diff --git a/internal/x509util/verify.go b/internal/x509util/verify.go
new file mode 100644
index 00000000..bb46612f
--- /dev/null
+++ b/internal/x509util/verify.go
@@ -0,0 +1,347 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package x509util
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// UnknownAuthorityError results when the certificate issuer is unknown
+type UnknownAuthorityError struct {
+	Cert *x509.Certificate
+	// hintErr contains an error that may be helpful in determining why an
+	// authority wasn't found.
+	hintErr error
+	// hintCert contains a possible authority certificate that was rejected
+	// because of the error in hintErr.
+	hintCert *x509.Certificate
+}
+
+func (e UnknownAuthorityError) Error() string {
+	s := "x509: certificate signed by unknown authority"
+	if e.hintErr != nil {
+		certName := e.hintCert.Subject.CommonName
+		if len(certName) == 0 {
+			if len(e.hintCert.Subject.Organization) > 0 {
+				certName = e.hintCert.Subject.Organization[0]
+			} else {
+				certName = "serial:" + e.hintCert.SerialNumber.String()
+			}
+		}
+		s += fmt.Sprintf(" (possibly because of %q while trying to verify candidate authority certificate %q)", e.hintErr, certName)
+	}
+	return s
+}
+
+// errNotParsed is returned when a certificate without ASN.1 contents is
+// verified. Platform-specific verification needs the ASN.1 contents.
+var errNotParsed = errors.New("x509: missing ASN.1 contents; use ParseCertificate")
+
+// VerifyOptions contains parameters for Certificate.Verify.
+type VerifyOptions struct {
+	// Intermediates is an optional pool of certificates that are not trust
+	// anchors, but can be used to form a chain from the leaf certificate to a
+	// root certificate.
+	Intermediates *x509.CertPool
+	// Roots is the set of trusted root certificates the leaf certificate needs
+	// to chain up to. If nil, the system roots or the platform verifier are used.
+	Roots *x509.CertPool
+	// KeyUsages specifies which Extended Key Usage values are acceptable. A
+	// chain is accepted if it allows any of the listed values. An empty list
+	// means ExtKeyUsageServerAuth. To accept any key usage, include ExtKeyUsageAny.
+	KeyUsages []x509.ExtKeyUsage
+}
+
+const (
+	leafCertificate = iota
+	intermediateCertificate
+	rootCertificate
+)
+
+// isValid performs validity checks on c given that it is a candidate to append
+// to the chain in currentChain.
+func isValid(c *x509.Certificate, certType int, currentChain []*x509.Certificate, opts *VerifyOptions) error {
+	// UnhandledCriticalExtension check deleted.
+	// Precertificates have the poison extension which the Go library code does
+	// not recognize; also the Go library code does not support the standard
+	// PolicyConstraints extension (which is required to be marked critical, RFC
+	// 5280 s4.2.1.11)
+	// TODO(phboneff): re-evaluate whether PolicyConstraints is still an issue.
+	if len(currentChain) > 0 {
+		child := currentChain[len(currentChain)-1]
+		if !bytes.Equal(child.RawIssuer, c.RawSubject) {
+			return x509.CertificateInvalidError{c, x509.NameMismatch, ""}
+		}
+	}
+
+	// Expired checks disabled.
+	// CT servers handle this at submission time, and accept certificates even
+	// if they are expired.
+
+	if certType == intermediateCertificate || certType == rootCertificate {
+		if len(currentChain) == 0 {
+			return errors.New("x509: internal error: empty chain when appending CA cert")
+		}
+	}
+
+	// CANotAuthorizedForThisName check deleted.
+	// Allow to log all certificates, even if they have been isued by a CA that
+	// is not auhotized to issue certs for a given domain.
+	// TODO(phboneff): check whether we can add this constraint back to be closer
+	// to the x509 library.
+
+	// KeyUsage status flags are ignored. From Engineering Security, Peter
+	// Gutmann: A European government CA marked its signing certificates as
+	// being valid for encryption only, but no-one noticed. Another
+	// European CA marked its signature keys as not being valid for
+	// signatures. A different CA marked its own trusted root certificate
+	// as being invalid for certificate signing. Another national CA
+	// distributed a certificate to be used to encrypt data for the
+	// country’s tax authority that was marked as only being usable for
+	// digital signatures but not for encryption. Yet another CA reversed
+	// the order of the bit flags in the keyUsage due to confusion over
+	// encoding endianness, essentially setting a random keyUsage in
+	// certificates that it issued. Another CA created a self-invalidating
+	// certificate by adding a certificate policy statement stipulating
+	// that the certificate had to be used strictly as specified in the
+	// keyUsage, and a keyUsage containing a flag indicating that the RSA
+	// encryption key could only be used for Diffie-Hellman key agreement.
