Is BitLocker susceptible to any known attacks other than bruteforcing when used with a very strong passphrase and no TPM?

Question

I have learned about attacks where the BitLocker master key can be sniffed on its way from the CPU to the TPM using a logic analyzer. However, in computer configurations without TPMs, this is obviously not possible. Furthermore, as I understand, the TPM will rate-limit the number of attempts on BitLocker passwords, and also verify the integrity of the hardware/firmware of the entire system before even allowing decryption attempts. I have also read that in some cases, Microsoft will automatically send the BitLocker recovery codes to its "Microsoft cloud backup" under the user's account, but others have contested that this is optional and/or not enabled by default. I need to continue work to find out if there is a way to find out if this is the case on a running system connected to a Microsoft account.

My question is, is there any known way, aside from bruteforce attacks, for an attacker to decrypt a drive protected by BitLocker with a very strong passphrase?

Answer 1

While there are more than one, the Cold Boot attack is probably the most infamous type of attack on encryption keys like Bitlocker’s.

The Cold Boot attack is a physical attack that can dump encryption keys- meaning that the length of the passphrase doesn’t matter.

But don’t get me wrong: this is a very sophisticated attack. It usually involves using an extremely cold chemical (such as liquid nitrogen) to capture the remanence of the RAM- which includes these encryption keys.

Answer 2

It depends totally on your threat model

If the attacker can physically access a powered on machine protected by only screen lock, the cold boot or DMA attacks will work.

If the attacker can physically access the machine to install hidden hardware and later the owner types the password, there are many attacks possible, such as the TPM sniffing, keyloggers and malicious USB devices. They could even completely replace the machine and have a fake BitLocker steal the password.

If the machine is connected to network, any exploitable security issues in the operating system or other software could be used to steal data with no regard to BitLocker.

If a wireless keyboard is used to log in, timing analysis can provide information about the password and make bruteforcing more efficient.

If the recovery key is physically stored somewhere, it can be found and used to open the encryption.

One can reasonably say that none of these are attacks against BitLocker itself but merely circumvent it. A security solution is only as strong as the weakest link.

Answer 3

Other than the Cold Boot attack outlined by @security_paranoid, another way to extract raw encryption key from the RAM is to perform DMA attack, where the key is leaked out through a DMA-capable peripheral port like Thunderbolt.

This can be mitigated by IOMMU-based DMA protection, where the IOMMU hardware restricts supported peripheral devices to accessing only the memory area assigned to them, similar to how the MMU imposes memory protection between software processes. For unsupported devices, the mitigation is usually requiring logging into the machine before the device can be connected and used.

Without TPMs, devices are also ineligible to Secure Boot and thus susceptible to rootkit attacks where early-launch malware sits under the OS and extracts arbitrary memory content from the machine. You can use the TPM + PIN authentication option to enable platform integrity validation and enforce physical presence (thus mitigating TPM-sniffing attacks) at the same time.

See BitLocker countermeasures article for more attack opportunities and defense measures.

Answer 4

Technically speaking, yes, there are attacks faster than brute force.

BitLocker uses the AES algorithm for encrypting the data on the disks, and there is a known attack against AES that provides about a three-fold speedup over brute force for AES-256, and a four-fold speedup for AES-128. Still completely impractical, so if you need to decrypt something without having access to the password, your best bet is still something like a side-channel attack or a $5 wrench.