Original Base64 value is edited but still it is giving same normal string

Question

I am encrypting the plain text using RSA and converting that value to base64 string.But while decrypting the I altered the base64 string and try to decrypt it...it given me same original text return. Is there any thing wrong ?

Original Plain Text :007189562312 Output Base64 string : VfZN7WXwVz7Rrxb+W08u9F0N9Yt52DUnfCOrF6eltK3tzUUYw7KgvY3C8c+XER5nk6yfQFI9qChAes/czWOjKzIRMUTgGPjPPBfAwUjCv4Acodg7F0+EwPkdnV7Pu7jmQtp4IMgGaNpZpt33DgV5AJYj3Uze0A3w7wSQ6/tIgL4=

Altered Base64 String : VfZN7WXwVz7Rrxb+W08u9F0N9Yt52DUnfCOrF6eltK3tzUUYw7KgvY3C8c+XER5nk6yfQFI9qChAes/czWOjKzIRMUTgGPjPPBfAwUjCv4Acodg7F0+EwPkdnV7Pu7jmQtp4IMgGaNpZpt33DgV5AJYj3Uze0A3w7wSQ6/tIgL4=55

Please explain. Thank you.

Answer 1

I'm assuming you're asking whether the altered ciphertext should have thrown an error when decrypting. It looks like the altered string only adds two characters to the end and is otherwise the same string.

Your Base 64 library probably makes some reasonable assumptions when parsing Base 64 data. Base 64 works by encoding 3 bytes into 4 characters. If at the end the data length is not a multiple of 3 it must be padded. That is signalized by the = at the end of the encoded string.

This also means that during parsing, the library knows that padding characters are at the end and stops parsing there. If the alteration appeared at the end of the string then the encoded ciphertext didn't effectively change.