put together a little online demo tutorial (in my teaching and learning programming language).
Also, it's not entirely clear just how bad a reuse actually is. For example, in AES-CBC, reusing the IV has much less impact than reusing the nonce with AES-GCM.
Aside from just not understanding it, it's plausible that someone would generate nonces weakly, say, from a weak source of randomness.
Even using a strong source of randomness for an AES-GCM nonce is weak over enough messages, since it only gets you 48 bits of collision resistance.
If you're not using random nonces, maybe you want to use a counter, and then you have to worry about race conditions, state resets, etc. (if your system lost power immediately after using nonce n, would it boot back up and reuse it?)
This is actually generally fine for nonces (used in CTR and GCM modes, and in ChaCha20). Typically the only requirement for a nonce is that it is only used once. It is even safe to use a simple incrementing counter.
IVs, on the other hand, are required to be cryptographically random.
I can imagine VPNs or other packetized communications potentially running into this problem, e.g. with N parties needing to encrypt messages under the same key to each other without coordination on nonces. The worst case I can think of is a large number of devices with a baked-in key and secure RNG but no non-volatile storage. They can't generate more than 2^48 messages with AES-GCM or risk collision.
Full disk encryption has always had a similar problem; generally a single long-lived master key that individual sectors or blocks are encrypted by, often without the additional storage set aside for IVs or nonces (which would break exact sector to sector mapping of encrypted virtual disk to plaintext disk). That leaves IV-derivation to be static per block offset/number, or key derivation on master key and block offset/number.
Devices without secure RNGs are also at risk (microcontrollers with no non-volatile storage that restart a lot, for example).
I'm curious if there are any other hard cases where nonce reuse becomes a risk in practice.
What do you think the ratios are regarding improper use of nonce with this mode?
Most implementations that I am familiar with intentionally generate a random nonce to help lower the percentage of app devs doing this very thing
If I had to, based on absolutely nothing but a gut feeling, guess, I'd think this may appear more frequently in IoT devices, where AES-GCM is attractive because of its speed, but randomness is sometimes in low supply?
I think one reason nonce-reuse is a problem in IoT is lack of experience and awareness. Up until relatively recently a lot of embedded development was constrained to just offline devices, so cryptography wasn’t really required.
1. CBC does not have the same class of vulnerability to Nonce/IV reuse. Reusing an IV would leak some information about the first block (or first few blocks which are the same), but it would not give your a XOR of two plaintext or let you recover the keystream. On the other hand, CBC is vulnerable when IVs are predictable (e.g. the BEAST attack).
2. CBC with a proper encrypt-then-MAC scheme (e.g. HMAC-SHA256 + HKDF-SHA256 for generating Authentication and Encryption Keys) can encrypt more data than GCM without rotating a key. GCM with random nonces are particularly problematic, since at one point you would run into a nonce collision.
Overall, AES-GCM is preferable to AES-CBC because it is quite hard to implement a good encrypt-then-MAC scheme on top of AES-CBC unless you know what you're doing. But it's not good enough as a general worry-free solution, even when you're using a library to wrap nonce generation for you. What you want is XChaCha20Poly1305, if you're going for an ubiquitous and mature cipher.
When I use AES-GCM I just use a bigger nonce and use a random one.
Last time I used AES-GCM I had a really hard time getting the person writing the other end to not re-use nonces.
I don't think nonces bigger than 12 bytes will help. My quick reading of the AES-GCM spec is that when using a nonce that's not 96 bits (12 bytes), it is hashed to 96 bits. So either the nonce (called iv in the spec) is carefully constructed from a counter and set to exactly 96 bits, or the number of invocations is limited. The spec still restricts use of a key to 2^32 total uses for random nonces of any bigger length (resulting in a re-use probability of about 1e-10):
https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpubli...
> unless an implementation only uses 96-bit IVs that are generated by the deterministic construction: The total number of invocations of the authenticated encryption function shall not exceed 2^32, including all IV lengths...
Which would be the conclusion if the hash was always reduced to 96 bits. What it actually goes on to say though is:
> For the RBG-based construction of IVs, the above requirement, in conjunction with the requirement that r(i)≥96, is sufficient to ensure the uniqueness requirement in Sec. 8
So, it's a bound. If there are at least 96 random bits, it should all be ok. But it strangely leaves open the possibility, without saying either way, that a longer iv, with r(i)>96 random bits might allow generating more iv's. As you point out, it will depend on the properties of GHASH (and potentially on how the result is used downstream from there). At this point I don't know, but the spec says:
> For IVs, it is recommended that implementations restrict support to the length of 96 bits, to promote interoperability, efficiency, and simplicity of design.
So personally, not being an expert, I'd follow the advice and use 96 bit iv's. And if using random iv's, re-key before using the key a billion times. I'd certainly want a reference to a careful analysis before assuming that I could ever use an AES-GCM key with longer random iv's more than that.
There is some details on the "GHASH as initial counter value" which seem to suggest that for larger nonces, the total number of messages shouldn't exceed 2^44.5 here: https://neilmadden.blog/2024/05/23/galois-counter-mode-and-r...
