AES_DECRYPT Weirdness

[Thomas]

I have a 9 character string (alphanumeric - only A-Z0-9) encrypted with AES_ENCRYPT. I am using AES_DECRYPT to retrieve it. In the table I'm storing in a Column defined as VARCHAR(255).

Things work pretty well but out of 200K records I have found about 500 that will not decrypt. For those records AES_DECRYPT returns a NULL value.

Also this returns the record I want:
SELECT Stuff
FROM MyTableName
WHERE (EncryptedColumn = AES_ENCRYPT("9CharString", "MyKey"));

This does not:
SELECT Stuff
FROM MyTableName
WHERE (AES_DECRYPT(EncryptedColumn, "MyKey") = "9CharString");

I'm running version 4.1.7-standard on Redhat Linux with Perl.

I'm at a loss to figure out what the problem might be.

Am I using the wrong character set? (how do I find that out?)

Is my table column not big enough?

This is not an actual value, but to illustrate:
if my string should be ABC
I AES_DECRYPT is returning NULL.

If I change the string to ABB (encrypting with AES_ENCRYPT)
That works, as does ABD, but ABC still returns NULL.

Huh?