lnd.xprv/lnwire/signature.go
Alex Akselrod a20594b0bf lnwire: switch to using a fixed 64-byte encoding for signatures (#86)
This commit modifies the encoding of signatures on the wire to use 
a fixed-size 64-byte format. This change is required as the current spec
draft dictates that all signatures be encoded as `R` and `S` as 32-byte
big-endian integers. With this, signatures are now always a _fixed_ size 
slice of bytes on the wire, which is nice to have. 

Fixes #83.
2016-12-08 12:56:37 -08:00

106 lines
3.2 KiB
Go

package lnwire
import (
"fmt"
"github.com/roasbeef/btcd/btcec"
)
// serializeSigToWire serializes a *Signature to [64]byte in the format
// specified by the Lightning RFC.
func serializeSigToWire(b *[64]byte, e *btcec.Signature) error {
// Serialize the signature with all the checks that entails.
sig := e.Serialize()
// Extract lengths of R and S. The DER representation is laid out as
// 0x30 <length> 0x02 <length r> r 0x02 <length s> s
// which means the length of R is the 4th byte and the length of S
// is the second byte after R ends. 0x02 signifies a length-prefixed,
// zero-padded, big-endian bigint. 0x30 sigifies a DER signature.
// See the Serialize() method for btcec.Signature for details.
rLen := uint8(sig[3])
sLen := uint8(sig[5+rLen])
// Check to make sure R and S can both fit into their intended buffers.
// We check S first because these code blocks decrement sLen and
// rLen in the case of a 33-byte 0-padded integer returned from
// Serialize() and rLen is used in calculating array indices for
// S. We can track this with additional variables, but it's more
// efficient to just check S first.
if sLen > 32 {
if (sLen > 33) || (sig[6+rLen] != 0x00) {
return fmt.Errorf("S is over 32 bytes long " +
"without padding")
} else {
sLen -= 1
copy(b[64-sLen:], sig[7+rLen:])
}
} else {
copy(b[64-sLen:], sig[6+rLen:])
}
// Do the same for R as we did for S
if rLen > 32 {
if (rLen > 33) || (sig[4] != 0x00) {
return fmt.Errorf("R is over 32 bytes long " +
"without padding")
} else {
rLen -= 1
copy(b[32-rLen:], sig[5:5+rLen])
}
} else {
copy(b[32-rLen:], sig[4:4+rLen])
}
return nil
}
// deserializeSigFromWire deserializes a *Signature from [64]byte in the format
// specified by the Lightning RFC.
func deserializeSigFromWire(e **btcec.Signature, b [64]byte) error {
// Extract canonically-padded bigint representations from buffer
r := extractCanonicalPadding(b[0:32])
s := extractCanonicalPadding(b[32:64])
rLen := uint8(len(r))
sLen := uint8(len(s))
// Create a canonical serialized signature. DER format is:
// 0x30 <length> 0x02 <length r> r 0x02 <length s> s
sigBytes := make([]byte, 6+rLen+sLen, 6+rLen+sLen)
sigBytes[0] = 0x30 // DER signature magic value
sigBytes[1] = 4 + rLen + sLen // Length of rest of signature
sigBytes[2] = 0x02 // Big integer magic value
sigBytes[3] = rLen // Length of R
sigBytes[rLen+4] = 0x02 // Big integer magic value
sigBytes[rLen+5] = sLen // Length of S
copy(sigBytes[4:], r) // Copy R
copy(sigBytes[rLen+6:], s) // Copy S
// Parse the signature with strict checks.
sig, err := btcec.ParseDERSignature(sigBytes, btcec.S256())
if err != nil {
return err
}
*e = sig
return nil
}
// extractCanonicalPadding is a utility function to extract the canonical
// padding of a big-endian integer from the wire encoding (a 0-padded
// big-endian integer) such that it passes btcec.canonicalPadding test.
func extractCanonicalPadding(b []byte) []byte {
for i := 0; i < len(b); i++ {
// Found first non-zero byte.
if b[i] > 0 {
// If the MSB is set, we need zero padding.
if b[i]&0x80 == 0x80 {
return append([]byte{0x00}, b[i:]...)
} else {
return b[i:]
}
}
}
return []byte{0x00}
}