/// <summary>
/// Compares a hash of the specified plain text value to a given hash
/// value. Plain text is hashed with the same salt value as the original
/// hash.
/// </summary>
/// <param name="plainText">
/// Plain text to be verified against the specified hash. The function
/// does not check whether this parameter is null.
/// </param>
/// <param name="hashAlgorithm">
/// Name of the hash algorithm. Allowed values are: "MD5", "SHA1",
/// "SHA256", "SHA384", and "SHA512" (if any other value is specified,
/// MD5 hashing algorithm will be used). This value is case-insensitive.
/// </param>
/// <param name="hashValue">
/// Base64-encoded hash value produced by ComputeHash function. This value
/// includes the original salt appended to it.
/// </param>
/// <returns>
/// If computed hash mathes the specified hash the function the return
/// value is true; otherwise, the function returns false.
/// </returns>
public static bool VerifyHash(string plainText, string hashValue,
HashAlgorihm hashAlgorithm = HashAlgorihm.SHA256)
{
// Convert base64-encoded hash value into a byte array.
byte[] hashWithSaltBytes = Convert.FromBase64String(hashValue);
// We must know size of hash (without salt).
int hashSizeInBits, hashSizeInBytes;
// Size of hash is based on the specified algorithm.
switch (hashAlgorithm)
{
case HashAlgorihm.SHA1:
hashSizeInBits = 160;
break;
case HashAlgorihm.SHA256:
hashSizeInBits = 256;
break;
case HashAlgorihm.SHA384:
hashSizeInBits = 384;
break;
case HashAlgorihm.SHA512:
hashSizeInBits = 512;
break;
default: // Must be MD5
hashSizeInBits = 128;
break;
}
// Convert size of hash from bits to bytes.
hashSizeInBytes = hashSizeInBits / 8;
// Make sure that the specified hash value is long enough.
if (hashWithSaltBytes.Length < hashSizeInBytes)
{
return(false);
}
// Allocate array to hold original salt bytes retrieved from hash.
byte[] saltBytes = new byte[hashWithSaltBytes.Length -
hashSizeInBytes];
// Copy salt from the end of the hash to the new array.
for (int i = 0; i < saltBytes.Length; i++)
{
saltBytes[i] = hashWithSaltBytes[hashSizeInBytes + i];
}
// Compute a new hash string.
string expectedHashString =
ComputeHash(plainText, hashAlgorithm, saltBytes);
// If the computed hash matches the specified hash,
// the plain text value must be correct.
return(hashValue == expectedHashString);
}
/// <summary>
/// Generates a hash for the given plain text value and returns a
/// base64-encoded result. Before the hash is computed, a random salt
/// is generated and appended to the plain text. This salt is stored at
/// the end of the hash value, so it can be used later for hash
/// verification.
/// </summary>
/// <param name="plainText">
/// Plaintext value to be hashed. The function does not check whether
/// this parameter is null.
/// </param>
/// <param name="hashAlgorithm">
/// Name of the hash algorithm. Allowed values are: "MD5", "SHA1",
/// "SHA256", "SHA384", and "SHA512" (if any other value is specified
/// MD5 hashing algorithm will be used). This value is case-insensitive.
/// </param>
/// <param name="saltBytes">
/// Salt bytes. This parameter can be null, in which case a random salt
/// value will be generated.
/// </param>
/// <returns>
/// Hash value formatted as a base64-encoded string.
/// </returns>
public static string ComputeHash(string plainText,
HashAlgorihm hashAlgorithm = HashAlgorihm.SHA256,
byte[] saltBytes = null)
{
// If salt is not specified, generate it on the fly.
if (saltBytes == null)
{
// Define min and max salt sizes.
int minSaltSize = 4;
int maxSaltSize = 8;
// Generate a random number for the size of the salt.
Random random = new Random();
int saltSize = random.Next(minSaltSize, maxSaltSize);
// Allocate a byte array, which will hold the salt.
saltBytes = new byte[saltSize];
// Initialize a random number generator.
RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
// Fill the salt with cryptographically strong byte values.
rng.GetNonZeroBytes(saltBytes);
}
// Convert plain text into a byte array.
byte[] plainTextBytes = Encoding.Default.GetBytes(plainText);
// Allocate array, which will hold plain text and salt.
byte[] plainTextWithSaltBytes =
new byte[plainTextBytes.Length + saltBytes.Length];
// Copy plain text bytes into resulting array.
for (int i = 0; i < plainTextBytes.Length; i++)
{
plainTextWithSaltBytes[i] = plainTextBytes[i];
}
// Append salt bytes to the resulting array.
for (int i = 0; i < saltBytes.Length; i++)
{
plainTextWithSaltBytes[plainTextBytes.Length + i] = saltBytes[i];
}
// Because we support multiple hashing algorithms, we must define
// hash object as a common (abstract) base class. We will specify the
// actual hashing algorithm class later during object creation.
HashAlgorithm hash;
// Initialize appropriate hashing algorithm class.
switch (hashAlgorithm)
{
case HashAlgorihm.SHA1:
hash = new SHA1Managed();
break;
case HashAlgorihm.SHA256:
hash = new SHA256Managed();
break;
case HashAlgorihm.SHA384:
hash = new SHA384Managed();
break;
case HashAlgorihm.SHA512:
hash = new SHA512Managed();
break;
default:
hash = new MD5CryptoServiceProvider();
break;
}
// Compute hash value of our plain text with appended salt.
byte[] hashBytes = hash.ComputeHash(plainTextWithSaltBytes);
// Create array which will hold hash and original salt bytes.
byte[] hashWithSaltBytes = new byte[hashBytes.Length +
saltBytes.Length];
// Copy hash bytes into resulting array.
for (int i = 0; i < hashBytes.Length; i++)
{
hashWithSaltBytes[i] = hashBytes[i];
}
// Append salt bytes to the result.
for (int i = 0; i < saltBytes.Length; i++)
{
hashWithSaltBytes[hashBytes.Length + i] = saltBytes[i];
}
// Convert result into a base64-encoded string.
string hashValue = Convert.ToBase64String(hashWithSaltBytes);
// Return the result.
return(hashValue);
}
