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d9b2482415
ChangeLog-Added: New `getsharedsecret` command, which lets you compute a shared secret with this node knowing only a public point. This implements the BOLT standard of hashing the ECDH point, and is incompatible with ECIES.
97 lines
3.1 KiB
Groff
Generated
97 lines
3.1 KiB
Groff
Generated
.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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