NaCl: Networking and Cryptography library
Computer Aided Cryptography Engineering
ECRYPT II
Introduction
Features
Installation
Internals
Validation
Public-key cryptography:
Authenticated encryption
Scalar multiplication
Signatures
Secret-key cryptography:
Authenticated encryption
Encryption
Authentication
One-time authentication
Low-level functions:
Hashing
String comparison

String comparison: crypto_verify

C NaCl provides a crypto_verify_16 function callable as follows: 
     #include "crypto_verify_16.h"

     const unsigned char x[16];
     const unsigned char y[16];

     crypto_verify_16(x,y);
The crypto_verify_16 function returns 0 if x[0], x[1], ..., x[15] are the same as y[0], y[1], ..., y[15]. Otherwise it returns -1.

This function is safe to use for secrets x[0], x[1], ..., x[15], y[0], y[1], ..., y[15]. The time taken by crypto_verify_16 is independent of the contents of x[0], x[1], ..., x[15], y[0], y[1], ..., y[15]. In contrast, the standard C comparison function memcmp(x,y,16) takes time that depends on the longest matching prefix of x and y, often allowing easy timing attacks.

C NaCl also provides a similar crypto_verify_32 function.

Version

This is version 2010.03.09 of the verify.html web page.