Platform crypto APIs for PKCS#1 RSA signatures vary between expecting the
caller to prepend the DigestInfo prefix (RSA_sign_raw) and prepending it
internally (RSA_sign). Currently, Chromium implements sign or sign_raw as
appropriate. To avoid needing both variants, the new asynchronous methods will
only expose the higher-level one, sign.
To satisfy ports which previously implemented sign_raw, expose the DigestInfo
prefix as a utility function.
BUG=347404
Change-Id: I04c397b5e9502b2942f6698ecf81662a3c9282e6
Reviewed-on: https://boringssl-review.googlesource.com/4940
Reviewed-by: Adam Langley <agl@google.com>
b0acb7743f