crypto: Use secure_allocator for `AES256_ctx` #31774

davidgumberg · 2025-01-31T20:10:56Z

Reuse secure_allocator for AES256_ctx in the aes 256 encrypters and decrypters and the iv of AES256CBC encrypters and decrypters. These classes are relevant to CCrypter, used for encrypting wallets, and my understanding is that if an attacker knows some or all of the contents of these data structures (AES256_ctx & iv) they might be able to decrypt a user's wallet.

Presently the secure_allocator tries to protect sensitive data with mlock() on POSIX systems and VirtualLock() on Windows to prevent memory being paged to disk, and by zero'ing out memory contents on deallocation with memory_cleanse() which is similar to OPENSSL_cleanse() by scaring compilers away from optimizing memset calls on non-Windows systems, and using SecureZeroMemory() on Windows.

DrahtBot · 2025-01-31T20:10:59Z

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/31774.

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DrahtBot · 2025-01-31T21:32:45Z

🚧 At least one of the CI tasks failed.
_{Debug: https://github.com/bitcoin/bitcoin/runs/36501336640}

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theStack · 2025-02-04T15:51:03Z

Concept ACK on clearing out the ctx/iv members

I'm wondering if a minimum-diff fix which simply replaces the memset calls in the dtors with memory_cleanse would be largely sufficient here? In #31166 (comment) one argument for not needing secure allocators was the short-lived nature of the secrets. Looking at the only usage in the wallet, this would imho apply here too (en/decrypting might take a while for larger inputs, but I still wouldn't classify that as long-lived):

bitcoin/src/wallet/crypter.cpp

Lines 85 to 91 in 94ca99a

    
           AES256CBCEncrypt enc(vchKey.data(), vchIV.data(), true); 
        
           size_t nLen = enc.Encrypt(vchPlaintext.data(), vchPlaintext.size(), vchCiphertext.data()); 
        
           if(nLen < vchPlaintext.size()) 
        
               return false; 
        
           vchCiphertext.resize(nLen); 
        
           return true;

bitcoin/src/wallet/crypter.cpp

Lines 102 to 108 in 94ca99a

    
           AES256CBCDecrypt dec(vchKey.data(), vchIV.data(), true); 
        
           int len = dec.Decrypt(ciphertext.data(), ciphertext.size(), plaintext.data()); 
        
           if (len == 0) { 
        
               return false; 
        
           } 
        
           plaintext.resize(len); 
        
           return true;

The reasoning might be a bit loose as there might be other uses of these classes in the future where the situation is different. Curious about other opinions.

sipa · 2025-02-04T16:35:20Z

I think the approach here might have an unacceptable performance impact, as it is allocating and deallocating a secure area for every individual key being encrypted/decrypted.

It may be better to:

Somehow make the AES256CBC classes members of CCrypter, surviving an individual encryption/decryption (e.g. by adding a reset function that can get called for each encryption/decryption, resetting the CBC state, but letting the key material survive).
Follow @theStack's suggestion of not locking the memory, but just securely wiping it after each operation.

DrahtBot · 2025-02-08T00:50:43Z

🚧 At least one of the CI tasks failed.
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davidgumberg · 2025-02-08T01:49:49Z

I'm wondering if a minimum-diff fix which simply replaces the memset calls in the dtors with memory_cleanse would be largely sufficient here? In #31166 (comment) one argument for not needing secure allocators was the short-lived nature of the secrets.

It seems right to me that this structure is always short lived, but I also felt in #31166 secure_allocator should have been used over just memory_cleanse(). I feel that the alloc/dealloc strategy for secrets in memory should be reused and applied universally unless there is a good reason not to.

I think the approach here might have an unacceptable performance impact, as it is allocating and deallocating a secure area for every individual key being encrypted/decrypted.

I added a benchmark for WalletEncrypt() which I ran a few times and it appears that the performance impact is neglible on my machine (Ryzen 7900x, Fedora 40), if the benchmark I wrote is actually representative:

wallet type	branch	EncryptWallet() (ns/key)	benchmark overhead (ns/key)	normalized value (total - overhead)	flamegraph
Descriptor	`15d8500`	42,106.55	36,235.03	5,871.53	link
Descriptor	master	42,050.82	36,362.40	5,688.42	link
Legacy	`15d8500`	17,067.82	2,779.16	14,288.6	link
Legacy	master	16,812.63	2,757.26	14,055.57	link

But if the short-lived nature of the key material makes just using memory_cleanse() a good-enough solution over having to reason about whether the benchmark is sufficiently correct, and whether the larger / more complicated diff required to reuse secure_allocator is worth the review effort / risk, I'm happy to change it.

laanwj · 2025-03-27T13:27:45Z

I think the approach here might have an unacceptable performance impact, as it is allocating and deallocating a secure area for every individual key being encrypted/decrypted.

This may still be true, though FWIW the idea behind the LockedPool is that it reduces the amount of locking/unlocking operations by mapping and locking memory in blocks, not every time it's requested.

But agree that for short-lived key material, it's less important, there is little to no chance of it ending up in swap anyway.

