加密字符串的长度

我需要创建一个数据库列,它将存储使用Triple DES加密的字符串.如何确定加密字符串列的长度？

(欢迎使用Triple DES以外的算法的答案.)

Block-ciphers such as DES, 3DES and AES can only operate on blocks of bytes. DES and 3DES operate on block of 8 bytes and AES on blocks of 16 bytes.

To handle this, you usually apply a reversible padding to your plaintext before encrypting it. It will mostly always be "PKCS"-padding (also called PKCS5- or PKCS7-padding).

PKCS-padding adds at least one byte such that the padded text has a length divisible with the block-length (8 bytes for 3DES). The value of the padding-bytes is the number of bytes added. Fx. ABCDEF is padded to ABCDEF0505050505 and 0011223344556677 is padded to 0011223344566770808080808080808. Note that this is easy to remove: you just look at the final byte of the padded bytes, verify that it is between 1 and the block-length, and remove that number of bytes from the end (verifying that each removed byte has the correct value).

Finally, to answer your question: Assuming you are using 3DES with CBC encryption and PKCS-padding - which you probably are - the encryption of a string of length n will have length:

n + 8 - (n % 8)