Specified key is not a valid size for this algorithm

Solution 1:

The string "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz012345678912" when base64-decoded yields 48 bytes (384 bits). RijndaelManaged supports 128, 192 and 256 bit keys.

A valid 128-bit key is new byte[]{ 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F } or if you need to get it from base64 : Convert.FromBase64String("AAECAwQFBgcICQoLDA0ODw==").

The default blocksize is 128 bits, so the same byte-array will work as the IV.

Solution 2:

Use the random number generator class (RNGCryptoServiceProvider) to fill a specified buffer with random bytes as follows:

var numberOfBits = 256; // or 192 or 128, however using a larger bit size renders the encrypted data harder to decipher

var ivBytes = new byte[numberOfBits / 8]; // 8 bits per byte

new RNGCryptoServiceProvider().GetBytes(ivBytes);

var rijndaelManagedCipher = new RijndaelManaged();

//Don't forget to set the explicitly set the block size for the IV if you're not using the default of 128

rijndaelManagedCipher.BlockSize = 256;

rijndaelManagedCipher.IV = ivBytes;

Note the same process could be used to derive a key. Hope this helps.