mkimage: Add support for signing with pkcs11

Add support for signing with the pkcs11 engine. This allows FIT images
to be signed with keys securely stored on a smartcard, hardware security
module, etc without exposing the keys.

Support for other engines can be added in the future by modifying
rsa_engine_get_pub_key() and rsa_engine_get_priv_key() to construct
correct key_id strings.

Signed-off-by: George McCollister <george.mccollister@gmail.com>

doc/uImage.FIT/signature.txt

@@ -385,6 +385,149 @@ Test Verified Boot Run: signed config with bad hash: OK
 Test passed
 
 
+Hardware Signing with PKCS#11
+-----------------------------
+
+Securely managing private signing keys can challenging, especially when the
+keys are stored on the file system of a computer that is connected to the
+Internet. If an attacker is able to steal the key, they can sign malicious FIT
+images which will appear genuine to your devices.
+
+An alternative solution is to keep your signing key securely stored on hardware
+device like a smartcard, USB token or Hardware Security Module (HSM) and have
+them perform the signing. PKCS#11 is standard for interfacing with these crypto
+device.
+
+Requirements:
+Smartcard/USB token/HSM which can work with the pkcs11 engine
+openssl
+libp11 (provides pkcs11 engine)
+p11-kit (recommended to simplify setup)
+opensc (for smartcards and smartcard like USB devices)
+gnutls (recommended for key generation, p11tool)
+
+The following examples use the Nitrokey Pro. Instructions for other devices may vary.
+
+Notes on pkcs11 engine setup:
+
+Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc.
+/usr/share/p11-kit/modules/opensc.module should be present on your system.
+
+
+Generating Keys On the Nitrokey:
+
+$ gpg --card-edit
+
+Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00
+Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+Version ..........: 2.1
+Manufacturer .....: ZeitControl
+Serial number ....: xxxxxxxx
+Name of cardholder: [not set]
+Language prefs ...: de
+Sex ..............: unspecified
+URL of public key : [not set]
+Login data .......: [not set]
+Signature PIN ....: forced
+Key attributes ...: rsa2048 rsa2048 rsa2048
+Max. PIN lengths .: 32 32 32
+PIN retry counter : 3 0 3
+Signature counter : 0
+Signature key ....: [none]
+Encryption key....: [none]
+Authentication key: [none]
+General key info..: [none]
+
+gpg/card> generate
+Make off-card backup of encryption key? (Y/n) n
+
+Please note that the factory settings of the PINs are
+  PIN = '123456' Admin PIN = '12345678'
+You should change them using the command --change-pin
+
+What keysize do you want for the Signature key? (2048) 4096
+The card will now be re-configured to generate a key of 4096 bits
+Note: There is no guarantee that the card supports the requested size.
+  If the key generation does not succeed, please check the
+  documentation of your card to see what sizes are allowed.
+What keysize do you want for the Encryption key? (2048) 4096
+The card will now be re-configured to generate a key of 4096 bits
+What keysize do you want for the Authentication key? (2048) 4096
+The card will now be re-configured to generate a key of 4096 bits
+Please specify how long the key should be valid.
+  0 = key does not expire
+  <n> = key expires in n days
+  <n>w = key expires in n weeks
+  <n>m = key expires in n months
+  <n>y = key expires in n years
+Key is valid for? (0)
+Key does not expire at all
+Is this correct? (y/N) y
+
+GnuPG needs to construct a user ID to identify your key.
+
+Real name: John Doe
+Email address: john.doe@email.com
+Comment:
+You selected this USER-ID:
+  "John Doe <john.doe@email.com>"
+
+Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
+
+
+Using p11tool to get the token URL:
+
+Depending on system configuration, gpg-agent may need to be killed first.
+
+$ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens
+Token 0:
+URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29
+Label: OpenPGP card (User PIN (sig))
+Type: Hardware token
+Manufacturer: ZeitControl
+Model: PKCS#15 emulated
+Serial: 000xxxxxxxxx
+Module: (null)
+
+
+Token 1:
+URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29
+Label: OpenPGP card (User PIN)
+Type: Hardware token
+Manufacturer: ZeitControl
+Model: PKCS#15 emulated
+Serial: 000xxxxxxxxx
+Module: (null)
+
+Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below.
+
+
+Use the URL of the token to list the private keys:
+
+$ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \
+"pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29"
+Token 'OpenPGP card (User PIN (sig))' with URL 'pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29' requires user PIN
+Enter PIN:
+Object 0:
+URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29;id=%01;object=Signature%20key;type=private
+Type: Private key
+Label: Signature key
+Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE;
+ID: 01
+
+Use the label, in this case "Signature key" as the key-name-hint in your FIT.
+
+Create the fitImage:
+$ ./tools/mkimage -f fit-image.its fitImage
+
+
+Sign the fitImage with the hardware key:
+
+$ ./tools/mkimage -F -k \
+"model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \
+-K u-boot.dtb -N pkcs11 -r fitImage
+
+
 Future Work
 -----------
 - Roll-back protection using a TPM is done using the tpm command. This can