DrahtBot · 2025-05-01T05:42:13Z

🚧 At least one of the CI tasks failed.
_{Task ARM, unit tests, no functional tests: https://github.com/bitcoin/bitcoin/runs/41468173464}
_{LLM reason (✨ experimental): The CI failure is caused by an assertion failure in wallet creation during the bench_sanity_check test.}

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davidgumberg · 2025-05-01T05:44:18Z

Rebased to address merge conflict, dropped legacy wallet encryption benchmark, and reduced number of keys since the benchmark was taking an unreasonably long time.

sipa

utACK 591764f, just coding style nits

src/wallet/wallet.cpp

src/bench/wallet_encrypt.cpp

davidgumberg · 2025-05-02T01:17:23Z

Rebase to address style feedback.

theStack

Code-review ACK 3d7d2c3

furszy

I don't see why the first commit is necessary. Couldn't you just mock the time so that it's fixed and the number of derivation rounds always stays at the default value?

davidgumberg · 2025-05-05T23:36:11Z

I don't see why the first commit is necessary. Couldn't you just mock the time so that it's fixed and the number of derivation rounds always stays at the default value?

Maybe I'm not thinking creatively enough, but I think because the timing takes place entirely inside of EncryptWallet()->EncryptMasterKey, from the wallet_encrypt.cpp benchmark, I could only mock the clock so that it's static during the derivation "benchmark"¹ runs, so the difference between the end time and the start time of a run will be 0 which will result in dividing by 0:

bitcoin/src/wallet/wallet.cpp

Lines 581 to 589 in 68ac9f1

    
           static bool EncryptMasterKey(const SecureString& wallet_passphrase, const CKeyingMaterial& plain_master_key, CMasterKey& master_key) 
        
           { 
        
               constexpr MillisecondsDouble target{100}; 
        
               auto start{SteadyClock::now()}; 
        
               CCrypter crypter; 
        
               crypter.SetKeyFromPassphrase(wallet_passphrase, master_key.vchSalt, master_key.nDeriveIterations, master_key.nDerivationMethod); 
        
               master_key.nDeriveIterations = static_cast<unsigned int>(master_key.nDeriveIterations * target / (SteadyClock::now() - start));

The ones that decide how many derivations to perform based on hardware speed. ↩

furszy · 2025-05-05T23:54:12Z

so the difference between the end time and the start time of a run will be 0 which will result in dividing by 0:

Yes, it seems simpler and more scoped to address that inside the encryption function (which could arguably be considered a 'fix', since we shouldn’t be dividing by zero regardless of whether the clock time is messed up) than to add a test-only field to the wallet and another function arg just just for this.

davidgumberg · 2025-05-06T00:11:05Z

since we shouldn’t be dividing by zero regardless of whether the clock time is messed up

AFAIK, std::chrono::steady_clock, unlike the system clock, cannot fail to move forward, and this will never happen.

than to add a test-only field to the wallet and another function arg just just for this.

I agree that adding a test-only field and arg is very bad, and would like to avoid it, but it seems to me worse to imbue secret meaning into the benchmark by handling a time difference of 0 in this special way which appears on the surface to be error-handling, but is really a "side-channel" for test code to sneak an extra parameter into EncryptWallet, but I don't feel strongly enough about which of those is worse, so I'll implement your approach.

DrahtBot · 2025-05-06T04:03:17Z

🚧 At least one of the CI tasks failed.
_{Task lint: https://github.com/bitcoin/bitcoin/runs/41695411207}
_{LLM reason (✨ experimental): The CI failure is due to issues identified by lint-includes.py and the all_python_linters check.}

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Try to run the tests locally, according to the documentation. However, a CI failure may still
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affected test.
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Leave a comment here, if you need help tracking down a confusing failure.

Adds a special case where if the elapsed time during measurement of DKF performance is 0, the default derive iterations are used so that behavior is stable for testing and benchmarks.

Allows `crypto` functions and classes to use `secure_allocator`.

davidgumberg · 2025-05-06T04:26:42Z

Refactored to address @furszy feedback to avoid adding a test-only parameter to CWallet, and added a somewhat opinionated refactor (7176b26) of CWallet::EncryptMasterKey() since I was touching the iteration measuring stuff anyways.

I don't like the idea of using a for loop that iterates once with 0 and once with 1, but it's shorter, I believe it is easier to understand how the weighted average calculation is working, and this version generalizes to any number of measurement runs.

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crypto: secure erase memory #31744

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wallet: refactor: various master key encryption cleanups #31398

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davidgumberg added 6 commits May 5, 2025 21:07

refactor: Generalize derivation target calculation

7176b26

wallet: Make encryption derivation clock mockable

38aa65f

Adds a special case where if the elapsed time during measurement of DKF performance is 0, the default derive iterations are used so that behavior is stable for testing and benchmarks.

bench: Add wallet encryption benchmark

26442fe

build: Move lockedpool.cpp from util -> crypto

d796803

Allows `crypto` functions and classes to use `secure_allocator`.

crypto: Use secure_allocator for AES256_ctx

1a2e08e

crypto: Use secure_allocator for AES256CBC*::iv

8bdcd12

davidgumberg force-pushed the 1-31-25-aes-secure-alloc branch from 90528e7 to 8bdcd12 Compare May 6, 2025 04:13

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DrahtBot added CI failed and removed CI failed labels May 16, 2025

crypto: Use secure_allocator for AES256_ctx #31774

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