Blob cryptoKey = Crypto.generateAesKey(128);
System.debug('KeyBlobSize:-'+cryptoKey.size());
System.debug('Blob:-'+EncodingUtil.base64Encode(cryptoKey));

Blob newcryptoKey128 = Blob.valueOf('xGhMrqIixKvQ4S1bqv8CYw==');
Blob newcryptoKey192 = Blob.valueOf('kBsCore1SUxZ8sTxSU5FCGqUGLhJ8IGq');
Blob newcryptoKey256 = Blob.valueOf('Wc0gT/1xfFAjlRwip7l7MmEdjw7DzMXamEHLjyCOQQ0=');

system.debug('Size1:-'+newcryptoKey128.size());
system.debug('Size2:-'+newcryptoKey192.size());
system.debug('Size3:-'+newcryptoKey256.size());

Blob data = Blob.valueOf('Test data to encrypted');
Blob encryptedData = Crypto.encryptWithManagedIV('AES256', newcryptoKey256, data);

EXCEPTION: System.InvalidParameterValueException: Invalid private key. Must be 32 bytes.
STACKTRACE: Class.System.Crypto.encryptWithManagedIV: line 55, column 1

String algorithmName = 'AES256';
    Blob privateKey = Crypto.generateAesKey(256);
    Blob data = Blob.valueOf('Test data to encrypted');
    Blob encryptedData = Crypto.encryptWithManagedIV(algorithmName, privateKey, data);
    String encodedString = EncodingUtil.base64Encode(encryptedData);

Blob decodedData=EncodingUtil.base64Decode(encodedString);
Blob decryptedData = Crypto.decryptWithManagedIV(algorithmName, privateKey, decodedData);