RSA Security Projection Television 5.2.2 Instrukcja Użytkownika Pobierz pdf (Strona 183)

Chapter 5 Non-Cryptographic Operations 161

Hash-Based Message Authentication Code

Hash-Based Message Authentication

Code (HMAC)

A hash-based message authentication code (HMAC) combines a secret key with a

message digest to create a message authentication code. See “Hash-Based Message

Authentication Codes (HMAC)” on page 49 for a description of the algorithm.

Crypto-C provides an HMAC implementation based on SHA1. Recall that SHA1

produces a 20-byte digest and takes input in 64-byte blocks.

The example in this section corresponds to the file

hmac.c.

Step 1: Creating an Algorithm Object

Declare a variable of type B_ALGORITHM_OBJ. As defined in the function prototype in

Chapter 4 of the Reference Manual, its address is the argument for

B_CreateAlgorithmObject:

Step 2: Setting the Algorithm Object

There is only one AI for hash-based message authentication codes, AI_HMAC. The

Reference Manual Chapter 2 entry for

AI_HMAC states that the format of

info

supplied to

B_SetAlgorithmInfo is a pointer to a B_DIGEST_SPECIFIER structure:

The only choice for

digestInfoType

in Crypto-C is AI_SHA1. In the case of AI_SHA1,

digestInfoParams

should be set to NULL_PTR:

B_ALGORITHM_OBJ HMACDigester = (B_ALGORITHM_OBJ)NULL_PTR;

if ((status = B_CreateAlgorithmObject (&HMACDigester)) != 0)

break;

typedef struct {

B_INFO_TYPE digestInfoType;

POINTER digestInfoParams;

} B_DIGEST_SPECIFIER;

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RSA Security Projection Television 5.2.2 Instrukcja Użytkownika Strona 183