Add AES encrypt/decrypt example #492
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Please note there are two comments in the code that we should probably discuss. First is exposing the AES block size from the AesKeyBits enum. The second is if we should add pkcs7 padding to the crate to prevent people from "rolling their own". |
| // REVIEW NOTE: Tss-esapi likely should expose these as constants from AesKeyBits::Aes128 | ||
| // to prevent ambiguity! | ||
| const AES_BLOCK_SIZE: usize = 16; |
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I agree that we need to expose this constant from somewhere in the code (and I'd welcome it as a separate commit in this PR!), but I disagree it should have anything to do with AesKeyBits because the block size for AES is not tied to key size.
Given that this would be an addition to what the spec provides, the abstraction module would probably be the best place to have it, perhaps as a method on this. But I wouldn't be opposed to also adding it as a method on SymmetricCipherParameters, for example. We just need to figure out what that method should return for Xor and Null, and document them.
See comment below.
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Wait what? I thought that block size was tied to the AESKeyBit size? IE AES-128 requires a 128bit key and a 16 byte block, where AES-256 requires a 256 bit key and a 32 byte block? I'm happy to be wrong, but all my other experience with openssl etc has the above assumptions enshrined.
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See NIST FIPS 197, paragraph three of section 1, first paragraph of section 3.1 ... Rijndael accepts different block sizes, but AES does not.
I'm not that great at digging through OpenSSL docs, the best I found was this.
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Ahhh yes, it's related to cipher selection not key. Maybe it should be on https://docs.rs/tss-esapi/latest/tss_esapi/interface_types/algorithm/enum.SymmetricMode.html or https://docs.rs/tss-esapi/latest/tss_esapi/interface_types/algorithm/enum.SymmetricAlgorithm.html 🤔
AES-GCM makes it tricky since it's block size is 1, but I think the way we have the parameters laid out we need the SymmetricMode + Key to really know what the block size is.
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I'm not actually sure if in the context of TPMs the same applies. I've skimmed a bit through the spec, couldn't find much, I'll need to test it.
| // REVIEW NOTE: Should we added PKCS7 padding as a function to MaxBuffer to prevent | ||
| // people needing to "roll their own"? |
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Yeah, agree, that would be good to have! But again, this seems like an abstraction kind of thing, perhaps it could go in the Cipher module - something like PaddedBuffer? We can then reference these new handy features in the documentation for the encryption/decryption methods, and, of course, in this example.
Actually, it strikes me that we could (maybe even should) offload these details to the cipher RustCrypto traits. For example if we allow Block, or a vector of Blocks, to be converted to MaxBuffer, then padding is free for us, and we can easily get the block size. This could be a first step towards tighter integration with those traits.
What do you think?
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Yeah, I think that's a good idea. But I suspect that is it's own PR?
Still even so, this example has highlighted a few places the api we have can be better :)
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Having that as its own PR makes sense! And we can update this example when that's pushed.
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I'm not even sure we'd need that since https://docs.rs/block-padding/0.3.2/block_padding/struct.Pkcs7.html already has .as_slice and we have https://docs.rs/tss-esapi/latest/tss_esapi/structures/struct.MaxBuffer.html#impl-TryFrom%3C%26%5Bu8%5D%3E-for-MaxBuffer
So we could just swap the padding process here.
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Actually, reading that API it seems to be really limited and complex.
We would need to take the array of bytes, then iterate over it as chunks exact with https://doc.rust-lang.org/std/slice/struct.ChunksExact.html to match the block size.
We need this because we need to check the remainder case. If the remainder is empty, we need to append another block because pkcs7 always pads to avoid ambiguity. So that means we can't just look at the vec and grow it if needed, we have to clone the full input everytime because blocks are effectively an array.
And then a vec of generic array can't do to_slice, so we have to iterate again to make a new vec that we can try_from for max_buffer.
So unless I'm really overlooking something obvious, this seems like a really rough api to want to interface with.
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Have you forgot to sign the commits? |
Very likely :) But it wasn't ready to merge yet either. |
This adds an example of how to use AES-128-CBC encryption and decryption with a TPM. Signed-off-by: William Brown <[email protected]>
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Okay, I've rebased an updated to latest main. I think that pkcs7/padding is "out of scope" for this crate, and I'm going to make an example of exposing the TPM via rust-crypto interfaces so we can inherit their padding options. I'll hopefully have this example "converted" shortly as a POC. |
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This has finally come up again so I should be looping back to improve this shortly :) |
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| impl<'a> cipher::BlockBackend for TpmEnc<'a> { | ||
| fn proc_block(&mut self, mut block: cipher::inout::InOut<'_, '_, cipher::Block<Self>>) { |
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I guess InOut needs to be added here 🙄
This adds an example of how to use AES-128-CBC encryption and decryption with a TPM.