RSA Security 5.2.2 manual AX931PARAMS

B_SIGN_VERIFY_PARAMS

The X9.31 Sample Program

•For digestMethodName, use "sha1"; currently this is the only digest supported.

•For formatMethodName, use "formatX931"; currently this is the only format method supported.

•formatParams requires a pointer to an A_X931_PARAMS structure, which is defined as follows:

typedef struct { unsigned int blockLen; unsigned int oidNum;

ITEM OID;

} A_X931_PARAMS;

The parameters are:

-blockLen: the smallest number of bytes you can use for your block.

Note that AI_SignVerify encodes the input data in blocks. Because of the requirements of the underlying RSA algorithm, the number of bits of data must be the same as the number of bits of the RSA modulus. However, the input block size is measured in bytes. Because the modulus size, which is stored in RSA_MODULUS_BITS, may not be an even number of bytes, you need to calculate the smallest number of bytes you can use for your block. This number is the integer part of (RSA_MODULUS_BITS + 7) / 8. For example, if your modulus is 514 bits long, the smallest block size you can use is the integer part of (514 + 7) / 8, or 65 bytes.

-OIDNum: the object identifier for the SHA1 hash algorithm. Currently this always equals 3 (SHA) for X9.31.

-OID: the object identifier to use for BER encoding. The OID is only used when “formatX932PKCS5” is specified in the formatMethodName of the

structure.

In this example, the formatMethodName is “formatX931”, so the BER encoding of the OID is unnecessary and will be left undefined.

/* Step 2: Set the algorithm object to AI_SignVerify */

x931params.blockLen = ((RSA_MODULUS_BITS + 7) / 8); x931params.oidNum = 3;

C h a p t e r 9 P u t t i n g I t A l l To g e t h e r : A n X 9 . 3 1 E x a m p l e 3 2 1