diff --git a/pdf/core/security/handlers.go b/pdf/core/security/handlers.go
index a72e6f2f..48857fb4 100644
--- a/pdf/core/security/handlers.go
+++ b/pdf/core/security/handlers.go
@@ -5,6 +5,8 @@
 
 package security
 
+import "fmt"
+
 // StdHandler is an interface for standard security handlers.
 type StdHandler interface {
 	// GenerateParams uses owner and user passwords to set encryption parameters and generate an encryption key.
@@ -30,3 +32,22 @@ type StdEncryptDict struct {
 	OE, UE []byte // R=6
 	Perms  []byte // An encrypted copy of P (16 bytes). Used to verify permissions. R=6
 }
+
+// checkAtLeast checks the size of the bytes field and returns a descriptive error if it doesn't.
+func checkAtLeast(fnc, field string, exp int, b []byte) error {
+	if len(b) < exp {
+		return errInvalidField{Func: fnc, Field: field, Exp: exp, Got: len(b)}
+	}
+	return nil
+}
+
+type errInvalidField struct {
+	Func  string
+	Field string
+	Exp   int
+	Got   int
+}
+
+func (e errInvalidField) Error() string {
+	return fmt.Sprintf("%s: expected %s field to be %d bytes, got %d", e.Func, e.Field, e.Exp, e.Got)
+}
diff --git a/pdf/core/security/standard_r4.go b/pdf/core/security/standard_r4.go
index c4d655c6..9efc18bf 100644
--- a/pdf/core/security/standard_r4.go
+++ b/pdf/core/security/standard_r4.go
@@ -10,6 +10,7 @@ import (
 	"crypto/md5"
 	"crypto/rand"
 	"crypto/rc4"
+	"encoding/binary"
 	"errors"
 
 	"github.com/unidoc/unidoc/common"
@@ -35,19 +36,11 @@ type stdHandlerR4 struct {
 	ID0    string
 }
 
-func (sh stdHandlerR4) paddedPass(pass []byte) []byte {
+func (stdHandlerR4) paddedPass(pass []byte) []byte {
 	key := make([]byte, 32)
-	if len(pass) >= 32 {
-		for i := 0; i < 32; i++ {
-			key[i] = pass[i]
-		}
-	} else {
-		for i := 0; i < len(pass); i++ {
-			key[i] = pass[i]
-		}
-		for i := len(pass); i < 32; i++ {
-			key[i] = padding[i-len(pass)]
-		}
+	i := copy(key, pass)
+	for ; i < 32; i++ {
+		key[i] = padding[i-len(pass)]
 	}
 	return key
 }
@@ -64,12 +57,9 @@ func (sh stdHandlerR4) alg2(d *StdEncryptDict, pass []byte) []byte {
 	h.Write(d.O)
 
 	// Pass P (Lower order byte first).
-	var p = uint32(d.P)
-	var pb []byte
-	for i := 0; i < 4; i++ {
-		pb = append(pb, byte(((p >> uint(8*i)) & 0xff)))
-	}
-	h.Write(pb)
+	var pb [4]byte
+	binary.LittleEndian.PutUint32(pb[:], uint32(d.P))
+	h.Write(pb[:])
 	common.Log.Trace("go P: % x", pb)
 
 	// Pass ID[0] from the trailer
@@ -82,8 +72,9 @@ func (sh stdHandlerR4) alg2(d *StdEncryptDict, pass []byte) []byte {
 	hashb := h.Sum(nil)
 
 	if d.R >= 3 {
+		h = md5.New()
 		for i := 0; i < 50; i++ {
-			h = md5.New()
+			h.Reset()
 			h.Write(hashb[0 : sh.Length/8])
 			hashb = h.Sum(nil)
 		}
diff --git a/pdf/core/security/standard_r6.go b/pdf/core/security/standard_r6.go
index 3ead14c9..3becf56c 100644
--- a/pdf/core/security/standard_r6.go
+++ b/pdf/core/security/standard_r6.go
@@ -21,6 +21,16 @@ import (
 
