When DES was originally developed, the NSA changed the s-boxes. For decades people thought that their changes introduced a backdoor but then it was discovered that their changes actually strengthened DES against differential cryptoanalysis.
The NSA is well known for it's placement of backdoors into algorithms. They kinda did this with DES when they reduced it's key strength but Dual_EC_DBRG is the most famous contemporary example of this.
Anyway, my question is... are there any other examples of the NSA strengthening non-classified cryptographic algorithms?