s3proxy: add intial implementation

INSECURE!
The proxy intercepts GetObject and PutObject.
A manual deployment guide is included.
The decryption only relies on a hardcoded, static key.
Do not use with sensitive data; testing only.
* Ticket to track ranged GetObject: AB#3466.

s3proxy/internal/crypto/crypto.go

@@ -0,0 +1,79 @@
+/*
+Copyright (c) Edgeless Systems GmbH
+
+SPDX-License-Identifier: AGPL-3.0-only
+*/
+
+/*
+Package crypto provides encryption and decryption functions for the s3proxy.
+It uses AES-256-GCM to encrypt and decrypt data.
+A new nonce is generated for each encryption operation.
+*/
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"io"
+)
+
+// Encrypt takes a 32 byte key and encrypts a plaintext using AES-256-GCM.
+// Output format is 12 byte nonce + ciphertext.
+func Encrypt(plaintext, key []byte) ([]byte, error) {
+	// Enforce AES-256
+	if len(key) != 32 {
+		return nil, aes.KeySizeError(len(key))
+	}
+
+	// None should not be reused more often that 2^32 times:
+	// https://pkg.go.dev/crypto/cipher#NewGCM
+	// Assuming n encryption operations per second, the key has to be rotated every:
+	// n=1: 2^32 / (60*60*24*365*10) = 135 years.
+	// n=10: 2^32 / (60*60*24*365*10) = 13.5 years.
+	// n=100: 2^32 / (60*60*24*365*10) = 1.3 years.
+	// n=1000: 2^32 / (60*60*24*365*10) = 50 days.
+	nonce := make([]byte, 12)
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, err
+	}
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	aesgcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+	ciphertext := aesgcm.Seal(nil, nonce, plaintext, nil)
+
+	// Prepend the nonce to the ciphertext.
+	ciphertext = append(nonce, ciphertext...)
+
+	return ciphertext, nil
+}
+
+// Decrypt takes a 32 byte key and decrypts a ciphertext using AES-256-GCM.
+// ciphertext is formatted as 12 byte nonce + ciphertext.
+func Decrypt(ciphertext []byte, key []byte) ([]byte, error) {
+	// Enforce AES-256
+	if len(key) != 32 {
+		return nil, aes.KeySizeError(len(key))
+	}
+
+	// Extract the nonce from the ciphertext.
+	nonce := ciphertext[:12]
+	ciphertext = ciphertext[12:]
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	aesgcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+
+	return aesgcm.Open(nil, nonce, ciphertext, nil)
+}