Fix decode_openssh_signature to respect offset

Author: vcsjones

lib/ssh_data/encoding.rb

@@ -329,13 +329,13 @@ def decode_string_public_key(raw, offset=0, algo=nil)
     def decode_openssh_signature(raw, offset=0)
       total_read = 0
 
-      magic = raw.byteslice(total_read, OPENSSH_SIGNATURE_MAGIC.bytesize)
+      magic = raw.byteslice(offset, OPENSSH_SIGNATURE_MAGIC.bytesize)
       unless magic == OPENSSH_SIGNATURE_MAGIC
         raise DecodeError, "bad OpenSSH signature"
       end
 
       total_read += OPENSSH_SIGNATURE_MAGIC.bytesize
-      offset = total_read
+      offset += total_read
       data, read = decode_fields(raw, OPENSSH_SIGNATURE_FIELDS, offset)
       total_read += read
       [data, total_read]

spec/encoding_spec.rb

@@ -806,4 +806,33 @@
       expect(read).to eq(0)
     end
   end
+
+  describe ("#decode_openssh_signature") do
+    let(:signature) {
+      Base64.decode64(%Q(
+        U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgoXgg2XgepEwBr3CQzkHXigy8T8
+        uvs69Dvox5fgnGgPMAAAAEZmlsZQAAAAAAAAAGc2hhNTEyAAAAUwAAAAtzc2gtZWQyNTUx
+        OQAAAECRxENUPwmbRveDvNFOc36EuyMIa6jXWbCVkEQ2dtORyFAnChmr1kHMFX4B9TQm6U
+        ssvYRRUo6ePL5DuAjLP+kD
+      ))
+    }
+
+    it "can decode a signature" do
+      data, total_read = SSHData::Encoding.decode_openssh_signature(signature)
+      expect(total_read).to eq(signature.bytesize)
+      expect(data[:sigversion]).to eq(1)
+      expect(data[:reserved]).to be_empty
+      expect(data[:namespace]).to eq("file")
+      expect(data[:hashalgorithm]).to eq("sha512")
+      expect(data[:signature]).not_to be_empty
+      expect(data[:publickey]).not_to be_empty
+    end
+
+    it "can decode a signature starting at offset" do
+      prefix = "helloworld"
+      signature_appended = prefix + signature
+      data, total_read = SSHData::Encoding.decode_openssh_signature(signature_appended, offset = prefix.bytesize)
+      expect(total_read).to eq(signature.bytesize)
+    end
+  end
 end