+
+	if certType == intermediateCertificate && (!c.BasicConstraintsValid || !c.IsCA) {
+		return x509.CertificateInvalidError{c, x509.NotAuthorizedToSign, ""}
+	}
+
+	// TooManyIntermediates check deleted.
+	// Path length checks get confused by the presence of an additional
+	// pre-issuer intermediate.
+
+	return nil
+}
+
+// Verify attempts to verify c by building one or more chains from c to a
+// certificate in opts.Roots, using certificates in opts.Intermediates if
+// needed. If successful, it returns one or more chains where the first
+// element of the chain is c and the last element is from opts.Roots.
+//
+// If opts.Roots is nil, the platform verifier might be used, and
+// verification details might differ from what is described below. If system
+// roots are unavailable the returned error will be of type SystemRootsError.
+//
+// Name constraints in the intermediates will be applied to all names claimed
+// in the chain, not just opts.DNSName. Thus it is invalid for a leaf to claim
+// example.com if an intermediate doesn't permit it, even if example.com is not
+// the name being validated. Note that DirectoryName constraints are not
+// supported.
+//
+// Name constraint validation follows the rules from RFC 5280, with the
+// addition that DNS name constraints may use the leading period format
+// defined for emails and URIs. When a constraint has a leading period
+// it indicates that at least one additional label must be prepended to
+// the constrained name to be considered valid.
+//
+// Extended Key Usage values are enforced nested down a chain, so an intermediate
+// or root that enumerates EKUs prevents a leaf from asserting an EKU not in that
+// list. (While this is not specified, it is common practice in order to limit
+// the types of certificates a CA can issue.)
+//
+// Certificates that use SHA1WithRSA and ECDSAWithSHA1 signatures are not supported,
+// and will not be used to build chains.
+//
+// Certificates other than c in the returned chains should not be modified.
+//
+// WARNING: this function doesn't do any revocation checking.
+func Verify(c *x509.Certificate, opts VerifyOptions) (chains [][]*x509.Certificate, err error) {
+	// Platform-specific verification needs the ASN.1 contents so
+	// this makes the behavior consistent across platforms.
+	if len(c.Raw) == 0 {
+		return nil, errNotParsed
+	}
+	for i := 0; i < opts.Intermediates.len(); i++ {
+		c, _, err := opts.Intermediates.cert(i)
+		if err != nil {
+			return nil, fmt.Errorf("crypto/x509: error fetching intermediate: %w", err)
+		}
+		if len(c.Raw) == 0 {
+			return nil, errNotParsed
+		}
+	}
+
+	// CT server roots MUST not be empty.
+	if opts.Roots == nil {
+		return nil, fmt.Errorf("opts.Roots == nil, roots MUST be provided")
+	}
+
+	err = isValid(c, leafCertificate, nil, &opts)
+	if err != nil {
+		return
+	}
+
+	var candidateChains [][]*x509.Certificate
+	if opts.Roots.contains(c) {
+		candidateChains = [][]*x509.Certificate{{c}}
+	} else {
+		candidateChains, err = buildChains(c, []*x509.Certificate{c}, nil, &opts)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	if len(opts.KeyUsages) == 0 {
+		opts.KeyUsages = []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth}
+	}
+
+	for _, eku := range opts.KeyUsages {
+		if eku == x509.ExtKeyUsageAny {
+			// If any key usage is acceptable, no need to check the chain for
+			// key usages.
+			return candidateChains, nil
+		}
+	}
+
+	if len(candidateChains) == 0 {
+		var details []string
+		err = x509.CertificateInvalidError{c, x509.IncompatibleUsage, strings.Join(details, ", ")}
+		return nil, err
+	}
+
+	return candidateChains, nil
+}
+
+func appendToFreshChain(chain []*x509.Certificate, cert *x509.Certificate) []*x509.Certificate {
+	n := make([]*x509.Certificate, len(chain)+1)
+	copy(n, chain)
+	n[len(chain)] = cert
+	return n
+}
+
+// alreadyInChain checks whether a candidate certificate is present in a chain.
