CODEWITHSUNDEEP
openssl
Kgshail
Kgshail

Reputation: 1

Key Computation using openSSL APIs in C

Please suggest me how to implement the following equations using openssl in C.

Out = A^x mod y;

Where Exponent x is random and y is Diffie-Hellman prime modulus.

A = SHA256 (Buffer1 concatenate with Buffer 2).

Upvotes: 0

Views: 294

Answers (1)

Chiara Hsieh
Chiara Hsieh

Reputation: 3393

Your question is not quite clear, so I'll just point out the parts you might be interested.

SHA256 digest

For making a SHA256 digest, look at EVP_Digest page. Also take a look at cyprto/sha1test.c for usage. Replace EVP_sha1() with EVP_sha256() in this line

EVP_Digest(*P,strlen((char *)*P),md,NULL,EVP_sha1(), NULL);

BIGNUMs

For x and y, they are stored as BIGNUMs in OpenSSL. Here is a clear and simple guide to BIGNUMs. Also look at crypto/bn/bntest.c for usage examples.

SHA256 digest to BIGNUM

Now you need to convert message digest A to a BIGNUM, look here and select an appropriate function that fits your needs. BN_bin2bn() might be what you're looking for. If the resulting BIGNUM bn_A is larger than modulus y, this function will do bn_A = bn_A mod y for you:

int BN_mod(BIGNUM *rem, const BIGNUM *a, const BIGNUM *m, BN_CTX *ctx);

, or if you expect both bn_A and y to be non-negative,

int BN_nnmod(BIGNUM *r, const BIGNUM *a, const BIGNUM *m, BN_CTX *ctx);

Random BIGNUM x

Suppose you already assigned modulus y and you want to come up with a random number x such that 0=<x<y, make use of this function

int BN_rand_range(BIGNUM *rnd, BIGNUM *range);

See here for BIGNUM rand functions information.

Modular Multiplication of BIGNUMs

Finally it's time to do the modulus multiplication, use this function (described here):

int BN_mod_mul(BIGNUM *r, BIGNUM *a, BIGNUM *b, const BIGNUM *m, BN_CTX *ctx);

For usage of this function, you can refer to function test_mod_mul in crypto/bn/bntest.c

And that should do it.

Upvotes: 1

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