sec1.go

     1  // Copyright 2012 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 x509
     6  
     7  import (
     8  	"crypto/ecdsa"
     9  	"crypto/elliptic"
    10  	"encoding/asn1"
    11  	"errors"
    12  	"fmt"
    13  	"math/big"
    14  )
    15  
    16  const ecPrivKeyVersion = 1
    17  
    18  // ecPrivateKey reflects an ASN.1 Elliptic Curve Private Key Structure.
    19  // References:
    20  //   RFC 5915
    21  //   SEC1 - http://www.secg.org/sec1-v2.pdf
    22  // Per RFC 5915 the NamedCurveOID is marked as ASN.1 OPTIONAL, however in
    23  // most cases it is not.
    24  type ecPrivateKey struct {
    25  	Version       int
    26  	PrivateKey    []byte
    27  	NamedCurveOID asn1.ObjectIdentifier `asn1:"optional,explicit,tag:0"`
    28  	PublicKey     asn1.BitString        `asn1:"optional,explicit,tag:1"`
    29  }
    30  
    31  // ParseECPrivateKey parses an EC private key in SEC 1, ASN.1 DER form.
    32  //
    33  // This kind of key is commonly encoded in PEM blocks of type "EC PRIVATE KEY".
    34  func ParseECPrivateKey(der []byte) (*ecdsa.PrivateKey, error) {
    35  	return parseECPrivateKey(nil, der)
    36  }
    37  
    38  // MarshalECPrivateKey converts an EC private key to SEC 1, ASN.1 DER form.
    39  //
    40  // This kind of key is commonly encoded in PEM blocks of type "EC PRIVATE KEY".
    41  // For a more flexible key format which is not EC specific, use
    42  // MarshalPKCS8PrivateKey.
    43  func MarshalECPrivateKey(key *ecdsa.PrivateKey) ([]byte, error) {
    44  	oid, ok := oidFromNamedCurve(key.Curve)
    45  	if !ok {
    46  		return nil, errors.New("x509: unknown elliptic curve")
    47  	}
    48  
    49  	return marshalECPrivateKeyWithOID(key, oid)
    50  }
    51  
    52  // marshalECPrivateKey marshals an EC private key into ASN.1, DER format and
    53  // sets the curve ID to the given OID, or omits it if OID is nil.
    54  func marshalECPrivateKeyWithOID(key *ecdsa.PrivateKey, oid asn1.ObjectIdentifier) ([]byte, error) {
    55  	privateKey := make([]byte, (key.Curve.Params().N.BitLen()+7)/8)
    56  	return asn1.Marshal(ecPrivateKey{
    57  		Version:       1,
    58  		PrivateKey:    key.D.FillBytes(privateKey),
    59  		NamedCurveOID: oid,
    60  		PublicKey:     asn1.BitString{Bytes: elliptic.Marshal(key.Curve, key.X, key.Y)},
    61  	})
    62  }
    63  
    64  // parseECPrivateKey parses an ASN.1 Elliptic Curve Private Key Structure.
    65  // The OID for the named curve may be provided from another source (such as
    66  // the PKCS8 container) - if it is provided then use this instead of the OID
    67  // that may exist in the EC private key structure.
    68  func parseECPrivateKey(namedCurveOID *asn1.ObjectIdentifier, der []byte) (key *ecdsa.PrivateKey, err error) {
    69  	var privKey ecPrivateKey
    70  	if _, err := asn1.Unmarshal(der, &privKey); err != nil {
    71  		if _, err := asn1.Unmarshal(der, &pkcs8{}); err == nil {
    72  			return nil, errors.New("x509: failed to parse private key (use ParsePKCS8PrivateKey instead for this key format)")
    73  		}
    74  		if _, err := asn1.Unmarshal(der, &pkcs1PrivateKey{}); err == nil {
    75  			return nil, errors.New("x509: failed to parse private key (use ParsePKCS1PrivateKey instead for this key format)")
    76  		}
    77  		return nil, errors.New("x509: failed to parse EC private key: " + err.Error())
    78  	}
    79  	if privKey.Version != ecPrivKeyVersion {
    80  		return nil, fmt.Errorf("x509: unknown EC private key version %d", privKey.Version)
    81  	}
    82  
    83  	var curve elliptic.Curve
    84  	if namedCurveOID != nil {
    85  		curve = namedCurveFromOID(*namedCurveOID)
    86  	} else {
    87  		curve = namedCurveFromOID(privKey.NamedCurveOID)
    88  	}
    89  	if curve == nil {
    90  		return nil, errors.New("x509: unknown elliptic curve")
    91  	}
    92  
    93  	k := new(big.Int).SetBytes(privKey.PrivateKey)
    94  	curveOrder := curve.Params().N
    95  	if k.Cmp(curveOrder) >= 0 {
    96  		return nil, errors.New("x509: invalid elliptic curve private key value")
    97  	}
    98  	priv := new(ecdsa.PrivateKey)
    99  	priv.Curve = curve
   100  	priv.D = k
   101  
   102  	privateKey := make([]byte, (curveOrder.BitLen()+7)/8)
   103  
   104  	// Some private keys have leading zero padding. This is invalid
   105  	// according to [SEC1], but this code will ignore it.
   106  	for len(privKey.PrivateKey) > len(privateKey) {
   107  		if privKey.PrivateKey[0] != 0 {
   108  			return nil, errors.New("x509: invalid private key length")
   109  		}
   110  		privKey.PrivateKey = privKey.PrivateKey[1:]
   111  	}
   112  
   113  	// Some private keys remove all leading zeros, this is also invalid
   114  	// according to [SEC1] but since OpenSSL used to do this, we ignore
   115  	// this too.
   116  	copy(privateKey[len(privateKey)-len(privKey.PrivateKey):], privKey.PrivateKey)
   117  	priv.X, priv.Y = curve.ScalarBaseMult(privateKey)
   118  
   119  	return priv, nil
   120  }
   121
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