Recently discussed: "Galois/Counter Mode and random nonces" (28.05.2024) https://news.ycombinator.com/item?id=40497525
Turns out the people who wrote the in house Go library didn't have any idea. There is no non-deterministic encryption function because that might be too complicated for non-senior engineers (afterall they wrote most of the actual application) to correctly choose.
The first version use AES-CFB. There's no authentication. It's probably copy pasted from a public Gist and nobody ever commented on it that it is insecure. I wonder if it was actually intended to be the non-deterministic version, but the higher level wrappers do not wrap this function so people didn't actually use it.
The second version use AES-GCM with nonce derived from the key and AD. Since nobody understand why AD is needed, AD is always nil. Essentially there's ever one nonce.
I think the problem is that many senior engineers know that encryption use "AES" library but the Go standard library doesn't tell you how to use it securely.
Surprisingly this mistake also happen in our Java stack that was written by a different team. A senior engineer did notice and quietly moved away from the vulnerable version without telling the Go version.
I wrote a POC to decrypt data of the Go version, then wrote the third version, perhaps it will be open source soon. The new library only implement envelope key management, encrypted string wrapper and ORM integration. The rest is Google's Tink.
[Edit: I was unaware of the existence of "deterministic AEAD" before I wrote this: "Deterministic" encryption is discouraged because it passes through block-aligned patterns in the plaintext to the ciphertext. There is a simple method to do what you're after: it's just feeding your data (with padding) directly into the cipher (so-called ECB mode). Go's standard library gives you the raw AES cipher to do this with, but it doesn't expose the standard padding mechanisms (and it's not authenticated). You should be aware that doing anything like this leaves your data open to certain kinds of cryptanalysis that can infer the plaintext without directly breaking the cipher.]
I largely agree that the standard library doesn't provide any solid guidance or higher-level APIs for any use case other than TLS. The implementations seem to be pretty high-quality but you quickly go from "it's hard to use this wrong" in some libraries to "here's a drawer full of sharp knives" in others.
I'll take things you should never do as a non-expert for $100.
> The first version use AES-CFB. There's no authentication. It's probably copy pasted from a public Gist and nobody ever commented on it that it is insecure. I wonder if it was actually intended to be the non-deterministic version, but the higher level wrappers do not wrap this function so people didn't actually use it.
Lack of authentication is probably the least of your concerns if your product is searching over encrypted data.
...a variant on that is DNDK-GCM in draft at https://datatracker.ietf.org/doc/draft-gueron-cfrg-dndkgcm/ and a recent presentation: https://youtu.be/GsFO4ZQlYS8 (this is Shay Gueron who worked on AES-GCM-SIV too).
But, AES-GCM-SIV requires two passes over the data, which isn't always ideal.
The goal of the CAESAR competition [1] was essentially to find alternatives. Whether that goal has been met is a bit unclear at the moment.
>What happens if you repeat the nonce? You’re going to mess up authenticity for all future messages, and you’re going to mess up privacy for the messages that use the repeated nonce.
The loss of privacy on OCB nonce reuse is not as severe. It would be more or less the same as with ECB mode.
> It is the user’s obligation to ensure that nonces don’t repeat within a session. In settings where this is infeasible, OCB should not be used.
But earlier in that section we have:
> […] The nonce doesn’t have to be random or secret or unpredictable. It does have to be something new with each message you encrypt. A counter value will work for a nonce, and that is what is recommended. […]
* https://www.cs.ucdavis.edu/~rogaway/ocb/ocb-faq.htm#nonce
So given that GCM uses a counter ("C"), and a counter is recommended for OCB, wouldn't it be simple enough to get the equivalent (?) security more efficiently?
GCM-SIV is not perfect for nonce reuse anyway. It reveals to the attacker that two messages are identical.
Nonce reuse in SIV is much less catastrophic. Reuse of a nonce can reveal if two packets are identical but doesn’t let you do anything else.
Of course there are categorically better modes than GCM but they are not widely supported. ChaChaPoly is better cryptographically but no hardware acceleration, which matters on small devices.
If you never had a plaintext you could potentially (depending on the content) collect enough ciphertexts to do frequency analysis on it. You'd recover the keystream at least partially, and then guess based on context to fill out the rest.
Shouldn't the result be ((U10 ⊕ U20) ⨂ H3) ⊕ ((U11 ⊕ U21) ⨂ H2) ⊕ ((U12 ⊕ U22) ⨂ H) ?
The same key + nonce generates the same keystream.
The ciphertext is generated by xoring the plaintext with the keystream.
The keystream can be recovered by xoring the ciphertext with the plain text.
To abuse it...
The defender needs to re-use both the same key and nonce.
The attacker needs to have a ciphertext/plaintext pair, know or find the position of that text in the keystream, and needs access to other ciphertexts generated with the same key/nonce.
We are, after all, talking about the country that terrorized an Austrian town into changing its name after decades of sign theft and jokes [0]