feat: support allowed signers parsing

This just adds the lowest level API to match the
golang.org/x/crypto/ssh package.

The library itself should offer a nicer API addition to also provide
proper verification while utilizing the principals and options from the
allowed_signers file.

Signed-off-by: Hidde Beydals <[email protected]>

allowed_signers.go

@@ -0,0 +1,115 @@
+package sshsig
+
+import (
+	"bytes"
+	"encoding/base64"
+	"errors"
+	"io"
+	"strings"
+
+	"golang.org/x/crypto/ssh"
+)
+
+// ParseAllowedSigner parses an entry in the format of the allowed_signers file.
+//
+// The allowed_signers format is documented in the ssh-keygen(1) manual page.
+// This function will parse a single entry from in. On successful return,
+// principals will contain the list of principals that this entry matches,
+// options will contain the list of options that this entry matches (i.e.
+// "cert-authority", "namespaces=file,git"), and pubKey will contain the
+// public key. See the ssh-keygen(1) manual page for the various forms that a
+// principal string can take, and further details on the options.
+//
+// The unparsed remainder of the input will be returned in rest. This function
+// can be called repeatedly to parse multiple entries.
+//
+// If no entries were found in the input then err will be io.EOF. Otherwise, a
+// non-nil err value indicates a parse error.
+//
+// This function is an addition to the golang.org/x/crypto/ssh package, which
+// does offer ssh.ParseAuthorizedKey and ssh.ParseKnownHosts, but not a parser
+// for allowed_signers files which has a slightly different format.
+func ParseAllowedSigner(in []byte) (principals []string, options []string, pubKey ssh.PublicKey, rest []byte, err error) {
+	for len(in) > 0 {
+		end := bytes.IndexByte(in, '\n')
+		if end != -1 {
+			rest = in[end+1:]
+			in = in[:end]
+		} else {
+			rest = nil
+		}
+
+		end = bytes.IndexByte(in, '\r')
+		if end != -1 {
+			in = in[:end]
+		}
+
+		in = bytes.TrimSpace(in)
+		if len(in) == 0 || in[0] == '#' {
+			in = rest
+			continue
+		}
+
+		i := bytes.IndexAny(in, " \t")
+		if i == -1 {
+			in = rest
+			continue
+		}
+
+		// Split the line into the principal list, options, and key.
+		// The options are not required, and may not be present.
+		keyFields := bytes.Fields(in)
+		if len(keyFields) < 3 || len(keyFields) > 4 {
+			return nil, nil, nil, nil, errors.New("ssh: invalid entry in allowed_signers data")
+		}
+
+		// The first field is the principal list.
+		principals := string(keyFields[0])
+
+		// If there are 4 fields, the second field is the options list.
+		var options string
+		if len(keyFields) == 4 {
+			options = string(keyFields[1])
+		}
+
+		// keyFields[len(keyFields)-2] contains the key type (e.g. "sha-rsa").
+		// This information is also available in the base64-encoded key, and
+		// thus ignored here.
+		key := bytes.Join(keyFields[len(keyFields)-1:], []byte(" "))
+		if pubKey, err = parseAuthorizedKey(key); err != nil {
+			return nil, nil, nil, nil, err
+		}
+		return strings.Split(principals, ","), strings.Split(options, ","), pubKey, rest, nil
+	}
+	return nil, nil, nil, nil, io.EOF
+}
+
+// parseAuthorizedKey parses a public key in OpenSSH authorized_keys format
+// (see sshd(8) manual page) once the options and key type fields have been
+// removed.
+//
+// This function is a modified copy of the parseAuthorizedKey function from the
+// golang.org/x/crypto/ssh package, and does not return any comments.
+//
+// xref: https://cs.opensource.google/go/x/crypto/+/refs/tags/v0.7.0:ssh/keys.go;l=88?q=parseAuthorizedKey
+func parseAuthorizedKey(in []byte) (out ssh.PublicKey, err error) {
+	in = bytes.TrimSpace(in)
+
+	i := bytes.IndexAny(in, " \t")
+	if i == -1 {
+		i = len(in)
+	}
+	base64Key := in[:i]
+
+	key := make([]byte, base64.StdEncoding.DecodedLen(len(base64Key)))
+	n, err := base64.StdEncoding.Decode(key, base64Key)
+	if err != nil {
+		return nil, err
+	}
+	key = key[:n]
+	out, err = ssh.ParsePublicKey(key)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}