Fix various keys issues (#117)

* Fix armouring headers for public keys
* Fix error for session key decryption failure

crypto/testdata/sessionkey_key

@@ -0,0 +1,31 @@
+-----BEGIN PGP PRIVATE KEY BLOCK-----
+Version: BCPG C# v1.8.8.0
+
+lQOyBF/H5jYBCACIUtGN3wGf8I4Kl+fwz54nrJcPYf88hyUDNOY2esSBqd7RrJCU
+RrWFKmuYHXdstfLwS9h8FLboLB5Gkq+hNnWq66CFC3H93rvmvm8IEU3DcJyUSIMr
+QlQn4+/lsMhHjerwhHJrY/moNaDzZ85WEvnkbptEy8AUJLw0t8wMbujyUfdQnmCK
+CRTB/s/Ol9gq+eRar6YPonr/8R/YM/kColk+7VKENayslb2217X8SZ99MzeWbafP
+9JRq/sl0dAfYH0Gbw+1tTyP5lB5LAbdZ0OVYPW0tnJ9yKjDBjrwn5C/3hDS4fYzk
+FZAix1baNBuqtgRUzCnu0AjZW9LfFrZts6aVAAUT/wkDAsUlnOW3SOQLYLnYo/Td
+zOwk7Myzttd++zQ7rCZOksGFk3qVJOA18z1CKMKg/NB+hDWpw7gw9HejnnFOxZ1x
+ONCQ09x7/nrza13XiLGoeOCgIz4VKkUDiup1l4x9/9P6jVNwSmtSdzgVb+4lv8X2
+YFkt3o/SxlztM2s9ZiFz2IBXMF4FgXogG9417RmXKJc0Ham5rZQ0PEVWqCVmXcPq
+pbb5z/TgwfScAzpKvjFXusR7s+jDYk2PZ8NN4ncOlFRSXOEBTJ98hyx0A520Kqkz
+NqB15SQRIUIwbpsHud+bEYS5jfEMb1fg4GAr/lCeu++g4ckmFeoehSUTRLiQcpZ8
+8iVUrF5VJqj3D4h6hJPaLplA1uqA29cVo8c1OO0RUtOnUEi4MRoGDyf2kFHcKvqk
+lPnb+UxeBI4LLiUWcAKGEHIZujTWfwvfJdBFM5aDZbHFAi7YIX7OBSrQPpa/3uSK
+V6I0jJWtXGw693EQ9BGdlScHp4/iSdSb6cjHjTxqe/jJOadnY5OPb+mHSGs/AWQ8
+4dGERmjRE9nfV8Pau1/3Hk7h5ELjDrDu3X2t4Gc/U8I7I1kWywmQbxJVvfsMjZio
+jOBnaBigjWT2sEFyvgNd96IEpWBIlF25y7AaR9f1oe7INzl0I48xXjyb8JPZyLU3
+9NbWob6MscqQw9ePai/xCeXS5VUYc/VqAp2SYrNuh5YFh52arWjABPWcZxAOSRmT
+OTXf8Oqs4XINYK2uuu25DMwAA7Y0NTEUYETkSOJQ0hrs+vXDabIJIunkr9V4o5kR
+l8ivGEFvKyuq4djPgINbybwtKg5wDAupik8x7vipiGNmkrICDJOaUqhy/JU+QXrU
+an1BvMoFz4bqI2yhjV5OqgLpZFJIoirWb6igkkMhXzbThtq/HrQAiQEcBBABAgAG
+BQJfx+Y2AAoJECkFqdVSI1GtKtAH/jzA+CkPkSaN6A29RP/BWSRSeCEMP9+ihrhh
+AZ2xZYUgeUnvv/UMqI6oq6ytvttMJYlaF8sRVLy/1IPCs3j8uHwqYZTbrfU3U3pC
+52NoalCzYV82QM1RViiOO+yYuR7uR66ETpMmKzl3J9l78756Ti6aYU/mY06QQLc7
+5iGmjv0vQTGLnrmTR60PBWoC1M66v/keJg1caW0o05dQCDScP8TSA/VhDyf2Sn84
+RMNkzQymCb/PpUcDbfcM/7izOsn1A/ypX3p1B7LRr044QylxjrisM0Yyk5ATHWkU
+s9IcG0hCAeASb2stvyioO4GMbelxFOdfDh/2bEWlfUYIMnDToHg=
+=grHo
+-----END PGP PRIVATE KEY BLOCK-----