Secure symmetric encryption algorithm for any-size base62 data I am searching for a secure algorithm to encrypt base62 (or any other base) data for ids in urls.

It should feature:


No blocksize etc, limiting the length of the message to a factor of n
If you have the decryted and the encrypted message you should not be able to guess the key
An avalanche effect
Any size of charset, not only 256 like in AES, DES etc.
High performance on modern CPUs


This is not required:


Streaming, it is only for small data


Is this possible?
If you define your encryption to be $C=E_{62}(E_k(D_{62}(P)))$ and your decryption to be $P=E_{62}(D_k(D_{62}({C})))$ where $P$ is your encoded URL then you've brought back your problem to finding an encryption function for $l$ bits, where $l$ is the size of $D_{62}(P)$. After that you can "just" look for a Format Preserving Encryption primitive for those $l$ bits. If the encrypted URL is allowed to be larger than the unencrypted URL then you could use CTR mode with a nonce of 8 random bytes.