I have more than 20 years experience in IT industry and hands on programming experience of more than 15 years in C, C++, C#, Java, etc.
I have written a number of Encryption algorithm like AES (including AESCBC, AESCtr, AesGCM, etc.), Blowfish, Camelia, Rsa, RsaOaep, RsaPkcs1, and so on.
Depending on what you are going to use the encryption for, you will have to choose between symmetric and asymmetric encryption (public/private key). Once you have chosen that, then you can decide on specific algorithm, key length and performance.
I can provide you with any type of encryption in C# on a WPF form.
If you contact me then I can send you a video of a application which does encryption/decryption including those of long strings.