RSA Security 5.2.2 manual BER-Encoding the Digest

Message Digests

Step 6: Destroy

Remember to destroy all objects when you are done with them:

B_DestroyAlgorithmObject (&digester);

BER-Encoding the Digest

If you want to send your digest to someone, you should BER-encode the algorithm identifier and the digest. The Crypto-C function B_EncodeDigestInfo offers a way to put together a string containing your information in BER format.

The example in this section corresponds to the file mdber.c.

The Reference Manual Chapter 4 entry for B_EncodeDigestInfo shows that this function takes six arguments:

int B_EncodeDigestInfo (
unsigned char	*digestInfo,	/* encoded output buffer */
unsigned int	*digestInfoLen,	/* length of encoded output */
unsigned int	maxDigestInfoLen,	/* size of digestInfo buffer */
ITEM	*algorithmID,	/* message digest algorithm identifier */
unsigned char	*digest,	/* message digest value */
unsigned int	digestLen	/* length of digest */
);

The first argument is an address where Crypto-C can drop the BER-encoded digest information. You will have to allocate the space for this buffer. This buffer will contain the algorithm identifier and the 16- or 20-byte digest, the total for MD2 and MD5 digests is 34; for a SHA1 digest, it is 35 bytes. If you want to be safe, you can make the buffer larger.

The second argument is the address of an unsigned int; Crypto-C will place the final length of the BER encoding at that address. The third argument is the buffer size. The fourth is a pointer to an ITEM containing the DER encoding of the message digest algorithm; you obtain the DER encoding by calling B_GetAlgorithmInfo with the appropriate AI with BER encoding. The fifth argument is the digest itself; the sixth is the length of the digest.

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