evp.go

package evp

import (
	"crypto/aes"
	"crypto/md5"
	"crypto/sha256"
	"fmt"
	"hash"
)

const (
	pkcs5SaltLen = 8
	aes265KeyLen = 32
)

// BytesToKey implements the Openssl EVP_BytesToKey logic.
// It takes the salt, data, a hash type and the key/block length used by that type.
// As such it differs considerably from the openssl method in C.
func BytesToKey(salt, data []byte, h hash.Hash, keyLen, blockLen int) (key, iv []byte) {
	saltLen := len(salt)
	if saltLen > 0 && saltLen != pkcs5SaltLen {
		panic(fmt.Sprintf("Salt length is %d, expected %d", saltLen, pkcs5SaltLen))
	}
	var (
		concat   []byte
		lastHash []byte
		totalLen = keyLen + blockLen
	)
	for ; len(concat) < totalLen; h.Reset() {
		// concatenate lastHash, data and salt and write them to the hash
		h.Write(append(lastHash, append(data, salt...)...))
		// passing nil to Sum() will return the current hash value
		lastHash = h.Sum(nil)
		// append lastHash to the running total bytes
		concat = append(concat, lastHash...)
	}
	return concat[:keyLen], concat[keyLen:totalLen]
}

// BytesToKeyAES256CBC implements the SHA256 version of EVP_BytesToKey using AES CBC
func BytesToKeyAES256CBC(salt, data []byte) (key []byte, iv []byte) {
	return BytesToKey(salt, data, sha256.New(), aes265KeyLen, aes.BlockSize)
}

// BytesToKeyAES256CBCMD5 implements the MD5 version of EVP_BytesToKey using AES CBC
func BytesToKeyAES256CBCMD5(salt, data []byte) (key []byte, iv []byte) {
	return BytesToKey(salt, data, md5.New(), aes265KeyLen, aes.BlockSize)
}