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// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package ecdsa
import (
"crypto/elliptic"
"errors"
"internal/cpu"
"io"
"math/big"
)
// kdsa invokes the "compute digital signature authentication"
// instruction with the given function code and 4096 byte
// parameter block.
//
// The return value corresponds to the condition code set by the
// instruction. Interrupted invocations are handled by the
// function.
//
//go:noescape
func kdsa(fc uint64, params *[4096]byte) (errn uint64)
// testingDisableKDSA forces the generic fallback path. It must only be set in tests.
var testingDisableKDSA bool
// canUseKDSA checks if KDSA instruction is available, and if it is, it checks
// the name of the curve to see if it matches the curves supported(P-256, P-384, P-521).
// Then, based on the curve name, a function code and a block size will be assigned.
// If KDSA instruction is not available or if the curve is not supported, canUseKDSA
// will set ok to false.
func canUseKDSA(c elliptic.Curve) (functionCode uint64, blockSize int, ok bool) {
if testingDisableKDSA {
return 0, 0, false
}
if !cpu.S390X.HasECDSA {
return 0, 0, false
}
switch c.Params().Name {
case "P-256":
return 1, 32, true
case "P-384":
return 2, 48, true
case "P-521":
return 3, 80, true
}
return 0, 0, false // A mismatch
}
func hashToBytes(dst, hash []byte, c elliptic.Curve) {
l := len(dst)
if n := c.Params().N.BitLen(); n == l*8 {
// allocation free path for curves with a length that is a whole number of bytes
if len(hash) >= l {
// truncate hash
copy(dst, hash[:l])
return
}
// pad hash with leading zeros
p := l - len(hash)
for i := 0; i < p; i++ {
dst[i] = 0
}
copy(dst[p:], hash)
return
}
// TODO(mundaym): avoid hashToInt call here
hashToInt(hash, c).FillBytes(dst)
}
func signAsm(priv *PrivateKey, csprng io.Reader, hash []byte) (sig []byte, err error) {
c := priv.Curve
functionCode, blockSize, ok := canUseKDSA(c)
if !ok {
return nil, errNoAsm
}
for {
var k *big.Int
k, err = randFieldElement(c, csprng)
if err != nil {
return nil, err
}
// The parameter block looks like the following for sign.
// +---------------------+
// | Signature(R) |
// +---------------------+
// | Signature(S) |
// +---------------------+
// | Hashed Message |
// +---------------------+
// | Private Key |
// +---------------------+
// | Random Number |
// +---------------------+
// | |
// | ... |
// | |
// +---------------------+
// The common components(signatureR, signatureS, hashedMessage, privateKey and
// random number) each takes block size of bytes. The block size is different for
// different curves and is set by canUseKDSA function.
var params [4096]byte
// Copy content into the parameter block. In the sign case,
// we copy hashed message, private key and random number into
// the parameter block.
hashToBytes(params[2*blockSize:3*blockSize], hash, c)
priv.D.FillBytes(params[3*blockSize : 4*blockSize])
k.FillBytes(params[4*blockSize : 5*blockSize])
// Convert verify function code into a sign function code by adding 8.
// We also need to set the 'deterministic' bit in the function code, by
// adding 128, in order to stop the instruction using its own random number
// generator in addition to the random number we supply.
switch kdsa(functionCode+136, ¶ms) {
case 0: // success
return encodeSignature(params[:blockSize], params[blockSize:2*blockSize])
case 1: // error
return nil, errZeroParam
case 2: // retry
continue
}
panic("unreachable")
}
}
func verifyAsm(pub *PublicKey, hash []byte, sig []byte) error {
c := pub.Curve
functionCode, blockSize, ok := canUseKDSA(c)
if !ok {
return errNoAsm
}
r, s, err := parseSignature(sig)
if err != nil {
return err
}
if len(r) > blockSize || len(s) > blockSize {
return errors.New("invalid signature")
}
// The parameter block looks like the following for verify:
// +---------------------+
// | Signature(R) |
// +---------------------+
// | Signature(S) |
// +---------------------+
// | Hashed Message |
// +---------------------+
// | Public Key X |
// +---------------------+
// | Public Key Y |
// +---------------------+
// | |
// | ... |
// | |
// +---------------------+
// The common components(signatureR, signatureS, hashed message, public key X,
// and public key Y) each takes block size of bytes. The block size is different for
// different curves and is set by canUseKDSA function.
var params [4096]byte
// Copy content into the parameter block. In the verify case,
// we copy signature (r), signature(s), hashed message, public key x component,
// and public key y component into the parameter block.
copy(params[0*blockSize+blockSize-len(r):], r)
copy(params[1*blockSize+blockSize-len(s):], s)
hashToBytes(params[2*blockSize:3*blockSize], hash, c)
pub.X.FillBytes(params[3*blockSize : 4*blockSize])
pub.Y.FillBytes(params[4*blockSize : 5*blockSize])
if kdsa(functionCode, ¶ms) != 0 {
return errors.New("invalid signature")
}
return nil
}
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