Question

public byte[] CryptDeriveKey(
    string algname,
    string alghashname,
    int keySize,
    byte[] rgbIV
)

Can someone please enlighten me as to what options there are in algname? If I want to specify an encryption algorithm for AES-128 and AES-256, what should I put in the algname?

Answer 1

I’m not 100% sure but, algname is your algorithm name. keySize is size of the key.

You should use AES-128 and AES-256 like this;

CryptDeriveKey("AES", "SHA1", 128, aes.IV)

and

CryptDeriveKey("AES", "SHA1", 256, aes.IV)

Check out for more details from MSDN.

Here is a decompiled code for PasswordDeriveBytes.CryptDeriveKey method.

[SecuritySafeCritical]
public byte[] CryptDeriveKey(string algname, string alghashname, int keySize, byte[] rgbIV)
{
    if (keySize < 0)
    {
        throw new CryptographicException(Environment.GetResourceString("Cryptography_InvalidKeySize"));
    }
    int algidHash = X509Utils.NameOrOidToAlgId(alghashname, OidGroup.HashAlgorithm);
    if (algidHash == 0)
    {
        throw new CryptographicException(Environment.GetResourceString("Cryptography_PasswordDerivedBytes_InvalidAlgorithm"));
    }
    int algid = X509Utils.NameOrOidToAlgId(algname, OidGroup.AllGroups);
    if (algid == 0)
    {
        throw new CryptographicException(Environment.GetResourceString("Cryptography_PasswordDerivedBytes_InvalidAlgorithm"));
    }
    if (rgbIV == null)
    {
        throw new CryptographicException(Environment.GetResourceString("Cryptography_PasswordDerivedBytes_InvalidIV"));
    }
    byte[] o = null;
    DeriveKey(this.ProvHandle, algid, algidHash, this._password, this._password.Length, keySize << 0x10, rgbIV, rgbIV.Length, JitHelpers.GetObjectHandleOnStack<byte[]>(ref o));
    return o;
}

Here is a decompiled code of NameOrOidToAlgId method.

internal static int NameOrOidToAlgId(string oid, OidGroup oidGroup)
{
    if (oid == null)
    {
        return 0x8004;
    }
    string str = CryptoConfig.MapNameToOID(oid, oidGroup);
    if (str == null)
    {
        str = oid;
    }
    int algIdFromOid = GetAlgIdFromOid(str, oidGroup);
    switch (algIdFromOid)
    {
        case 0:
        case -1:
            throw new CryptographicException(Environment.GetResourceString("Cryptography_InvalidOID"));
    }
    return algIdFromOid;
}