(system/keymaster is using them now.)

Change-Id: I8fba501005b9318b7d3a76bf1715fb772b23c49d
Reviewed-on: https://boringssl-review.googlesource.com/4092
Reviewed-by: Adam Langley <agl@google.com>

This change extracts two, common parts of RSA_decrypt and RSA_sign into
a function called |private_transform|. It also allows this to be
overridden in a method, which is convenient for opaque keys that only
expose the raw RSA transform as it means that the padding code from
BoringSSL can be easily reimplemented.

One significant change here is that short RSA ciphertexts will no longer
be accepted. I think this is correct and OpenSSL has a comment about PGP
mistakenly stripping leading zeros. However, these is the possibility
that it could break something.

Change-Id: I258c5cbbf21314cc9b6e8d2a2b898fd9a440cd40
Reviewed-on: https://boringssl-review.googlesource.com/1554
Reviewed-by: Adam Langley <agl@google.com>

It's unused with SSLv2 gone. Also, being a decryption padding check, it really
should be constant-time and isn't.

Change-Id: I96be02cb50f9bf0229b9174eccd80fa338bf8e3e
Reviewed-on: https://boringssl-review.googlesource.com/1254
Reviewed-by: Adam Langley <agl@google.com>

This change adds a new function, BN_bn2bin_padded, that attempts, as
much as possible, to serialise a BIGNUM in constant time.

This is used to avoid some timing leaks in RSA decryption.

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)