package azureidentity

import (
	"crypto/x509"
	"encoding/base64"
	"encoding/json"
	"encoding/pem"
	"regexp"

	"go.mozilla.org/pkcs7"
	"golang.org/x/xerrors"
)

// allowedSigners matches valid common names listed here:
// https://docs.microsoft.com/en-us/azure/virtual-machines/windows/instance-metadata-service?tabs=linux#tabgroup_14
var allowedSigners = regexp.MustCompile(`^(.*\.)?metadata\.(azure\.(com|us|cn)|microsoftazure\.de)$`)

type metadata struct {
	VMID string `json:"vmId"`
}

// Validate ensures the signature was signed by an Azure certificate.
// It returns the associated VM ID if successful.
func Validate(signature string, options x509.VerifyOptions) (string, error) {
	data, err := base64.StdEncoding.DecodeString(signature)
	if err != nil {
		return "", xerrors.Errorf("decode base64: %w", err)
	}
	pkcs7Data, err := pkcs7.Parse(data)
	if err != nil {
		return "", xerrors.Errorf("parse pkcs7: %w", err)
	}
	signer := pkcs7Data.GetOnlySigner()
	if signer == nil {
		return "", xerrors.New("no signers for signature")
	}
	if !allowedSigners.MatchString(signer.Subject.CommonName) {
		return "", xerrors.Errorf("unmatched common name of signer: %q", signer.Subject.CommonName)
	}
	if options.Intermediates == nil {
		options.Intermediates = x509.NewCertPool()
		for _, cert := range certificates {
			block, rest := pem.Decode([]byte(cert))
			if len(rest) != 0 {
				return "", xerrors.Errorf("invalid certificate. %d bytes remain", len(rest))
			}
			cert, err := x509.ParseCertificate(block.Bytes)
			if err != nil {
				return "", xerrors.Errorf("parse certificate: %w", err)
			}
			options.Intermediates.AddCert(cert)
		}
	}

	_, err = signer.Verify(options)
	if err != nil {
		return "", xerrors.Errorf("verify certificates: %w", err)
	}

	var metadata metadata
	err = json.Unmarshal(pkcs7Data.Content, &metadata)
	if err != nil {
		return "", xerrors.Errorf("unmarshal metadata: %w", err)
	}
	return metadata.VMID, nil
}

var certificates = []string{
	`-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----`,
	`-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----`,
}