Signing - encrypting, tomato - tomato. and they are RSA-encryptet if I remember correctly...
No. Card encryption is definitely based on Blowfish, I just looked it up in gbatek. The encryption was somewhat flawed, however, and the header was unencrypted. This allowed PassMes to execute code, and the encryption was then able to be reverse engineered. Signing is bulletproof provided Nintendo's private key is never leaked.
Edit: And provided they don't use strcmp()
Edit 2: Also, Blowfish is symmetric key. Digital signatures are asymmetric (public-key crypto).
If they really just used blowfish the encryption would have been cracked a LOT sooner. And the DS does use RSA, atleast for downloadplay. Nintendo would have been really stupid if they didn't use it for their cards (which I wouldn't put past them.
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Maybe they used a combination of both. I read somewhere that most of the time RSA is combined with a symetric encryption...