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56 lines
2.3 KiB
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56 lines
2.3 KiB
Plaintext
IMPORTANT
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=========
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1. Do not use any of the OpenSSL binaries
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2. Do not compile OpenSSL binaries yourself
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(except for doing some research into the deep internals of OpenSSL)
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REASONS
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=======
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All OpenSSL binaries included are highly experimental and are likely to fail
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in many cases. To get some real benefit from OpenSSL (or PolarSSL) it should
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handle all crypting/hashing.
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However this is not possible because Microsoft has slightly altered AES
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encryption in KMSv6 and uses a non-AES variant of the Rijndael CMAC in
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KMSv4. OpenSSL is not able to handle this if you use it correctly.
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This means OpenSSL can be used safely only for SHA256 and HMAC SHA256
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calculations used in KMSv5 and KMSv6 but the code size benefit is only
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100 to 300 bytes (depending on the architecture).
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To benefit more from OpenSSL (getting it performing the AES stuff) I do
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the first phase of AES encryption/decryption (called key expansion) with my
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own code. I then poke the expanded key into internal OpenSSL structs to make
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it behave in a way not intended by the OpenSSL developers but in a way to
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perform non-standard AES crypting as required by KMSv4 and KMSv6. KMSv5 is
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the only protocol that could use OpenSSL without hacking the OpenSSL internals.
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That means vlmcsd still needs about 40% of the internal AES code plus some
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OpenSSL hacking code to poke the expanded key into OpenSSL.
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The entire OpenSSL hacking does not work in every case because the internal
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OpenSSL structs differ depending on the OpenSSL version, OpenSSL configuration
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at compile time (whether it is configured to use compiled C code or assembler
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code), CPU architecture and CPU features (whether it can perform AES in
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hardware).
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SUMMARY
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=======
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If you use OpenSSL in a safe way (compile with CRYPTO=openssl), there is not
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much benefit from it. The binary may become bigger or smaller and you
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definitely need more RAM when you run vlmcsd or vlmcs.
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If you use hacked OpenSSL (compile with CRYPTO=openssl_with_aes or
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CRYPTO=openssl_with_aes_soft) you risk malfunction of vlmcs/vlmcsd even if it
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performed correctly several times before.
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Both vlmcs and vlmcsd do not have more features when compiled with OpenSSL
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support. It may be faster (especially on CPUs with hardware assisted AES) but
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uses more memory and may fail or perform unreliably.
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