Hi,
I've visited bitaddress.org to generate a paper wallet, but all the private adddresses generated start with '5H', '5J' or '5K'. Anyone knows if this is a bug of the RNG, javascript, browser... or whatever?
This is not a bug.
When converting a private key from a 256 bit binary number to Wallet Import Format, a byte with the value of 128 is added to the front of the number as a version number.
Then a 4 byte checksum is added to the end of the number.
The version number combined with the following byte result in a base58check value between 5H and 5K. As such, the wallet import format bitcoin private for an uncompressed public key will always start with '5H', '5J', or '5K'.