Method createSessionKey

src/SRP.cpp:155–181

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void SRP6A::createSessionKey(const uint8_t *publicKey, size_t len){

  TempBuffer<uint8_t> tBuf(768);                  // temporary buffer for staging
  TempBuffer<uint8_t> tHash(64);                  // temporary buffer for storing SHA-512 results

  mbedtls_mpi_read_binary(&A,publicKey,len);      // load client PublicKey into A

  // compute u = SHA512( PAD(A) | PAD(B) )
  
  mbedtls_mpi_write_binary(&A,tBuf,384);          // write A into first half of staging buffer (padding with initial zeros is less than 384 bytes)
  mbedtls_mpi_write_binary(&B,tBuf+384,384);      // write B into second half of staging buffer (padding with initial zeros is less than 384 bytes)
  mbedtls_sha512(tBuf,768,tHash,0);               // create hash of data
  mbedtls_mpi_read_binary(&u,tHash,64);           // load hash result into mpi structure u

  // compute S = (A * v^u)^b %N

  mbedtls_mpi_exp_mod(&t1,&v,&u,&N,&_rr);         // t1 = v^u %N
  mbedtls_mpi_mul_mpi(&t2,&A,&t1);                // t2 = A*t1
  mbedtls_mpi_mod_mpi(&t1,&t2,&N);                // t1 = t2 %N  (this is needed to reduce size of t2 before next calculation)
  mbedtls_mpi_exp_mod(&S,&t1,&b,&N,&_rr);         // S = t1^b %N

  // compute K = SHA512( PAD(S) )
  
  mbedtls_mpi_write_binary(&S,tBuf,384);          // write S into staging buffer (only first half of buffer will be used)
  mbedtls_sha512(tBuf,384,K,0);                   // create hash of data - this is the SRP SHARED SESSION KEY, K
  
}

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