From 9d5f99a5b4f86e5a880296c0d52d55848791871a Mon Sep 17 00:00:00 2001
From: Mike Crute <mike@crute.us>
Date: Tue, 24 Oct 2023 22:19:22 -0700
Subject: crypto: add pkcs7

---
 crypto/pkcs7/padding.go | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)
 create mode 100644 crypto/pkcs7/padding.go

diff --git a/crypto/pkcs7/padding.go b/crypto/pkcs7/padding.go
new file mode 100644
index 0000000..1f90643
--- /dev/null
+++ b/crypto/pkcs7/padding.go
@@ -0,0 +1,41 @@
+package pkcs7
+
+// Shamelessly stolen from: https://wgallagher86.medium.com/pkcs-7-padding-in-go-6da5d1d14590
+
+import "errors"
+
+func Unpad(input []byte) ([]byte, error) {
+	if input == nil || len(input) == 0 {
+		return nil, nil
+	}
+
+	pc := input[len(input)-1]
+	pl := int(pc)
+	err := checkPaddingIsValid(input, pl)
+	if err != nil {
+		return nil, err
+	}
+	return input[:len(input)-pl], nil
+}
+
+func checkPaddingIsValid(input []byte, paddingLength int) error {
+	if len(input) < paddingLength {
+		return errors.New("invalid padding")
+	}
+	p := input[len(input)-(paddingLength):]
+	for _, pc := range p {
+		if uint(pc) != uint(len(p)) {
+			return errors.New("invalid padding")
+		}
+	}
+	return nil
+}
+
+func Pad(input []byte, blockSize int) []byte {
+	r := len(input) % blockSize
+	pl := blockSize - r
+	for i := 0; i < pl; i++ {
+		input = append(input, byte(pl))
+	}
+	return input
+}
-- 
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