crypto/x509: deprecate legacy PEM encryption

It's unfortunate that we don't implement PKCS#8 encryption (#8860)
so we can't recommend an alternative but PEM encryption is so broken
that it's worth deprecating outright.

Fixes #41949
Fixes #32777

Change-Id: Ieb46444662adec108d0de3550b693a50545c2344
Reviewed-on: https://go-review.googlesource.com/c/go/+/264159
Trust: Filippo Valsorda <filippo@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

Author: FiloSottile

src/crypto/x509/pem_decrypt.go

@@ -95,7 +95,12 @@ func (c rfc1423Algo) deriveKey(password, salt []byte) []byte {
 	return out
 }
 
-// IsEncryptedPEMBlock returns if the PEM block is password encrypted.
+// IsEncryptedPEMBlock returns whether the PEM block is password encrypted
+// according to RFC 1423.
+//
+// Deprecated: Legacy PEM encryption as specified in RFC 1423 is insecure by
+// design. Since it does not authenticate the ciphertext, it is vulnerable to
+// padding oracle attacks that can let an attacker recover the plaintext.
 func IsEncryptedPEMBlock(b *pem.Block) bool {
 	_, ok := b.Headers["DEK-Info"]
 	return ok
@@ -104,14 +109,18 @@ func IsEncryptedPEMBlock(b *pem.Block) bool {
 // IncorrectPasswordError is returned when an incorrect password is detected.
 var IncorrectPasswordError = errors.New("x509: decryption password incorrect")
 
-// DecryptPEMBlock takes a password encrypted PEM block and the password used to
-// encrypt it and returns a slice of decrypted DER encoded bytes. It inspects
-// the DEK-Info header to determine the algorithm used for decryption. If no
-// DEK-Info header is present, an error is returned. If an incorrect password
-// is detected an IncorrectPasswordError is returned. Because of deficiencies
-// in the encrypted-PEM format, it's not always possible to detect an incorrect
-// password. In these cases no error will be returned but the decrypted DER
-// bytes will be random noise.
+// DecryptPEMBlock takes a PEM block encrypted according to RFC 1423 and the
+// password used to encrypt it and returns a slice of decrypted DER encoded
+// bytes. It inspects the DEK-Info header to determine the algorithm used for
+// decryption. If no DEK-Info header is present, an error is returned. If an
+// incorrect password is detected an IncorrectPasswordError is returned. Because
+// of deficiencies in the format, it's not always possible to detect an
+// incorrect password. In these cases no error will be returned but the
+// decrypted DER bytes will be random noise.
+//
+// Deprecated: Legacy PEM encryption as specified in RFC 1423 is insecure by
+// design. Since it does not authenticate the ciphertext, it is vulnerable to
+// padding oracle attacks that can let an attacker recover the plaintext.
 func DecryptPEMBlock(b *pem.Block, password []byte) ([]byte, error) {
 	dek, ok := b.Headers["DEK-Info"]
 	if !ok {
@@ -178,8 +187,12 @@ func DecryptPEMBlock(b *pem.Block, password []byte) ([]byte, error) {
 }
 
 // EncryptPEMBlock returns a PEM block of the specified type holding the
-// given DER-encoded data encrypted with the specified algorithm and
-// password.
+// given DER encoded data encrypted with the specified algorithm and
+// password according to RFC 1423.
+//
+// Deprecated: Legacy PEM encryption as specified in RFC 1423 is insecure by
+// design. Since it does not authenticate the ciphertext, it is vulnerable to
+// padding oracle attacks that can let an attacker recover the plaintext.
 func EncryptPEMBlock(rand io.Reader, blockType string, data, password []byte, alg PEMCipher) (*pem.Block, error) {
 	ciph := cipherByKey(alg)
 	if ciph == nil {