ssh: support ECDSA private keys in OpenSSH format

This adds support for parsing OpenSSH ECDSA private keys. It
implements parsing for P-256, P-384, and P-521 keys.

Fixes golang/go#36722

Change-Id: I77c8e0a23ed6353f6667686cc79ec14661cb10db
GitHub-Last-Rev: 2324b920d080fc7ac35fbcf0a79e25161b6a7f82
GitHub-Pull-Request: golang/crypto#114
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/215540
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/testdata/keys.go

@@ -113,6 +113,37 @@ pY2QA+P3QlrKxT/VWUMjHUbNNdYfJm48xu0SGNMRdKMAAABBAORh2NP/06JUV3J9W/2Hju
 X1ViJuqqcQnJPVzpgSL826EC2xwOECTqoY8uvFpUdD7CtpksIxNVqRIhuNOlz0lqEAAABB
 ANkaHTTaPojClO0dKJ/Zjs7pWOCGliebBYprQ/Y4r9QLBkC/XaWMS26gFIrjgC7D2Rv+rZ
 wSD0v0RcmkITP1ZR0AAAAYcHF1ZXJuYUBMdWNreUh5ZHJvLmxvY2FsAQID
+-----END OPENSSH PRIVATE KEY-----`),
+	"p256-openssh-format": []byte(`-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAaAAAABNlY2RzYS
+1zaGEyLW5pc3RwMjU2AAAACG5pc3RwMjU2AAAAQQSN5Ld/DFy8LJK0yrWg+Ryhq4/ifHry
+QyCQeT4UXSB+UGdRct7kWA0hARbTaSCh+8U/Gs5O+IkDNoTKVsgxKUMQAAAAsO3C7nPtwu
+5zAAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBI3kt38MXLwskrTK
+taD5HKGrj+J8evJDIJB5PhRdIH5QZ1Fy3uRYDSEBFtNpIKH7xT8azk74iQM2hMpWyDEpQx
+AAAAAhAIHB48R+goZaiXndfYTrwk4BT1+MeLPC2/dwe0J5d1QDAAAAE21hcmlhbm9AZW5k
+b3IubG9jYWwBAgME
+-----END OPENSSH PRIVATE KEY-----`),
+	"p384-openssh-format": []byte(`-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAiAAAABNlY2RzYS
+1zaGEyLW5pc3RwMzg0AAAACG5pc3RwMzg0AAAAYQTZb2VzEPs2NN/i1qHddKTVfwoIq3Tf
+PeQ/kcWBvuCVJfIygvpm9MeusawEPuLSEXwiNDew+YHZ9xHIvFjCmZsLuEOzuh9t9KotwM
+57H+7N+RDFzhM2j8hAaOuT5XDLKfUAAADgn/Sny5/0p8sAAAATZWNkc2Etc2hhMi1uaXN0
+cDM4NAAAAAhuaXN0cDM4NAAAAGEE2W9lcxD7NjTf4tah3XSk1X8KCKt03z3kP5HFgb7glS
+XyMoL6ZvTHrrGsBD7i0hF8IjQ3sPmB2fcRyLxYwpmbC7hDs7ofbfSqLcDOex/uzfkQxc4T
+No/IQGjrk+Vwyyn1AAAAMQDg0hwGKB/9Eq+e2FeTspi8QHW5xTD6prqsHDFx4cKk0ccgFV
+61dhFhD/8SEbYlHzEAAAATbWFyaWFub0BlbmRvci5sb2NhbAECAwQ=
+-----END OPENSSH PRIVATE KEY-----`),
+	"p521-openssh-format": []byte(`-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAArAAAABNlY2RzYS
+1zaGEyLW5pc3RwNTIxAAAACG5pc3RwNTIxAAAAhQQBKzI3QSp1a2e1zMulZl1uFF1Y2Dnv
+LSIwEu837hOV1epYEgNveAhGNm57TuBqYtnZeVfd2pzaz7CKX6N4B33N1XABQ5Ngji7lF2
+dUbmhNqJoMh43ioIsQNBaBenhmRpYP6f5k8P/7JZMIsLhkJk2hykb8maSZ+B3PYwPMNBdS
+vP+0sHQAAAEYIsr2CCLK9ggAAAATZWNkc2Etc2hhMi1uaXN0cDUyMQAAAAhuaXN0cDUyMQ
+AAAIUEASsyN0EqdWtntczLpWZdbhRdWNg57y0iMBLvN+4TldXqWBIDb3gIRjZue07gamLZ
+2XlX3dqc2s+wil+jeAd9zdVwAUOTYI4u5RdnVG5oTaiaDIeN4qCLEDQWgXp4ZkaWD+n+ZP
+D/+yWTCLC4ZCZNocpG/Jmkmfgdz2MDzDQXUrz/tLB0AAAAQgEdeH+im6iRcP/juTAoeSHo
+ExLtWhgL4JYqRwcOnzCKuLOPjEY/HSOuc+HRrbN9rbjsq+PcPHYe1NnkzXk0IW8hxQAAAB
+NtYXJpYW5vQGVuZG9yLmxvY2FsAQIDBAUGBw==
 -----END OPENSSH PRIVATE KEY-----`),
 	"user": []byte(`-----BEGIN EC PRIVATE KEY-----
 MHcCAQEEILYCAeq8f7V4vSSypRw7pxy8yz3V5W4qg8kSC3zJhqpQoAoGCCqGSM49