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I'm trying to move cryptographic key operations in the Monero Python module from pure Python to pynacl which is a wrapper around libsodium.

The first problem I stumbled upon is that Monero secret keys are 256 bit (32 bytes) while nacl.bindings.crypto_sign_ed25519_sk_to_pk expects the secret key to be twice as large (64 bytes).

I also see that unclamped scalar is also 64-byte long. Perhaps the reason for key size difference is related to that?

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Looking at the source you can see it's reading the end bytes from the parameter, and as crypto_sign_SEEDBYTES is defined (1,2) as 32, only the last 32 bytes of the supplied 64 byte parameter are used.

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    This is impressive how hours of looking int the source code may be saved by a simple remark of fellow developer. Thx!
    – emesik
    Dec 3 '21 at 8:20

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