+// Rather than doing a direct byte for byte equivalency check, we check if the
+// subject, public key, and SAN, if present, are equal. This prevents loops that
+// are created by mutual cross-signatures, or other cross-signature bridge
+// oddities.
+func alreadyInChain(candidate *x509.Certificate, chain []*x509.Certificate) bool {
+	type pubKeyEqual interface {
+		Equal(crypto.PublicKey) bool
+	}
+
+	var candidateSAN *pkix.Extension
+	for _, ext := range candidate.Extensions {
+		if ext.Id.Equal(oidExtensionSubjectAltName) {
+			candidateSAN = &ext
+			break
+		}
+	}
+
+	for _, cert := range chain {
+		if !bytes.Equal(candidate.RawSubject, cert.RawSubject) {
+			continue
+		}
+		if !candidate.PublicKey.(pubKeyEqual).Equal(cert.PublicKey) {
+			continue
+		}
+		var certSAN *pkix.Extension
+		for _, ext := range cert.Extensions {
+			if ext.Id.Equal(oidExtensionSubjectAltName) {
+				certSAN = &ext
+				break
+			}
+		}
+		if candidateSAN == nil && certSAN == nil {
+			return true
+		} else if candidateSAN == nil || certSAN == nil {
+			return false
+		}
+		if bytes.Equal(candidateSAN.Value, certSAN.Value) {
+			return true
+		}
+	}
+	return false
+}
+
+// maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
+// that an invocation of buildChains will (transitively) make. Most chains are
+// less than 15 certificates long, so this leaves space for multiple chains and
+// for failed checks due to different intermediates having the same Subject.
+const maxChainSignatureChecks = 100
+
+func buildChains(c *x509.Certificate, currentChain []*x509.Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*x509.Certificate, err error) {
+	var (
+		hintErr  error
+		hintCert *x509.Certificate
+	)
+
+	considerCandidate := func(certType int, candidate potentialParent) {
+		if candidate.cert.PublicKey == nil || alreadyInChain(candidate.cert, currentChain) {
+			return
+		}
+
+		if sigChecks == nil {
+			sigChecks = new(int)
+		}
+		*sigChecks++
+		if *sigChecks > maxChainSignatureChecks {
+			err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
+			return
+		}
+
+		if err := c.CheckSignatureFrom(candidate.cert); err != nil {
+			if hintErr == nil {
+				hintErr = err
+				hintCert = candidate.cert
+			}
+			return
+		}
+
+		err = isValid(candidate.cert, certType, currentChain, opts)
+		if err != nil {
+			if hintErr == nil {
+				hintErr = err
+				hintCert = candidate.cert
+			}
+			return
+		}
+
+		if candidate.constraint != nil {
+			if err := candidate.constraint(currentChain); err != nil {
+				if hintErr == nil {
+					hintErr = err
+					hintCert = candidate.cert
+				}
+				return
+			}
+		}
+
+		switch certType {
+		case rootCertificate:
+			chains = append(chains, appendToFreshChain(currentChain, candidate.cert))
+		case intermediateCertificate:
+			var childChains [][]*x509.Certificate
+			childChains, err = buildChains(candidate.cert, appendToFreshChain(currentChain, candidate.cert), sigChecks, opts)
+			chains = append(chains, childChains...)
+		}
+	}
+
+	for _, root := range opts.Roots.findPotentialParents(c) {
+		considerCandidate(rootCertificate, root)
+	}
+	for _, intermediate := range opts.Intermediates.findPotentialParents(c) {
+		considerCandidate(intermediateCertificate, intermediate)
+	}
+
+	if len(chains) > 0 {
+		err = nil
+	}
+	if len(chains) == 0 && err == nil {
+		err = UnknownAuthorityError{c, hintErr, hintCert}
+	}
+
+	return
+}
diff --git a/internal/x509util/x509.go b/internal/x509util/x509.go
new file mode 100644
index 00000000..01f85f37
--- /dev/null
+++ b/internal/x509util/x509.go
@@ -0,0 +1,5 @@
+package x509util
+
+var (
+	oidExtensionSubjectAltName = []int{2, 5, 29, 17}
+)