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97 lines
3.1 KiB
97 lines
3.1 KiB
5 years ago
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.TH "LIGHTNING-GETSHAREDSECRET" "7" "" "" "lightning-getsharedsecret"
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.SH NAME
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lightning-getsharedsecret - Command for computing an ECDH
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.SH SYNOPSIS
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\fBgetsharedsecret\fR \fIpoint\fR
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.SH DESCRIPTION
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The \fBgetsharedsecret\fR RPC command computes a shared secret from a
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given public \fIpoint\fR, and the secret key of this node\.
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The \fIpoint\fR is a hexadecimal string of the compressed public
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key DER-encoding of the SECP256K1 point\.
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.SH RETURN VALUE
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On success, \fBgetsharedsecret\fR returns a field \fIshared_secret\fR,
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which is a hexadecimal string of the 256-bit SHA-2 of the
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compressed public key DER-encoding of the SECP256K1 point
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that is the shared secret generated using the
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Elliptic Curve Diffie-Hellman algorithm\.
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This field is 32 bytes (64 hexadecimal characters in a string)\.
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This command may fail if communications with the HSM has a
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problem;
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by default lightningd uses a software "HSM" which should
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never fail in this way\.
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(As of the time of this writing there is no true hardware
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HSM that lightningd can use, but we are leaving this
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possibilty open in the future\.)
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In that case, it will return with an error code of 800\.
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.SH CRYPTOGRAPHIC STANDARDS
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This serves as a key agreement scheme in elliptic-curve based
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cryptographic standards\.
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However, note that most key agreement schemes based on
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Elliptic-Curve Diffie-Hellman do not hash the DER-compressed
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point\.
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Standards like SECG SEC-1 ECIES specify using the X coordinate
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of the point instead\.
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The Lightning BOLT standard (which \fBlightningd\fR uses), unlike
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most other cryptographic standards, specifies the SHA-256 hash
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of the DER-compressed encoding of the point\.
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It is not possible to extract the X coordinate of the ECDH point
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via this API, since there is no known way to reverse the 256-bit
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SHA-2 hash function\.
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Thus there is no way to implement ECIES and similar standards using
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this API\.
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If you know the secret key behind \fIpoint\fR, you do not need to
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even call \fBgetsharedsecret\fR, you can just multiply the secret key
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with the node public key\.
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Typically, a sender will generate an ephemeral secret key
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and multiply it with the node public key,
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then use the result to derive an encryption key
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for a symmetric encryption scheme
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to encrypt a message that can be read only by that node\.
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Then the ephemeral secret key is multiplied
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by the standard generator point,
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and the ephemeral public key and the encrypted message is
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sent to the node,
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which then uses \fBgetsharedsecret\fR to derive the same key\.
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The above sketch elides important details like
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key derivation function, stream encryption scheme,
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message authentication code, and so on\.
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You should follow an established standard and avoid
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rolling your own crypto\.
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.SH AUTHOR
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ZmnSCPxj \fI<ZmnSCPxj@protonmail.com\fR> is mainly responsible\.
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.SH SEE ALSO
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.SH RESOURCES
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.RS
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.IP \[bu]
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BOLT 4: \fIhttps://github.com/lightningnetwork/lightning-rfc/blob/master/04-onion-routing.md#shared-secret\fR
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.IP \[bu]
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BOLT 8: \fIhttps://github.com/lightningnetwork/lightning-rfc/blob/master/08-transport.md#handshake-state\fR
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.IP \[bu]
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SECG SEC-1 ECIES: \fIhttps://secg.org/sec1-v2.pdf\fR
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.IP \[bu]
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Main web site: \fIhttps://github.com/ElementsProject/lightning\fR
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.RE
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