Creating CSR programmatically

I am trying to create a CSR programmatically. I've read about ASN.1, RFC 2986, X.509.

I have开发者_JAVA百科 also parsed several DER-encoded CSR files manually which were created using OpenSSL.

Everything looks clear except for a couple of things:

1. Public key part contains next bytes 8D 00 30 81 89 02 81 81 before the BIT STRING content (and after 03 81). What is this? I noticed that all CSR files encoded with DER contain them. I didn't find anything about them in the RFC.

2. Signature part contains next unclear bytes before the signature content but after 03 81. As I understand this part contains information about the last octet in BIT STRING (how much bites in last byte actually should be taken). But I don't understand how to decode these bytes. For example the signature could look like the following:

   03 81 81 00 64 12 ... 24 B1 28

   where 03h is a BIT STRING format, 81h length of the bit string, 64 12 ... 24 B1 28 is a signature (but it has length 80h). I don't understand the part 81 00.

Thanks in advance.

1. The BIT STRING in the SubjectPublicKeyInfo depends on your public key algorithm. The contents are again DER-encoded, see RFC 3370 for possibilities.

2. If your signature looks like

   03 81 81 00 64 12 ... 24 B1 28

this is to be interpreted as follows. DER is a TLV (Tag - Length - Value) encoding. So the first byte (octet in their parlance) represents the tag - 03 for BIT STRING as you noticed correctly.

The second byte determines the length:

In the long form, the length octets shall consist of an initial octet and one or more subsequent octets. The initial octet shall be encoded as follows:

a) bit 8 shall be one;

b) bits 7 to 1 shall encode the number of subsequent octets in the length octets, as an unsigned binary integer with bit 7 as the most significant bit;

81has bit 8 set to one, so the remaining bits indicate the number of bytes that determine the overall length. In your case it's simply 1 byte. So the next byte is your lenght - 81 being equal to signature length of 129 bytes. So the following 129 bytes represent your value, starting with the 00.

When you say 'programmatically', what exactly do you mean? In code? If so, what language are you using? The BouncyCastle JCE provider contains classes for generating a PKCS#10 request, provided you're using Java. All you need to do is specify the necessary components (DN, public key, etc.) There is also a .Net implementation, I believe, which may be more suitable for Microsoft environments.