lnd.xprv/lnwire/signature.go

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package lnwire
import (
"fmt"
"github.com/roasbeef/btcd/btcec"
)
// Sig is a fixed-sized ECDSA signature. Unlike Bitcoin, we use fixed sized
// signatures on the wire, instead of DER encoded signatures. This type
// provides several methods to convert to/from a regular Bitcoin DER encoded
// signature (raw bytes and *btcec.Signature).
type Sig [64]byte
// NewSigFromRawSignature returns a Sig from a Bitcoin raw signature encoded in
// the canonical DER encoding.
func NewSigFromRawSignature(sig []byte) (Sig, error) {
var b Sig
if len(sig) == 0 {
return b, fmt.Errorf("cannot decode empty signature")
}
// 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 signifies a DER signature.
// See the Serialize() method for btcec.Signature for details.
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rLen := sig[3]
sLen := 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 b, fmt.Errorf("S is over 32 bytes long " +
"without padding")
}
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sLen--
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 b, fmt.Errorf("R is over 32 bytes long " +
"without padding")
}
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rLen--
copy(b[32-rLen:], sig[5:5+rLen])
} else {
copy(b[32-rLen:], sig[4:4+rLen])
}
return b, nil
}
// NewSigFromSignature creates a new signature as used on the wire, from an
// existing btcec.Signature.
func NewSigFromSignature(e *btcec.Signature) (Sig, error) {
if e == nil {
return Sig{}, fmt.Errorf("cannot decode empty signature")
}
// Serialize the signature with all the checks that entails.
return NewSigFromRawSignature(e.Serialize())
}
// ToSignature converts the fixed-sized signature to a btcec.Signature objects
// which can be used for signature validation checks.
func (b *Sig) ToSignature() (*btcec.Signature, error) {
// Parse the signature with strict checks.
sigBytes := b.ToSignatureBytes()
sig, err := btcec.ParseDERSignature(sigBytes, btcec.S256())
if err != nil {
return nil, err
}
return sig, nil
}
// ToSignatureBytes serializes the target fixed-sized signature into the raw
// bytes of a DER encoding.
func (b *Sig) ToSignatureBytes() []byte {
// 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
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sigBytes := make([]byte, 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
return sigBytes
}
// 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:]...)
}
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return b[i:]
}
}
return []byte{0x00}
}