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lightning-getsharedsecret - Command for computing an ECDH

getsharedsecret point

The getsharedsecret RPC command computes a shared secret from a given public point, and the secret key of this node. The point is a hexadecimal string of the compressed public key DER-encoding of the SECP256K1 point.

On success, an object is returned, containing:

shared_secret (hex): the SHA-2 of the compressed encoding of the shared secp256k1 point (always 64 characters)

This command may fail if communications with the HSM has a problem; by default lightningd uses a software "HSM" which should never fail in this way. (As of the time of this writing there is no true hardware HSM that lightningd can use, but we are leaving this possibilty open in the future.) In that case, it will return with an error code of 800.

This serves as a key agreement scheme in elliptic-curve based cryptographic standards.

However, note that most key agreement schemes based on Elliptic-Curve Diffie-Hellman do not hash the DER-compressed point. Standards like SECG SEC-1 ECIES specify using the X coordinate of the point instead. The Lightning BOLT standard (which lightningd uses), unlike most other cryptographic standards, specifies the SHA-256 hash of the DER-compressed encoding of the point.

It is not possible to extract the X coordinate of the ECDH point via this API, since there is no known way to reverse the 256-bit SHA-2 hash function. Thus there is no way to implement ECIES and similar standards using this API.

If you know the secret key behind point, you do not need to even call getsharedsecret, you can just multiply the secret key with the node public key.

Typically, a sender will generate an ephemeral secret key and multiply it with the node public key, then use the result to derive an encryption key for a symmetric encryption scheme to encrypt a message that can be read only by that node. Then the ephemeral secret key is multiplied by the standard generator point, and the ephemeral public key and the encrypted message is sent to the node, which then uses getsharedsecret to derive the same key.

The above sketch elides important details like key derivation function, stream encryption scheme, message authentication code, and so on. You should follow an established standard and avoid rolling your own crypto.

ZmnSCPxj <> is mainly responsible.

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