public CngKey Build()
{
if (_theReader.First().Header == "PUBLIC KEY")
{
using (var ms = new MemoryStream(_theReader.First().Body))
using (var br = new BinaryReader(ms))
{
// should have a sequence
br.Require(Asn1Token.Sequence);
// ignore the length
br.ReadLengthField();
// should have an inner sequence
br.Require(Asn1Token.Sequence);
// ignore the length
br.ReadLengthField();
//should have an OID
br.Require(Asn1Token.Oid);
// get its length
int oidLength = br.ReadByte();
var oid = Asn1Token.GetOid(br.ReadBytes(oidLength));
var builder = GetBuilderFor(oid);
return(builder.Build(br));
}
}
throw new NotImplementedException();
}
private CngKeyBuilderImpl GetBuilderFor(Asn1Token.KnownOid oid)
{
switch(oid)
{
case Asn1Token.KnownOid.EcPublicKey:
return new EllipticCurveCngKeyBuilder();
case Asn1Token.KnownOid.RsaEncryptionPkcs1:
return new RsaCngKeyBuilder();
}
throw new UnsupportedCurveException("Unsupported key type with oid");
}
public override CngKey Build(BinaryReader reader)
{
reader.Require(Asn1Token.Oid);
var length = reader.ReadLengthField();
var curve = Asn1Token.GetOid(reader.ReadBytes(length));
if (!isSupportedCurve(curve))
{
throw new UnsupportedCurveException("Unsupported curve oid");
}
// We need to build a key blob structured as follows:
// BCRYPT_ECCKEY_BLOB header
// byte[cbPublicExp] publicExponent - Exponent
// byte[cbModulus] modulus - Modulus
// -- Private only --
// byte[cbPrime1] prime1 - P
// byte[cbPrime2] prime2 - Q
//
// Where
// typedef struct _BCRYPT_ECCKEY_BLOB {
// ULONG Magic; //BCRYPT_ECDSA_PUBLIC_P256_MAGIC = 0x31534345
// ULONG cbKey; // Key length in bytes
//} B
reader.Require(Asn1Token.BitString);
var keyLength = reader.ReadLengthField() - 2;
//ignore the zero byte
reader.Require(0x00);
// if this isn't an uncompressed curve, then panic
reader.Require(0x04);
var x = reader.ReadBytes(keyLength / 2);
var y = reader.ReadBytes(keyLength / 2);
return(BuildEcKey(x, y, curve));
}
private unsafe CngKey BuildEcKey(byte[] x, byte[] y, Asn1Token.KnownOid curve)
{
int headerSize = Marshal.SizeOf(typeof (BCRYPT_ECCKEY_BLOB));
int blobSize = headerSize + x.Length + y.Length;
byte[] blobBytes = new byte[blobSize];
fixed(byte* pBlobBytes = blobBytes)
{
BCRYPT_ECCKEY_BLOB* pBcryptEccBlob = (BCRYPT_ECCKEY_BLOB*) pBlobBytes;
pBcryptEccBlob->KeyBlobMagicNumber = GetMagicNumber(curve);
pBcryptEccBlob->KeySizeBytes = x.Length;
Buffer.BlockCopy(x, 0, blobBytes, headerSize, x.Length);
Buffer.BlockCopy(y, 0, blobBytes, headerSize+x.Length, y.Length);
}
new KeyContainerPermission(KeyContainerPermissionFlags.Import).Assert();
var key = CngKey.Import(blobBytes, CngKeyBlobFormat.EccPublicBlob);
CodeAccessPermission.RevertAssert();
return key;
}
private static bool isSupportedCurve(Asn1Token.KnownOid curve)
{
return curve == Asn1Token.KnownOid.AnsiX9P256R1 ||
curve == Asn1Token.KnownOid.Secp384R1 ||
curve == Asn1Token.KnownOid.Secp521R1;
}
private static int GetMagicNumber(Asn1Token.KnownOid curve)
{
switch(curve)
{
case Asn1Token.KnownOid.AnsiX9P256R1:
return 0x31534345;
case Asn1Token.KnownOid.Secp384R1:
return 0x33534345;
case Asn1Token.KnownOid.Secp521R1:
return 0x35534345;
}
throw new UnsupportedCurveException("Unsupported elliptic curve domain");
}
