How does RFC2898DeriveBytes generate an AES key?

Question

I saw some code like

string password = "11111111";
byte[] salt = Encoding.ASCII.GetBytes("22222222");
Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(password, salt);
RijndaelAlg.Key = key.GetBytes(RijndaelAlg.KeySize / 8);

I see that the key is generated by Rfc2898DeriveBytes with a passphrase and salt. AES then retrieves the key using GetBytes.

But the question is what does RFC2898DeriveBytes do and what does key.GetBytes (cb) do? Can anyone clarify this? I could not get it from the documentation.

Answer 1

RFC2898 , 2000 . , Rfc2898DeriveBytes . PBKDF2 ( № 2) 5.2 RFC2898. 5.2:

PBKDF2 (. B.1 ) . . ( , . . B.1 .) PBKDF2 .

. RFC2898.

, Rfc2898DeriveBytes.GetBytes, ; PBKDF2 , .

RFC, PBKDF2

PBKDF2 (P, S, c, dkLen)

P - , S - , c - , dkLen - .

RFCs . RFC 1149 , RFC 2324.

Answer 2

Reflector:

public Rfc2898DeriveBytes(string password, byte[] salt) : this(password, salt, 0x3e8)
{
}

public Rfc2898DeriveBytes(string password, int saltSize, int iterations)
{
    if (saltSize < 0)
    {
        throw new ArgumentOutOfRangeException("saltSize", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
    }
    byte[] data = new byte[saltSize];
    Utils.StaticRandomNumberGenerator.GetBytes(data);
    this.Salt = data;
    this.IterationCount = iterations;
    this.m_hmacsha1 = new HMACSHA1(new UTF8Encoding(false).GetBytes(password));
    this.Initialize();
}


public override byte[] GetBytes(int cb)
{
    if (cb <= 0)
    {
        throw new ArgumentOutOfRangeException("cb", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
    }
    byte[] dst = new byte[cb];
    int dstOffset = 0;
    int count = this.m_endIndex - this.m_startIndex;
    if (count > 0)
    {
        if (cb < count)
        {
            Buffer.InternalBlockCopy(this.m_buffer, this.m_startIndex, dst, 0, cb);
            this.m_startIndex += cb;
            return dst;
        }
        Buffer.InternalBlockCopy(this.m_buffer, this.m_startIndex, dst, 0, count);
        this.m_startIndex = this.m_endIndex = 0;
        dstOffset += count;
    }
    while (dstOffset < cb)
    {
        byte[] src = this.Func();
        int num3 = cb - dstOffset;
        if (num3 > 20)
        {
            Buffer.InternalBlockCopy(src, 0, dst, dstOffset, 20);
            dstOffset += 20;
        }
        else
        {
            Buffer.InternalBlockCopy(src, 0, dst, dstOffset, num3);
            dstOffset += num3;
            Buffer.InternalBlockCopy(src, num3, this.m_buffer, this.m_startIndex, 20 - num3);
            this.m_endIndex += 20 - num3;
            return dst;
        }
    }
    return dst;
}