 var _ StdHandler = stdHandlerR6{}
 
+// newAESCipher creates a new AES block cipher.
+// The size of a buffer should be exactly 16, 24 or 32 bytes, in other cases the function will panic.
+func newAESCipher(b []byte) cipher.Block {
+	c, err := aes.NewCipher(b)
+	if err != nil {
+		panic(err)
+	}
+	return c
+}
+
 // NewHandlerR6 creates a new standard security handler for R=5 and R=6.
 func NewHandlerR6() StdHandler {
 	return stdHandlerR6{}
@@ -34,6 +44,12 @@ type stdHandlerR6 struct{}
 // 7.6.4.3.2 Algorithm 2.A (page 83)
 func (sh stdHandlerR6) alg2a(d *StdEncryptDict, pass []byte) ([]byte, Permissions, error) {
 	// O & U: 32 byte hash + 8 byte Validation Salt + 8 byte Key Salt
+	if err := checkAtLeast("alg2a", "O", 48, d.O); err != nil {
+		return nil, 0, err
+	}
+	if err := checkAtLeast("alg2a", "U", 48, d.U); err != nil {
+		return nil, 0, err
+	}
 
 	// step a: Unicode normalization
 	// TODO(dennwc): make sure that UTF-8 strings are normalized
@@ -90,6 +106,9 @@ func (sh stdHandlerR6) alg2a(d *StdEncryptDict, pass []byte) ([]byte, Permission
 		ekey = d.UE
 		ukey = nil
 	}
+	if err := checkAtLeast("alg2a", "Key", 32, ekey); err != nil {
+		return nil, 0, err
+	}
 	ekey = ekey[:32]
 
 	// intermediate key
@@ -164,10 +183,7 @@ func alg2b(data, pwd, userKey []byte) []byte {
 		repeat(K1, n)
 
 		// step b: encrypt K1 with AES-128 CBC
-		ac, err := aes.NewCipher(K[0:16])
-		if err != nil {
-			panic(err)
-		}
+		ac := newAESCipher(K[0:16])
 		cbc := cipher.NewCBCEncrypter(ac, K[16:32])
 		cbc.CryptBlocks(K1, K1)
 		E = K1
@@ -225,6 +241,9 @@ func (sh stdHandlerR6) alg2b(R int, data, pwd, userKey []byte) []byte {
 // alg8 computes the encryption dictionary's U (user password) and UE (user encryption) values (R>=5).
 // 7.6.4.4.6 Algorithm 8 (page 86)
 func (sh stdHandlerR6) alg8(d *StdEncryptDict, ekey []byte, upass []byte) error {
+	if err := checkAtLeast("alg8", "Key", 32, ekey); err != nil {
+		return err
+	}
 	// step a: compute U (user password)
 	var rbuf [16]byte
 	if _, err := io.ReadFull(rand.Reader, rbuf[:]); err != nil {
@@ -254,10 +273,7 @@ func (sh stdHandlerR6) alg8(d *StdEncryptDict, ekey []byte, upass []byte) error
 
 	h = sh.alg2b(d.R, str, upass, nil)
 
-	ac, err := aes.NewCipher(h[:32])
-	if err != nil {
-		panic(err)
-	}
+	ac := newAESCipher(h[:32])
 
 	iv := make([]byte, aes.BlockSize)
 	cbc := cipher.NewCBCEncrypter(ac, iv)
@@ -271,6 +287,12 @@ func (sh stdHandlerR6) alg8(d *StdEncryptDict, ekey []byte, upass []byte) error
 // alg9 computes the encryption dictionary's O (owner password) and OE (owner encryption) values (R>=5).
 // 7.6.4.4.7 Algorithm 9 (page 86)
 func (sh stdHandlerR6) alg9(d *StdEncryptDict, ekey []byte, opass []byte) error {
+	if err := checkAtLeast("alg9", "Key", 32, ekey); err != nil {
+		return err
+	}
+	if err := checkAtLeast("alg9", "U", 48, d.U); err != nil {
+		return err
+	}
 	// step a: compute O (owner password)
 	var rbuf [16]byte
 	if _, err := io.ReadFull(rand.Reader, rbuf[:]); err != nil {
@@ -303,10 +325,7 @@ func (sh stdHandlerR6) alg9(d *StdEncryptDict, ekey []byte, opass []byte) error
 
 	h = sh.alg2b(d.R, str, opass, userKey)
 
