diff --git a/precerts.go b/precerts.go
new file mode 100644
index 0000000..d58df56
--- /dev/null
+++ b/precerts.go
@@ -0,0 +1,105 @@
+package ctwatch
+
+import (
+	"fmt"
+	"errors"
+	"bytes"
+	"encoding/asn1"
+)
+
+func bitStringEqual (a, b *asn1.BitString) bool {
+	return a.BitLength == b.BitLength && bytes.Equal(a.Bytes, b.Bytes)
+}
+
+var (
+	oidExtensionAuthorityKeyId	= []int{2, 5, 29, 35}
+	oidExtensionCTPoison		= []int{1, 3, 6, 1, 4, 1, 11129, 2, 4, 3}
+)
+func ValidatePrecert (precertBytes []byte, tbsBytes []byte) error {
+	precert, err := ParseCertificate(precertBytes)
+	if err != nil {
+		return errors.New("failed to parse pre-certificate: " + err.Error())
+	}
+	precertTBS, err := precert.ParseTBSCertificate()
+	if err != nil {
+		return errors.New("failed to parse pre-certificate TBS: " + err.Error())
+	}
+	tbs, err := ParseTBSCertificate(tbsBytes)
+	if err != nil {
+		return errors.New("failed to parse TBS: " + err.Error())
+	}
+
+	// Everything must be equal except:
+	//  issuer
+	//  Authority Key Identifier extension (both must have it OR neither can have it)
+	//  CT poison extension (precert must have it, TBS must not have it)
+	if precertTBS.Version != tbs.Version {
+		return errors.New("version not equal")
+	}
+	if !bytes.Equal(precertTBS.SerialNumber.FullBytes, tbs.SerialNumber.FullBytes) {
+		return errors.New("serial number not equal")
+	}
+	sameIssuer := bytes.Equal(precertTBS.Issuer.FullBytes, tbs.Issuer.FullBytes)
+	if !bytes.Equal(precertTBS.SignatureAlgorithm.FullBytes, tbs.SignatureAlgorithm.FullBytes) {
+		return errors.New("SignatureAlgorithm not equal")
+	}
+	if !bytes.Equal(precertTBS.Validity.FullBytes, tbs.Validity.FullBytes) {
+		return errors.New("Validity not equal")
+	}
+	if !bytes.Equal(precertTBS.Subject.FullBytes, tbs.Subject.FullBytes) {
+		return errors.New("Subject not equal")
+	}
+	if !bytes.Equal(precertTBS.PublicKey.FullBytes, tbs.PublicKey.FullBytes) {
+		return errors.New("PublicKey not equal")
+	}
+	if !bitStringEqual(&precertTBS.UniqueId, &tbs.UniqueId) {
+		return errors.New("UniqueId not equal")
+	}
+	if !bitStringEqual(&precertTBS.SubjectUniqueId, &tbs.SubjectUniqueId) {
+		return errors.New("SubjectUniqueId not equal")
+	}
+
+	precertHasPoison := false
+	tbsIndex := 0
+	for precertIndex := range precertTBS.Extensions {
+		precertExt := &precertTBS.Extensions[precertIndex]
+
+		if precertExt.Id.Equal(oidExtensionCTPoison) {
+			if !precertExt.Critical {
+				return errors.New("pre-cert poison extension is not critical")
+			}
+			if !bytes.Equal(precertExt.Value, []byte{0x05, 0x00}) {
+				return errors.New("pre-cert poison extension contains incorrect value")
+			}
+			precertHasPoison = true
+			continue
+		}
+
+		if tbsIndex >= len(tbs.Extensions) {
+			return errors.New("pre-cert contains extension not in TBS")
+		}
+		tbsExt := &tbs.Extensions[tbsIndex]
+
+		if !precertExt.Id.Equal(tbsExt.Id) {
+			return fmt.Errorf("pre-cert and TBS contain different extensions (%v vs %v)", precertExt.Id, tbsExt.Id)
+		}
+		if precertExt.Critical != tbsExt.Critical {
+			return fmt.Errorf("pre-cert and TBS %v extension differs in criticality", precertExt.Id)
+		}
+		if !precertExt.Id.Equal(oidExtensionAuthorityKeyId) || sameIssuer {
+			if !bytes.Equal(precertExt.Value, tbsExt.Value) {
+				return fmt.Errorf("pre-cert and TBS %v extension differs in value", precertExt.Id)
+			}
+		}
+
+		tbsIndex++
+	}
+	if tbsIndex < len(tbs.Extensions) {
+		return errors.New("TBS contains extension not in pre-cert")
+	}
+	if !precertHasPoison {
+		return errors.New("pre-cert does not have poison extension")
+	}
+
+	return nil
+}