Adding TLS 1.2 Support to Your TLS Library - Implementing SSL/TLS Using Cryptography and PKI - page 529

The parser must split the data in half and recognize that the fi rst half is x and
the last half is y .
What about the compressed format? Well, if you think about it, it's somewhat
redundant to provide the y value. After all, an elliptic curve is defi ned by an
algebraic formula that describes y in terms of x . Technically speaking, all you
really need to know is the x value, along with the curve parameters themselves,
and you can compute the y value. The only wrinkle here is that there are two
possible y values for a given x — one positive and one negative — because the
formula is y 2 b . A compressed point, then, is just the x value with

a single extra bit indicating whether the y value is the positive or the negative
one; the implementation must multiply out the elliptic curve to recover y . This
topic only deals with uncompressed points.
Modify the algorithm identifi er as shown in Listing 9-34 to recognize the
new public key algorithm type, and modify parse_public_key_info from
Listing 5-19 as shown in Listing 9-35 to properly parse ECDSA public keys.

x 3 ax

Listing 9-34: “x509.h” ECDSA algorithm identifi er

typedef enum
{
rsa,
dsa,
dh,
ecdsa
}
algorithmIdentifier;

typedef struct
{
algorithmIdentifier algorithm;
…
elliptic_curve ecdsa_curve;
point ecdsa_public_key;
}
public_key_info;

Listing 9-35: “x509.c” parse_public_key_info with ECDSA support

static const unsigned char OID_RSA[] =
{ 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x01, 0x01, 0x01 };
static const unsigned char OID_DSA[] =
{ 0x2A, 0x86, 0x48, 0xCE, 0x38, 0x04, 0x01 };
static const unsigned char OID_DH[] =
{ 0x2A, 0x86, 0x48, 0xCE, 0x3E, 0x02, 0x01 };
static const unsigned char OID_ECDSA[] =
{ 0x2A, 0x86, 0x48, 0xCE, 0x3D, 0x02, 0x01 };

(Continued)