-	ac, err := aes.NewCipher(h[:32])
-	if err != nil {
-		panic(err)
-	}
+	ac := newAESCipher(h[:32])
 
 	iv := make([]byte, aes.BlockSize)
 	cbc := cipher.NewCBCEncrypter(ac, iv)
@@ -320,6 +339,9 @@ func (sh stdHandlerR6) alg9(d *StdEncryptDict, ekey []byte, opass []byte) error
 // alg10 computes the encryption dictionary's Perms (permissions) value (R=6).
 // 7.6.4.4.8 Algorithm 10 (page 87)
 func (sh stdHandlerR6) alg10(d *StdEncryptDict, ekey []byte) error {
+	if err := checkAtLeast("alg10", "Key", 32, ekey); err != nil {
+		return err
+	}
 	// step a: extend permissions to 64 bits
 	perms := uint64(uint32(d.P)) | (math.MaxUint32 << 32)
 
@@ -346,10 +368,7 @@ func (sh stdHandlerR6) alg10(d *StdEncryptDict, ekey []byte) error {
 	}
 
 	// step f: encrypt permissions
-	ac, err := aes.NewCipher(ekey[:32])
-	if err != nil {
-		panic(err)
-	}
+	ac := newAESCipher(ekey[:32])
 
 	ecb := newECBEncrypter(ac)
 	ecb.CryptBlocks(Perms, Perms)
@@ -360,6 +379,9 @@ func (sh stdHandlerR6) alg10(d *StdEncryptDict, ekey []byte) error {
 
 // alg11 authenticates the user password (R >= 5) and returns the hash.
 func (sh stdHandlerR6) alg11(d *StdEncryptDict, upass []byte) ([]byte, error) {
+	if err := checkAtLeast("alg11", "U", 48, d.U); err != nil {
+		return nil, err
+	}
 	str := make([]byte, len(upass)+8)
 	i := copy(str, upass)
 	i += copy(str[i:], d.U[32:40]) // user Validation Salt
@@ -375,6 +397,12 @@ func (sh stdHandlerR6) alg11(d *StdEncryptDict, upass []byte) ([]byte, error) {
 // alg12 authenticates the owner password (R >= 5) and returns the hash.
 // 7.6.4.4.10 Algorithm 12 (page 87)
 func (sh stdHandlerR6) alg12(d *StdEncryptDict, opass []byte) ([]byte, error) {
+	if err := checkAtLeast("alg12", "U", 48, d.U); err != nil {
+		return nil, err
+	}
+	if err := checkAtLeast("alg12", "O", 48, d.O); err != nil {
+		return nil, err
+	}
 	str := make([]byte, len(opass)+8+48)
 	i := copy(str, opass)
 	i += copy(str[i:], d.O[32:40]) // owner Validation Salt
@@ -391,12 +419,18 @@ func (sh stdHandlerR6) alg12(d *StdEncryptDict, opass []byte) ([]byte, error) {
 // alg13 validates user permissions (P+EncryptMetadata vs Perms) for R=6.
 // 7.6.4.4.11 Algorithm 13 (page 87)
 func (sh stdHandlerR6) alg13(d *StdEncryptDict, fkey []byte) error {
+	if err := checkAtLeast("alg13", "Key", 32, fkey); err != nil {
+		return err
+	}
+	if err := checkAtLeast("alg13", "Perms", 16, d.Perms); err != nil {
+		return err
+	}
 	perms := make([]byte, 16)
 	copy(perms, d.Perms[:16])
 
 	ac, err := aes.NewCipher(fkey[:32])
 	if err != nil {
-		panic(err)
+		return err
 	}
 
 	ecb := newECBDecrypter(ac)