ecdsa_s390x.go

     1  // Copyright 2020 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package ecdsa
     6  
     7  import (
     8  	"crypto/elliptic"
     9  	"errors"
    10  	"internal/cpu"
    11  	"io"
    12  	"math/big"
    13  )
    14  
    15  // kdsa invokes the "compute digital signature authentication"
    16  // instruction with the given function code and 4096 byte
    17  // parameter block.
    18  //
    19  // The return value corresponds to the condition code set by the
    20  // instruction. Interrupted invocations are handled by the
    21  // function.
    22  //
    23  //go:noescape
    24  func kdsa(fc uint64, params *[4096]byte) (errn uint64)
    25  
    26  // testingDisableKDSA forces the generic fallback path. It must only be set in tests.
    27  var testingDisableKDSA bool
    28  
    29  // canUseKDSA checks if KDSA instruction is available, and if it is, it checks
    30  // the name of the curve to see if it matches the curves supported(P-256, P-384, P-521).
    31  // Then, based on the curve name, a function code and a block size will be assigned.
    32  // If KDSA instruction is not available or if the curve is not supported, canUseKDSA
    33  // will set ok to false.
    34  func canUseKDSA(c elliptic.Curve) (functionCode uint64, blockSize int, ok bool) {
    35  	if testingDisableKDSA {
    36  		return 0, 0, false
    37  	}
    38  	if !cpu.S390X.HasECDSA {
    39  		return 0, 0, false
    40  	}
    41  	switch c.Params().Name {
    42  	case "P-256":
    43  		return 1, 32, true
    44  	case "P-384":
    45  		return 2, 48, true
    46  	case "P-521":
    47  		return 3, 80, true
    48  	}
    49  	return 0, 0, false // A mismatch
    50  }
    51  
    52  func hashToBytes(dst, hash []byte, c elliptic.Curve) {
    53  	l := len(dst)
    54  	if n := c.Params().N.BitLen(); n == l*8 {
    55  		// allocation free path for curves with a length that is a whole number of bytes
    56  		if len(hash) >= l {
    57  			// truncate hash
    58  			copy(dst, hash[:l])
    59  			return
    60  		}
    61  		// pad hash with leading zeros
    62  		p := l - len(hash)
    63  		for i := 0; i < p; i++ {
    64  			dst[i] = 0
    65  		}
    66  		copy(dst[p:], hash)
    67  		return
    68  	}
    69  	// TODO(mundaym): avoid hashToInt call here
    70  	hashToInt(hash, c).FillBytes(dst)
    71  }
    72  
    73  func signAsm(priv *PrivateKey, csprng io.Reader, hash []byte) (sig []byte, err error) {
    74  	c := priv.Curve
    75  	functionCode, blockSize, ok := canUseKDSA(c)
    76  	if !ok {
    77  		return nil, errNoAsm
    78  	}
    79  	for {
    80  		var k *big.Int
    81  		k, err = randFieldElement(c, csprng)
    82  		if err != nil {
    83  			return nil, err
    84  		}
    85  
    86  		// The parameter block looks like the following for sign.
    87  		// 	+---------------------+
    88  		// 	|   Signature(R)      |
    89  		//	+---------------------+
    90  		//	|   Signature(S)      |
    91  		//	+---------------------+
    92  		//	|   Hashed Message    |
    93  		//	+---------------------+
    94  		//	|   Private Key       |
    95  		//	+---------------------+
    96  		//	|   Random Number     |
    97  		//	+---------------------+
    98  		//	|                     |
    99  		//	|        ...          |
   100  		//	|                     |
   101  		//	+---------------------+
   102  		// The common components(signatureR, signatureS, hashedMessage, privateKey and
   103  		// random number) each takes block size of bytes. The block size is different for
   104  		// different curves and is set by canUseKDSA function.
   105  		var params [4096]byte
   106  
   107  		// Copy content into the parameter block. In the sign case,
   108  		// we copy hashed message, private key and random number into
   109  		// the parameter block.
   110  		hashToBytes(params[2*blockSize:3*blockSize], hash, c)
   111  		priv.D.FillBytes(params[3*blockSize : 4*blockSize])
   112  		k.FillBytes(params[4*blockSize : 5*blockSize])
   113  		// Convert verify function code into a sign function code by adding 8.
   114  		// We also need to set the 'deterministic' bit in the function code, by
   115  		// adding 128, in order to stop the instruction using its own random number
   116  		// generator in addition to the random number we supply.
   117  		switch kdsa(functionCode+136, &params) {
   118  		case 0: // success
   119  			return encodeSignature(params[:blockSize], params[blockSize:2*blockSize])
   120  		case 1: // error
   121  			return nil, errZeroParam
   122  		case 2: // retry
   123  			continue
   124  		}
   125  		panic("unreachable")
   126  	}
   127  }
   128  
   129  func verifyAsm(pub *PublicKey, hash []byte, sig []byte) error {
   130  	c := pub.Curve
   131  	functionCode, blockSize, ok := canUseKDSA(c)
   132  	if !ok {
   133  		return errNoAsm
   134  	}
   135  
   136  	r, s, err := parseSignature(sig)
   137  	if err != nil {
   138  		return err
   139  	}
   140  	if len(r) > blockSize || len(s) > blockSize {
   141  		return errors.New("invalid signature")
   142  	}
   143  
   144  	// The parameter block looks like the following for verify:
   145  	// 	+---------------------+
   146  	// 	|   Signature(R)      |
   147  	//	+---------------------+
   148  	//	|   Signature(S)      |
   149  	//	+---------------------+
   150  	//	|   Hashed Message    |
   151  	//	+---------------------+
   152  	//	|   Public Key X      |
   153  	//	+---------------------+
   154  	//	|   Public Key Y      |
   155  	//	+---------------------+
   156  	//	|                     |
   157  	//	|        ...          |
   158  	//	|                     |
   159  	//	+---------------------+
   160  	// The common components(signatureR, signatureS, hashed message, public key X,
   161  	// and public key Y) each takes block size of bytes. The block size is different for
   162  	// different curves and is set by canUseKDSA function.
   163  	var params [4096]byte
   164  
   165  	// Copy content into the parameter block. In the verify case,
   166  	// we copy signature (r), signature(s), hashed message, public key x component,
   167  	// and public key y component into the parameter block.
   168  	copy(params[0*blockSize+blockSize-len(r):], r)
   169  	copy(params[1*blockSize+blockSize-len(s):], s)
   170  	hashToBytes(params[2*blockSize:3*blockSize], hash, c)
   171  	pub.X.FillBytes(params[3*blockSize : 4*blockSize])
   172  	pub.Y.FillBytes(params[4*blockSize : 5*blockSize])
   173  	if kdsa(functionCode, &params) != 0 {
   174  		return errors.New("invalid signature")
   175  	}
   176  	return nil
   177  }
   178
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